© 2015 Mariela E. Pajueloufdcimages.uflib.ufl.edu/UF/E0/04/96/21/00001/PAJUELO_M.pdf · Mariela E....
Transcript of © 2015 Mariela E. Pajueloufdcimages.uflib.ufl.edu/UF/E0/04/96/21/00001/PAJUELO_M.pdf · Mariela E....
THE FORAGING ECOLOGY OF LOGGERHEAD TURTLES IN THE NORTH ATLANTIC:
EVIDENCE FROM STABLE ISOTOPE VALUES
By
MARIELA E. PAJUELO
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2015
© 2015 Mariela E. Pajuelo
To my ever-supporting parents, Furtiño and Rica, my wonderful husband Lucas, and sweetest
son Gabriel
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ACKNOWLEDGMENTS
I thank my advisors Drs. Karen A. Bjorndal and Alan B. Bolten, for their constant and
generous support, guidance, utmost patience, and enthusiasm throughout all these years. I feel
privileged to have been one of their students. I also thank my committee member Dr. Jeffrey A.
Seminoff who supported me from the very beginning of my graduate studies, and who along
with Drs. Bruce MacFadden and Mark Brenner helped with ideas to develop my project.
I also thank the members of the Archie Carr Center for Sea Turtle Research: Hannah
Vander Zanden, Melania López-Castro, Patricia Zárate Bustamante, Luciano Soares, Joseph
Pfaller, Robert Brown, and Peter Eliazar for their unconditional support and friendship during
the PhD program. I am very thankful to my collaborators Michael D. Arendt and his crew from
South Carolina Division of Natural Resources, Kimberly Reich, Allen Foley, Barbara A.
Schroeder, Blair E. Witherington, Pearse Webster, Robbert Prescot, Peter Dutton, Lucy A.
Hawkes, Jeffrey Seminoff, and Cynthia Lagueux for their invaluable support. I am also thankful
to Jason Curtis and the Stable Isotope Geochemistry Lab at UF, who helped with guidance
through sample processing. Undergraduates Gaithe St. Cyr, Kelseanne Breder, Laura Palomino,
Devan Patel, and Fabio Biondolillo were of tremendous help during sample preparation and
processing and for that I am very thankful to them. I am also thankful to Chelsie Papiez for
sample collection and for her friendship.
My parents Furtiño and Rica deserve a lot of gratitude for their continued support and
unconditional love all these years. My love to and from my siblings Elí, Ethel and Edgar has
always been felt present and for that I am very grateful. I also thank my best friend Jorge Lingán
for always being present for me. To my husband Lucas, I give my love and gratitude for being
the best teammate and for sharing with me and our sweet son Gabriel his passion for nature. I am
also thankful to my parents in law, Diana and Terry, for their love and support.
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Financial support for this project was provided by Fisheries and Wildlife Foundation,
U.S. Fish and Wildlife Service, U.S. National Marine Fisheries Service, Western Pacific
Regional Fishery Management Council, the Knight Vision Foundation, Lerner Gray Fund for
Marine Research, Cynthia A. Melnick scholarship, PADI grant, UF Department travel grants, UF
College of Liberal Arts and Sciences travel grants, and UF Graduate Student Council travel
grants.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES ...........................................................................................................................8
LIST OF FIGURES .........................................................................................................................9
LIST OF ABBREVIATIONS ........................................................................................................11
ABSTRACT ...................................................................................................................................12
CHAPTER
1 INTRODUCTION ..................................................................................................................14
Background and Research Problem ........................................................................................14 Stable Isotopes as Tracers of Resource Use in Marine Environments ...................................15
Research Objectives ................................................................................................................17
2 DISTRIBUTION OF FORAGING HABITATS OF MALE LOGGERHEAD
TURTLES (Caretta caretta) AS REVEALED BY STABLE ISOTOPES AND
SATELLITE TELEMETRY ..................................................................................................19
Background .............................................................................................................................19 Material and Methods .............................................................................................................21
Data and Sample Collection ............................................................................................21
Sample Preparation and Analysis ....................................................................................23 Statistical analysis ...........................................................................................................24
Results.....................................................................................................................................25 Discussion ...............................................................................................................................27
Isotopic Signatures Among Different Tissues of Adult Male Loggerheads ...................27
Variation in 13
C and 15
N Among Satellite-Tracked Male Loggerheads.....................28
Relation Between Body Size and 15
N Signatures .........................................................32 Implications of the Geographic Variation of Isotopic Signatures ...................................33
Conclusions.............................................................................................................................35
3 ASSIGNMENT OF NESTING LOGGERHEADS TURTLES TO THEIR FORAGING
AREAS IN THE NORTHWEST ATLANTIC USING STABLE ISOTOPES......................45
Background .............................................................................................................................45 Material and Methods .............................................................................................................48
Data and Sample Collection ............................................................................................48 Sample Preparation and Analysis ....................................................................................51 Statistical Analyses ..........................................................................................................51
Results.....................................................................................................................................52
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Discussion ...............................................................................................................................54
Isotopic Characterization of the Geographic Areas Used by Adult Loggerheads in
the NWA ......................................................................................................................54 Foraging Locations of Adult Female Loggerheads in the NWA ....................................58
Conservation Implications ...............................................................................................62 Conclusions.............................................................................................................................63
4 LONG-TERM RESOURCE USE AND FORAGING SPECIALIZATION IN ADULT
MALE AND FEMALE LOGGERHEAD TURTLES ...........................................................73
Background .............................................................................................................................73
Methods ..................................................................................................................................75 Data Collection ................................................................................................................75
Scute Preparation and Analysis .......................................................................................76 Data Analysis ...................................................................................................................77
Results.....................................................................................................................................78 Discussion ...............................................................................................................................80
Temporal Consistency in Resource Use ..........................................................................80 Individual Specialization .................................................................................................83
Conclusions.............................................................................................................................87
5 SUMMARY AND FUTURE RESEARCH ............................................................................94
Summary .................................................................................................................................94
Significance and Implications .................................................................................................96
Future Research ......................................................................................................................97
APPENDIX EPIDERMIS AND PLASMA ISOTOPIC VALUES ...........................................100
LIST OF REFERENCES .............................................................................................................101
BIOGRAPHICAL SKETCH .......................................................................................................114
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LIST OF TABLES
Table
page
2-1 Foraging area, sample size (N), and body size (SCL) of the five groups determined
according to the migration that satellite-tracked male loggerheads followed after the
mating season. ....................................................................................................................36
2-2 Adult male loggerhead population range (maximum – minimum) and variance (Var)
of 13
C and 15
N values in red blood cells (RBC), epidermis (EPI), and plasma
(PLA) samples. ..................................................................................................................37
3-1 Location (state, breeding/foraging area, and latitude), year of collection, and sample
size of epidermis samples from adult loggerhead turtles with known and unknown
foraging grounds used in this study. ..................................................................................65
3-2 Assignment of adult female loggerheads of unknown foraging location to a
geographic area with ≥ 80% probability of group membership. Values in parentheses
are additional turtles assigned with a probability < 80% of group membership. ..............66
4-1 Mean and range of carapace length for individual loggerhead turtles, and mean and
total range of 15N and 13C values of loggerhead turtle scute tissue sampled at two
foraging areas, South Carolina/Georgia (SC/GA) and Florida Bay (FLB). For isotope
values, mean is the mean range of isotopic values within individual turtles and total
is the range of isotopic values across all individuals. ........................................................89
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LIST OF FIGURES
Figure page
2-1 Stable isotope ratios (13
C and 15
N) of adult female and male loggerheads in
Florida. ...............................................................................................................................38
2-2 Spatial distribution of satellite-tracked male loggerheads from Cape Canaveral,
Florida after mating season in April 2006 and 2007. .........................................................40
2-3 Linear relationships between epidermis and red blood cells for 13
C and 15
N of
adult male loggerheads.. ....................................................................................................41
2-4 Stable isotope ratios of carbon and nitrogen of red blood cells, epidermis, and plasma
from adult male loggerheads collected at Cape Canaveral, FL, versus the latitude to
which turtles migrated after the mating season..................................................................42
2-5 Red blood cell 15
N versus straight carapace length in male loggerhead turtles that
remained off Cape Canaveral.............................................................................................43
2-6 Comparison of stable isotope ratios of 13
C and 15
N of food web organisms at the
different foraging locations visited by male loggerheads after the mating season. ...........44
3-1 Distribution of stable isotope ratios (13
C and 15
N) of adult male and female
loggerheads in the Northwest Atlantic. ..............................................................................67
3-2 Map showing the locations of the six nesting areas and single foraging ground
sampled in this study: Bald Head Island, Wassaw Island, Sapelo Island, Blackbeard
Island, Jekyll Island, Cumberland Island, and Florida Bay.. .............................................68
3-3 Stable isotope ratios (13
C and 15
N) of adult female loggerheads in the Northwest
Atlantic. ..............................................................................................................................69
3-4 Relationship between carbon and nitrogen stable isotope values of adult female
loggerheads and the latitude to which they migrated after the nesting season. .................70
3-5 Stable isotope ratios of carbon and nitrogen of adult loggerhead turtles with known
foraging location showing three groups representing the three geographic areas used
by adult loggerheads in the Northwest Atlantic: Mid-Atlantic Bight, South Atlantic
Bight, and Subtropical Northwest Atlantic. .......................................................................71
3-6 Breeding population structure according to foraging area used by loggerheads
nesting along the U.S. Atlantic coast as determined through discriminant analysis
using carbon and nitrogen stable isotope values of adult loggerhead turtles with
known foraging grounds as reference data.. ......................................................................72
4-1 Foraging locations, South Carolina/Georgia and Florida Bay, where male
loggerheads turtles were sampled for scute tissue.. ...........................................................90
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4-2 Resource use of individual male loggerhead turtles as indicated by 15
N and 13
C
values of successive scute layers. ......................................................................................91
4-3 Thickness of scute in relation to straight carapace length in loggerhead turtles from
South Carolina/Georgia and Florida Bay. ..........................................................................92
4-4 Comparison of temporal consistency and degree of individual specialization between
loggerhead turtles from two foraging areas, South Carolina/Georgia and Florida
Bay. ....................................................................................................................................93
A-1 Stable isotope ratios (13
C and 15
N) of epidermis and plasma samples of adult male
loggerheads in Florida......................................................................................................100
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LIST OF ABBREVIATIONS
BIC Between individual component
CSIA-AA Compound-specific stable isotope analysis of amino acids
EPI Epidermis
GoM Gulf of Mexico
MAB Mid-Atlantic Bight
MANOVA Multivariate analysis of variance
NWA Northwest Atlantic
PLA Plasma
POM Particulate organic matter
RBC Red blood cells
SAB South Atlantic Bight
SCL Straight carapace length
SI Stable Isotopes
SNWA Subtropical Northwest Atlantic
TNW Trophic niche width
WIC Within individual component
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Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
THE FORAGING ECOLOGY OF LOGGERHEAD TURTLES IN THE NORTH
ATLANTIC: EVIDENCE FROM STABLE ISOTOPE VALUES
By
Mariela E. Pajuelo
December 2015
Chair: Karen A. Bjorndal
Major: Zoology
Sea turtles spend the majority of their lives in the marine environment but are more easily
accessible at their nesting beaches. Thus, understanding the relationships between their various
foraging grounds and breeding areas is essential to assess population dynamics. In the particular
case of the Northwest Atlantic (NWA) population, its life history stages and the habitats it
occupies have been identified. What still remains poorly understood is the role loggerheads play
within their ecosystems. Understanding the foraging ecology of loggerheads is necessary for
revealing this role. The goal of this study was to further our understanding of the foraging
ecology of loggerhead turtles in the North Atlantic Ocean using stable isotope analysis.
First, I investigated the foraging habitats of highly elusive male loggerhead turtles using
stable isotopes and satellite telemetry data. Male loggerhead isotopic data varied with foraging
location and this is explained by geographic isotopic variation at the base of the food web. Also,
comparison of male loggerhead isotopic values with those of female loggerheads revealed that
males may exhibit similar foraging strategies (diet and habitat use) to those of females. Next, I
characterized isotopically the foraging regions for loggerhead turtles in the NWA using a
combination of satellite telemetry and stable isotope analysis, thus validating the use of stable
isotope analysis to identify foraging areas of loggerhead turtles in the NWA. The largest
13
assignment of nesting loggerheads to their foraging locations revealed that turtles segregate
geographically in their use of foraging areas. I also examined the long-term consistency in
resource use and degree of foraging specialization in male loggerheads. Individual male
loggerheads exhibit a specialized foraging behavior similar to females that is consistent for up to
17 years. My results also revealed that resource diversity has an effect on the degree of
individual specialization in loggerhead turtles.
In summary, this study expanded our knowledge of loggerhead foraging strategies and
demonstrated that carbon and nitrogen stable isotopes of loggerhead turtles are effective
biochemical tags to link loggerhead foraging grounds and breeding areas.
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CHAPTER 1
INTRODUCTION
Background and Research Problem
As widely distributed marine organisms, species of sea turtles are composed of
populations that can vary in size, geographic distribution, and population dynamics, and some
populations of sea turtles have shown dramatic long-term changes in population numbers. A
clear example is the loggerhead sea turtle (Caretta caretta) nesting population in the Northwest
Atlantic (NWA), one of the largest nesting aggregations for the species in the world (Ehrhart et
al. 2003), whose numbers declined markedly from 1998 to 2007 (Witherington et al. 2009) but
have increased steadily ever since (Arendt et al. 2013).
Sea turtles have complex life histories, with ontogenetic diet and habitat shifts, long
migrations to developmental habitats and breeding areas, and fidelity to their nesting areas and
foraging grounds. While most studies of sea turtles have been conducted on nesting beaches
where female turtles are more easily accessible, sea turtles spend the majority of their lives in the
marine environment. Thus, understanding the relationships among the various developmental
and foraging areas to their breeding areas is essential to accurately assess population dynamics,
which in turn will help improve the design of management strategies for the conservation of sea
turtle populations.
In the case of the NWA population, its life history stages and the habitats it occupies have
been identified (Bolten 2003). After hatching in beaches along the U.S. east coast, hatchlings
embark on one of their first long migrations to oceanic developmental habitats across the
Atlantic Ocean (Bolten 2003), where they forage and grow for 7–15 years (Avens et al. 2013).
Juvenile loggerheads migrate from oceanic habitats to neritic areas where the majority will
remain, but some juvenile loggerheads still rely on the use of oceanic waters (McClellan et al.
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2007; Mansfield et al. 2009). In any case, NWA adult loggerhead turtles exclusively use coastal
foraging areas (Hawkes et al. 2011; Ceriani et al. 2012; Foley et al. 2014; Griffin et al. 2014) that
are widely distributed along the U.S. east coast, the Gulf of Mexico, the Bahamas, and Cuba.
What still remains poorly understood is the role loggerhead turtles play within their
ecosystems. Furthering our understanding of the foraging ecology of loggerheads will allow us to
better understand this role, as a primary way that organisms interact with other species and with
their environment is through their diet (Bjorndal 2003). Loggerhead turtles use a wide range of
foraging areas with distinct biotic and abiotic factors that could potentially have an effect on
their foraging strategies, which in turn can have profound effects on the demographic parameters
of populations.
This research focuses on understanding the foraging strategies, including diet, habitat use
and foraging area use of loggerhead turtles in the North Atlantic Ocean using stable isotope
analysis.
Stable Isotopes as Tracers of Resource Use in Marine Environments
Stable isotope analysis is a technique that uses ratios of stable isotopes of naturally
occurring elements. In the marine environment, carbon and nitrogen stable isotopes (13
C and
15
N, respectively) have been widely used to evaluate the foraging and migratory ecology of
highly migratory marine animals (Schell et al. 1989; Jaeger et al. 2010; McKenzie et al. 2011;
Zbinden et al. 2011; Pajuelo et al. 2012a,b; Lorrain et al. 2012; Seminoff et al. 2012). This is
based on the fact that the stable isotope composition of an organism’s tissue reflects that of the
food or nutrients assimilated at its foraging area and that such stable isotope composition can
vary geographically because of biogeochemical processes (Hobson 1999).
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During assimilation, there is a stepwise increase of 15
N between the diet and consumer
such that 15
N values are typically used to estimate an organism’s trophic position (DeNiro and
Epstein 1981; Post 2002). However, discrimination factors in 13
C (the difference in 13
C
between diet and consumer) are smaller (DeNiro and Epstein 1978) and vary with primary
production (Michener and Kaufmann 2007) and habitat type: pelagic versus benthic or oceanic
versus neritic (Hobson et al. 1994). Thus, 13
C has been mainly used to identify carbon sources
and habitats utilized (Hobson et al. 1994). Ultimately, the 13
C and 15
N values of a marine
consumer are not just a function of the isotopic values of its diet, but also the isotopic values of
primary producers at the base the food web (Schell et al. 1989; Minami and Ogi 1997; Burton
and Koch 1999; Cherel and Hobson 2007; Pajuelo et al. 2010), which can vary with geographic
location as a result of biogeochemical processes (Goericke and Fry 1994; Montoya 2007;
McMahon et al. 2013).
Nitrogen fixation and denitrification are two of the main processes by which nitrogen
cycles in the marine environment and that affect the 15
N composition of primary producers at
the base of the food web (e.g., marine phytoplankton, seagrass, algae). Nitrogen fixation lowers
and denitrification increases the 15
N values of primary producers (Montoya 2007). Additionally,
13
C values of marine phytoplankton and particulate organic matter (POM, a proxy for primary
production) decrease from the equatorial zones toward the polar regions (Goericke and Fry 1994)
as a result of differential plankton growth, sea water temperature, and dissolved CO2 (Goericke
and Fry 1994; Graham et al. 2010). These isotopic differences at the base of the food web create
isotopically distinct areas (McMahon et al. 2013) that allow us to use 13
C and 15
N to
distinguish foraging areas used and assess residency and migration patterns of marine organisms
(Graham et al. 2010). Therefore, it is essential to understand how baseline isotopic values vary
17
across the range of foraging areas used by highly migratory organisms before using stable
isotopes to make comparison between populations using distant foraging areas.
Research Objectives
This study aimed to improve our understanding of the foraging ecology of loggerhead
turtles in the North Atlantic Ocean. My primary research objectives were to: 1) use stable isotope
composition to investigate the foraging habitats of adult male loggerheads, 2) identify key
foraging areas of loggerheads using stable isotope analysis and assign nesting loggerheads to
these areas, and 3) assess temporal trends in resource use and degree of individualization in
resource use in adult male and female loggerheads.
These objectives are addressed and discussed in the following 4 chapters. In Chapter 2, I
provide insights into the foraging habitats of elusive male loggerhead turtles sampled at one
breeding area in the NWA using stable isotope analysis. I compared the stable isotope values of
male loggerheads with those previously reported for female loggerheads in the region, finding
that males may also exhibit foraging strategies (diet and habitat use) similar to those of female
loggerheads. Male turtles were satellite tracked after sampling for stable isotopes and followed
back to their foraging locations. Thus, I reveal how loggerhead isotopic values vary with
foraging location, which can be explained by differences in isotopic values at the base of the
food web rather than differences in loggerhead’s trophic levels. Chapter 2 has been published in
the journal Marine Biology (Pajuelo M, Bjorndal KA, Reich KJ, Arendt MD, Bolten AB (2012)
Distribution of foraging habitats of male loggerhead turtles (Caretta caretta) as revealed by
stable isotopes and satellite telemetry. Marine Biology 159:1255–1267).
In Chapter 3, I characterize isotopically the main foraging regions for loggerheads in the
NWA using a combination of satellite telemetry and stable isotope analysis, thus validating the
use of stable isotope analysis to identify foraging areas of loggerhead turtles in the NWA. This
18
allowed me to assign a large number of female loggerhead turtles from various nesting beaches
to their foraging locations using stable isotope values alone, finding that loggerhead turtles
segregate geographically in their use of foraging areas. Chapter 3 has been published in
Ecosphere (Pajuelo MA, Bjorndal KA, Reich KJ, Vander Zanden HB, Hawkes LA, Bolten AB
(2012) Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic
using stable isotopes. Ecosphere 3:art89, doi:10.1890/ES12-00220.1).
In Chapter 4, I evaluate the long-term consistency in resource use and degree of
individual foraging specialization in male loggerhead turtles. Using stable isotopes as a proxy for
diet and habitat use, I reaffirm that long assumed generalist loggerhead turtles are composed of
individuals with specialized foraging behaviors. The wide isotopic variation at the population
level revealed that male loggerheads are part of a generalist population, while the small variation
in isotopic values within individual male turtles compared to that of the population revealed that
males exhibit specialization in diet and habitat use. Similar to studies in female loggerhead and
green turtles (Chelonia mydas), this specialized foraging behavior in male loggerheads is
maintained consistently over time. I also present initial results on the effect of resource diversity
on the individual specialization in male and female loggerhead turtles. Chapter 4 will be
submitted to Marine Biology under the authorship of Pajuelo M, Bjorndal KA, Arendt MD,
Foley AM, Schroeder BA, Witherington BE, and Bolten AB. Finally, in Chapter 5, I provide a
summary of my research to date, discuss its significance, and suggest areas for further research.
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CHAPTER 2
DISTRIBUTION OF FORAGING HABITATS OF MALE LOGGERHEAD TURTLES
(Caretta caretta) AS REVEALED BY STABLE ISOTOPES AND SATELLITE TELEMETRY1
Background
Knowledge of foraging ground distribution of highly migratory animals is critical for
understanding their foraging behavior and habitat use. Identification of key habitats not only
helps to characterize life history features of populations (Block et al. 2001), but also to assess the
impact of threats that populations may face (Hays et al. 2003). Most efforts to identify key
habitats and movement patterns have used flipper tags (Limpus et al. 1992), genetic markers
(Bolker et al. 2007), chemical analysis (Thorrold et al. 2001), and electronic tagging (Block et al.
2005; Hawkes et al. 2011). Electronic tagging is an excellent tool for assessing the movement
and foraging behavior of marine animals, but this tool is constrained to small sample sizes due to
expense and to organisms large enough to be tagged (but see Block et al. 2005). Additional
information on migration patterns and foraging habitat use of highly migratory and elusive
marine animals can be obtained using stable isotopes (SI) (Reich et al. 2007; Rooker et al. 2008;
Newsome et al. 2009).
In marine ecosystems, δ15
N values vary predictably with trophic level (Minagawa and
Wada 1984), while δ13
C values vary with source of primary production (Michener and
Kaufmann 2007) and habitat type: pelagic versus benthic or oceanic versus neritic (Hobson et al.
1994). Additionally, both δ15
N and δ13
C may vary with geographical location as a result of the
effect of oceanic processes on baseline δ15
N and δ13
C (Goericke and Fry 1994; Montoya et al.
Reprinted with permission from the Publisher. The final publication is available at link.springer.com. Original
publication: Pajuelo M, Bjorndal KA, Reich KJ, Arendt MA, Bolten AB (2012a) Distribution of foraging habitats of
male loggerhead turtles (Caretta caretta) as revealed by stable isotopes and satellite telemetry. Mar Biol
159:1255−1267
20
2007), which in turn are reflected in higher trophic level organisms (Cherel and Hobson 2007;
Pajuelo et al. 2010). Marine phytoplankton and particulate organic matter δ13
C values decrease
from the equatorial zones toward the polar regions (Goericke and Fry 1994) as a result of
differential plankton growth rates, dissolved CO2 concentration in seawater, and seawater
temperature, among other factors (Goericke and Fry 1994; Graham et al. 2010). Characteristic
nitrogen cycle regimes—nitrogen fixation and denitrification—lower and increase, respectively,
the δ15
N of primary producers (Montoya 2007). These baseline isotopic differences create
isotopically distinct regions, which can then be used to assess movements of individuals
migrating among them (Graham et al. 2010). Therefore, it is important to know the community
baseline δ15
N and δ13
C when identifying foraging habitats in highly migratory organisms.
Loggerhead sea turtles (Caretta caretta) are listed as Endangered on the IUCN Red List
(IUCN 2011). Understanding the foraging ecology and movement patterns of loggerheads
improves their conservation outlook. While several studies have addressed various aspects of the
foraging ecology of adult female and juvenile loggerheads in the North Atlantic (Hawkes et al.
2007, 2011; Wallace et al. 2009; Frick et al. 2010; McClellan et al. 2010; Vander Zanden et al.
2010), little is known about the foraging strategies of adult male loggerhead turtles (but see
Arendt et al. 2012a, 2012b).
Recent work by Reich et al. (2010) revealed a large range in the δ15
N and δ13
C signatures
of skin samples in nesting loggerheads in Florida, USA. Two clusters were identified based on
δ13
C signatures that were consistent with differences in habitat use (oceanic versus neritic waters;
Figure 2-1a). However, the authors could not rule out the possibility of other factors affecting
carbon signatures, such as geographical location or differential source of primary production.
Furthermore, even though the range from 2 to 15‰ in δ15
N of females was not assessed, this
21
could represent differences in trophic levels (Post 2002) or baseline isotopic values (Pajuelo et
al. 2010). Recent observations of migration patterns of adult male loggerheads in Florida through
satellite telemetry have revealed the use of different geographic foraging areas after the mating
season (Figure 2-2; Arendt et al. 2012a, 2012b). Do adult male loggerheads in Florida exhibit a
pattern in SI values similar to that in nesting loggerheads? If so, how do patterns in δ15
N and
δ13
C of male loggerheads compare with their satellite tracking data? To answer these questions I
collected tissues from 29 satellite-tracked adult male loggerheads at one breeding area in Florida
and analyzed them for δ15
N and δ13
C. Tissues from additional male turtles not fitted with satellite
transmitters were used to compare SI patterns between adult males and females and to assess SI
values of different tissues within individual males. By integrating SI with satellite telemetry, I
seek to reveal the foraging strategy of adult male loggerheads in the Northwest Atlantic.
Material and Methods
Data and Sample Collection
Thirty-seven adult male loggerhead turtles (straight carapace length, SCL >86 cm) were
captured by trawling from the Port Canaveral shipping channel, Florida, USA (28.38N,
80.53W) during mating season in April 2006 and 2007. Sexual maturity of each turtle was
confirmed by laparoscopy (Blanvillain et al. 2008). All turtles were used to evaluate SI values
within and among adult male loggerhead tissues, while 29 turtles were used to assess turtle
movements by combining satellite telemetry and SI data.
Satellite transmitters (ST-20, Model A2020; Telonics, Inc., Mesa, Arizona, USA) were
attached to 29 males (see Arendt et al. 2012a), and their movements were tracked during and
after the mating season. Arendt et al. (2012a, 2012b) characterized the distinct movement
patterns of these males that I classify here into five groups: (1) residency in waters near the
22
breeding area in Cape Canaveral, FL, (2) northern migration and residency in waters off South
Carolina in the South Atlantic Bight, (3) northern migration to foraging grounds along the
continental shelf from North Carolina to New Jersey in the Mid-Atlantic Bight (MAB), (4)
southern migration to shallow waters of the Florida Keys and the Bahamas in the subtropical
NWA, and (5) southern migration and ultimate residency in coastal waters of the northeast Gulf
of Mexico (Figure 2-2). I grouped turtles based on their migration patterns and foraging locations
with similar oceanographic conditions.
Body size (SCL) was recorded using tree calipers marked with 0.1 cm units, and blood
samples were collected for each adult male loggerhead. Epidermis (EPI) samples were also
collected for 26 turtles, 20 of them corresponding to satellite-tracked turtles. Only one turtle was
recaptured in consecutive years, and blood samples were collected in both years.
Epidermis samples (N = 26) were collected from the dorsal surface of the neck using a 6 mm
biopsy punch. Blood samples were collected from the dorsal cervical sinus (Owens and Ruiz
1980) using a vacutainer tube with sodium heparin, fitted with a 21-gauge needle. Sodium
heparin does not affect isotopic values (Lemons et al. 2012). Blood was centrifuged for 5 min
within 1 h of collection and separated into red blood cells (RBC, N =38) and plasma (PLA, N =
38), which were stored in cryovials. All samples were stored frozen until dried at 60C prior to
sample preparation and analysis. EPI and RBC reflect the turtle’s dietary history over a longer
period of time (at least 4.2 months) than PLA (at least 2 months) based on studies conducted
with growing juvenile loggerheads (Reich et al. 2008). Isotope turnover (i.e., the time the
isotopic composition in the consumer tissue reaches equilibrium after a shift in resource use) for
adult loggerheads may be longer because rates of isotopic incorporation slow with reduced
23
growth rates (Reich et al. 2008), and because these rates are allometrically dependent on body
mass (Carleton and Martínez del Rio 2005).
To evaluate turtle movements, turtles from Group 1 (resident males) were only included
if they stayed near Cape Canaveral for at least 60 days, well after all migrants left the breeding
area. Turtles from Groups 2 through 5 (migratory males) were included if transmissions lasted a
minimum of 30 days at the foraging ground. Following these criteria, movements of 4 turtles
could not be determined because transmissions failed before the 30 or 60 day cut offs, and a total
of 25 turtles (Table 2-1) was used to relate SI with satellite telemetry data. I used the median
latitude where turtles occurred at the foraging grounds to evaluate the relationship between SI
and geographic location. Location data were extensively filtered (see Arendt et al. 2012a, 2012b
for details). Additionally, I compiled isotopic data from the literature on lower trophic-level
organisms from the geographic areas where male loggerheads traveled after the mating season.
Sample Preparation and Analysis
Turtle EPI samples were washed with deionized water and alcohol swabs to remove
epibionts and extraneous particles. The outermost layer of the turtle epidermis was separated
from the underlying tissue, finely diced with a scalpel blade, and dried at 60C for 24 h. Blood
samples (RBC and PLA) were dried for 24 h at 60C and then ground to a fine powder using a
mortar and pestle. Lipids were extracted from EPI samples with petroleum ether using an
accelerated solvent extractor. Lipids were not extracted from RBC and PLA samples because,
for these tissues, C:N ≤ 3.5. According to Post et al. (2007) no extraction of lipids is necessary
when tissue C:N < 3.5. Lipids were extracted from EPI samples to allow for comparison with
previously published EPI isotopic data.
24
For stable isotope analysis, approximately 500 to 600 μg of each sample was weighed
and sealed in a tin capsule. Samples were analyzed for δ13
C and δ15
N by combustion in a
COSTECH ECS 4010 elemental analyzer interfaced via a Finnigan-MAT ConFlow III device to
a Finnigan-MAT DeltaPlus XL isotope ratio mass spectrometer in the Stable Isotope
Geochemistry Lab at the University of Florida, Gainesville, USA. Results are presented as stable
isotope ratios of a sample relative to an international standard and reported in the conventional
notation: X = [(Rsample/Rstandard) –1] x 1000, where X is the relative abundance of 13
C or 15
N in
the sample expressed in parts per thousand (‰); Rsample and Rstandard are the ratios of heavy to light
isotope (13
C/12
C and 15
N/14
N) in the sample and international standard, respectively. The standard
used for 13
C was Vienna Pee Dee Belemnite and for 15
N was atmospheric N2. Working standards
L-glutamic acid USGS40 (13
C = -26.39‰ and 15
N = - 4.52‰) were calibrated monthly against
international standards and were inserted in all runs at regular intervals to calibrate the system.
In addition, a loggerhead scute standard (13
C = -18.36‰ and 15
N = 7.68‰) was used in all
runs. The analytical accuracy of my measurements—calculated as the SD of replicates of
standards—was 0.11 and 0.12‰ for 13
C and 15
N of working standards (N = 29), respectively,
and 0.12 and 0.16‰ for 13
C and 15
N of scute standard (N = 10), respectively.
Statistical analysis
Levene’s test was used to assess homogeneity of variances of 13
C and 15
N among the
three tissues sampled. The relationships between the isotopic signatures of the two tissues with
similar temporal isotopic assimilation, EPI and RBC, were evaluated with linear regressions. To
explore the effect of geographical location on 13
C and 15
N, the correlation between isotopic
signatures and the latitude of the foraging grounds of the turtles was evaluated using Spearman
rank test. Additionally, Wilcoxon rank sum test was used to assess body size differences between
25
two main migration patterns, northern (Group 3) versus southern (Group 4). Body size
differences among remaining Groups (1, 2, and 5) were not assessed because samples sizes were
too small (N < 3), or because turtles did not migrate to a different geographic area after the
mating season. Finally, the relationship between body size and isotopic signatures of males
within a foraging area was evaluated using linear regression whenever sample size allowed. All
data were analyzed using program R (R Development Core Team 2009) with an level of 0.05.
Results
The ranges (the difference between maximum and minimum values) of isotopic
signatures for each tissue from all adult male loggerhead samples varied between 7.53 and
8.19‰ for 13
C, while 15
N ranges varied from 8.96 to 9.68‰ (Table 2-2). The variance of
isotopic values was similar among tissues for both 13
C (Levene’s test, F = 0.19, P = 0.827) and
15
N (Levene’s test, F = 0.58, P = 0.562) (Table 2). Also, visual inspection of Figure 1b reveals a
pattern in the SI values of male turtles similar to that of female loggerheads.
When EPI and RBC—tissues reflecting the turtle’s longer-term foraging history—were
compared, I found that RBC had lower values for both 13
C and 15
N among all turtles with both
tissues sampled. Furthermore, there was a significant positive relationship between RBC and EPI
samples for both 13
C (Linear regression, r2 = 0.96, F1,24 = 623.8, P < 0.001; Fig 3a) and
15N (r
2
= 0.98, F1,24 = 1208, P < 0.001; Figure 2-3b). However, I should be cautious when using the 13
C
correction factor to obtain specific carbon isotopic values, as the data distribution is unequal
(Figure 2-3a).
I analyzed three different tissues collected from satellite-tracked loggerheads, but my
conclusions about turtle movements will be based on RBC because this was the only tissue
available for all turtles sampled that reflect the longer-term foraging history of the turtle prior to
26
capture. Male turtles that migrated south to the Bahamas and the Florida Keys had the highest
13
C and lowest 15
N values (Figure 2-1b). In contrast, the lowest values of 13
C were found in
males that migrated northward to New Jersey, while the highest 15
N value was found in a turtle
that established residency in Maryland (Figure 2-1b). A strong negative correlation was found
between the latitude of the residential foraging location and the 13
C RBC values (Spearman
rank correlation, rs = -0.73, N = 25, P < 0.001; Fig 2-4Aa). Similarly, a strong but positive
correlation was found between the latitude and the 15
N RBC values (rs = 0.78, N = 25, P <
0.001; Fig 2-4Ba). Moreover, EPI and PLA had similar correlation patterns as that of RBC. EPI
and PLA 13
C values were correlated negatively with latitude (EPI: rs = -0.77, N = 17, P < 0.001;
PLA: rs = -0.82, N = 25, P < 0.001; Fig 2-4Ab, 2-4Ac), and 15
N was positively correlated with
latitude (EPI: rs = 0.86, N = 17, P < 0.001; PLA: rs = 0.75, N = 25, P < 0.001; Fig 2-4Bb, 2-4Bc).
Only one male turtle (from Group 1) was recaptured in Port Canaveral in consecutive years and
had similar RBC 13
C (-15.57 and -15.55‰) and 15
N (10.95 and 10.80‰) values in both years.
Body size had a significant negative relationship with 15
N (Linear regression: r2 = 0.62,
F1,8 = 15.7, P = 0.004; Figure 2-5), but not with 13
C (Linear regression: r2 = 0.047, F1,8 = 1.445,
P = 0.264) in the turtles that remained near Cape Canaveral (Group 1). Sample size in other
foraging areas was too small to analyze a relationship between body size and isotopic signatures.
Additionally, body sizes were not significantly different between turtles with northernmost
foraging grounds (Group 3: using waters in the MAB) and turtles using southernmost foraging
areas (Group 4: using waters in the subtropical NWA) (Wilcoxon rank sum test, W = 7, N1 = 3,
N2 = 8, P = 0.376). However, sample sizes between north and south were unequal with only 3
turtles migrating to southernmost foraging areas (Figure 2-2).
27
Discussion
Isotopic Signatures Among Different Tissues of Adult Male Loggerheads
I found that the isotopic values of male tissues reflecting different temporal integration of
diet and habitat use (RBC, EPI, and PLA) were similar in range and variance. RBC and EPI
isotopic values were expected to be similar in range and variation because these tissues provide
the turtle’s longer-term dietary information. On the other hand, PLA reflects a relatively more
recent foraging history of a turtle (up to at least 2 months). Therefore, if males had been feeding
in waters off Cape Canaveral when captured, then PLA signatures would have been similar
among turtles. The fact that PLA samples presented a similar pattern as RBC and EPI samples
indicates that either 1) turtles had not spent enough time to allow the Cape Canaveral isotopic
signature to be incorporated, or 2) turtles had not been feeding in the breeding area.
The significant positive relationship between 13
C and 15
N of RBC and EPI (Figure 2-
3), allows me to predict EPI signatures for a given RBC sample and vice versa. EPI samples are
more easily collected than RBC samples. Correction factors of this nature can be useful when
trying to compare isotopic values among individuals and the same tissues are not available. In
this study, these correction factors allowed for general comparison between male and female
turtles. However, a systematic experiment in captivity in which turtles are consistently fed a diet
with known isotopic signatures would be the ideal way to evaluate isotopic discrimination
among different tissues, especially when specific tissue isotopic values are required to estimate
diet composition. Few such experiments have been conducted in sea turtles (Seminoff et al.
2006, 2009; Reich et al. 2008; Vander Zanden et al. unpubl. data), and no such experiments have
been conducted in adult loggerheads.
28
Variation in 13
C and 15
N Among Satellite-Tracked Male Loggerheads
In this study I found a significant relationship between both the 13
C and 15
N of male
loggerheads and the latitude of their foraging grounds. The ranges of isotopic values were 8.2‰
and 8.9‰ for RBC 13
C and 15
N, respectively, over a range of 16° latitude (Figure 2-4Aa, 2-
Ba). Large differences in the isotopic signatures of turtles in the NWA could be attributed to
differences in (1) trophic level (based on 15
N), (2) habitat type (i.e., pelagic versus benthic or
oceanic versus neritic, based on 13
C) and/or (3) geographical location (based on both 13
C and
15
N).
Turtle–diet isotopic discrimination factors for 15
N are not available for adult loggerheads.
However, given the discrimination factor of 2.5‰ for RBC in adult green turtles, Chelonia
mydas (Vander Zanden et al. 2012), the observed range in 15
N values in male loggerheads
would imply a variation of approximately 3.6 trophic levels in male loggerheads, if nitrogen
baseline signatures were equal across all foraging areas. Such trophic level differences are
unlikely to occur in NWA loggerhead turtles because they are known to prey mainly on benthic
invertebrates in coastal waters (see references cited in Hopkins-Murphy et al. 2003). A historic
shift in diet from horseshoe crabs to crustaceans, and then to mostly fish has been recently
reported in loggerheads in Chesapeake Bay, Virginia, USA, through analysis of stomach
contents; however, no turtle with SCL > 90 cm showed this diet change (Seney and Musick
2007). Male loggerhead RBC 15
N signature (mean ± SD = 12.3 ± 1.70‰) is higher than 15
N of
horseshoe crabs and blue crabs (10.3 and mean ± SD = 11.5 ± 2.40‰, respectively; Knoff et al.
2001) and lower than those of fish (range: 13.9 to 18.0‰; Buchheister and Latour 2011), which
suggests that adult male loggerheads in this region may be relying more on benthic invertebrates
than on fish.
29
The second possible explanation relates the large variation in 13
C to differential habitat
use. Because all male loggerheads dispersed to coastal locations (Figure 2-2), the variation in
13
C may be reflecting a pelagic versus benthic habitat use in neritic waters, which would result
in low versus high 13
C values, respectively. Indeed, lowest values of 13
C were found in turtles
that migrated to high latitude areas (New Jersey; Figure 2-1b), where turtles used deeper waters
(mean water depth = 28 m; M. Arendt, unpubl. data) contrasted with high 13
C turtles that
migrated to lower latitude foraging areas (Bahamas and Florida Keys; Figure 2-1b) and used
shallow waters (mean water depth = 8 m; M. Arendt, unpubl. data). Diving data for two northern
turtles revealed use of both bottom and surface waters suggesting that these feed throughout the
water column (M. Arendt, unpubl. data). However, low 13
C signatures were also present in
turtles using shallow waters in North and South Carolina (mean water depth = 5 and 7 m,
respectively; M. Arendt, unpubl. data) (Figure 2-1b). Reich et al. (2010) found similar results in
some nesting loggerheads in Florida which presented low 13
C values as well as neritic/benthic
epibionts suggesting use of shallow waters. Thus, although I cannot rule out the pelagic versus
benthic foraging strategy in male loggerheads, I propose that 13
C signatures are being affected
primarily by other factors.
The third possible explanation for the large variation of 13
C and 15
N is geographical
location. The differences in 15
N and 13
C at the base of the food web are conserved through
higher trophic levels (Cherel and Hobson 2007; Pajuelo et al. 2010). Thus, if baseline signatures
change with geographical area then the isotopic differences observed in males would reflect the
location of the foraging area rather than differences in diet or habitat use.
Indeed, different oceanographic processes and nutrient sources influence the baseline
signatures of the foraging areas used by male loggerheads. In the NWA, nitrogen fixation, which
30
lowers the 15
N signature of primary producers, is highest in the tropical and subtropical NWA
(Montoya et al. 2002). On the other hand, highly productive coastal waters near estuaries in the
Mid-Atlantic Bight are characterized by high 15
N values in primary producers apparently due to
the high 15
N contribution from human sources to these waters (McKinney et al. 2010). Also,
denitrification, a process that increases values of 15
N (Montoya et al. 2007), has been reported
in the MAB (Fennel et al. 2006). To what extent denitrification affects the 15
N of coastal biota
of the MAB has not yet been assessed (McKinney et al. 2010).
Therefore, I would expect male loggerheads foraging from Virginia to New Jersey to
have the highest 15
N signatures, while turtles foraging in areas with high rates of N2 fixation
(e.g., the Bahamas) will present the lowest 15
N signatures. This clearly corresponds with the
increase of male 15
N with latitude (Figure 2-4B), but does not necessarily support a latitudinal
effect on coastal waters from equator to polar regions because 15
N of primary producers slowly
decreases at latitudes north of the Delaware estuary (McKinney et al. 2010). The probable human
influence revealed in the 15
N signatures of males in northern waters corresponds with the
elevated concentrations of persistent organic pollutants recently found in the same male
loggerheads (Ragland et al. 2011).
Carbon isotope signatures can also reflect geographical location. High latitude primary
producers have much lower 13
C than primary producers at lower latitudes (Goericke and Fry
1994). Water temperature has recently been proposed as a proxy for baseline 13
C values
because it affects plankton growth rates and dissolved CO2 concentrations in seawater—which in
return have an effect on baseline 13
C values—(Mackenzie et al. 2011), and could explain the
13
C latitudinal gradient. This latitudinal gradient in 13
C agrees with the lowest male 13
C value
31
found in cooler waters off New Jersey (39.4°N) and highest male 13
C value found in warmer
waters of the Bahamas (23.3°N) (Figure 2-4A). This gradient could also explain why turtles
foraging in shallow waters off North and South Carolina (35.3 and 33.3°N, respectively) that
were expected to have high 13
C—reflecting benthic feeding—had low 13
C values.
Ultimately, variations in the isotopic signatures by geographical location should also be
reflected in other food web organisms. Isotopic data available in the literature for lower trophic-
level organisms in those geographic locations where male loggerheads migrated reveal that they
follow a similar pattern to that of male loggerheads (Figure 2-6). For instance, known trophic-
level organisms such as omnivorous shrimps and lobsters in the Florida Keys (~ 25°N) show
lower 15
N than those of omnivorous crabs and horseshoe crabs off North Carolina (~ 35°N), and
even similar or lower to those of filter feeding bivalves in Virginia and Delaware (~ 37° and
39°N, respectively). Food-web baseline signatures along the latitudinal gradient used by
loggerheads are scarce; however, nitrogen isotopic signatures of particulate organic matter
(proxy for primary producer) are available for the Florida Keys (~25°N) and coastal waters of
Virginia and Delaware (~ 37° and 39°N, respectively). Nitrogen values range from -0.9 (Macko
et al. 1984) to 3.6‰ in waters of the Florida Keys (Behringer and Butler 2006; Evans et al. 2006;
Lamb and Swart 2007) and 7.2 to 7.7‰ in near-shore waters off Virginia and Delaware
(McKinney et al. 2010). Hence, although I did not assess the isotopic signatures of loggerhead
prey items in all the geographic locations visited by the turtles, the results indicate that the
variation in the 13
C and 15
N of male loggerheads is due to geographic location. Additionally,
because the range of values in males is similar to that of female loggerheads (Figure 2-1b), I
believe that the 2-cluster females probably represent a gradient of North to South geographical
locations used by adult female loggerheads in the NWA. The relationship between female
32
isotopic signatures and geographic areas used are being addressed in another study. My results
highlight the need for knowledge of baseline isotopic signatures when identifying foraging
habitats of highly migratory organisms.
Even though there appears to be a separation of geographic location for at least
northernmost versus southernmost foraging areas (i.e., foraging grounds in the MAB versus
subtropical NWA, respectively) using combined 13
C and 15
N values, isotopic signatures from
the northeast Gulf of Mexico overlap with those of Cape Canaveral (Figure 2-1b). The use of
other markers (such as trace elements) could help reveal unique characteristics of the distinct
geographical locations in the NWA for which carbon and nitrogen are not informative.
Because isotopic signatures of male loggerhead tissues reflect integrated diet and habitat use of
the turtles before their capture, the agreement between isotopic signatures and migration
patternswhich reflects the foraging history after the mating seasonsuggests site fidelity to
foraging areas. Hatase et al. (2002; 2006), McClellan et al. (2010), and Zbinden et al. (2010)
have reported similar agreements between isotopic signatures and migration patterns in female
and juvenile sea turtles. Foraging fidelity in NWA adult female loggerheads has also been
observed through satellite telemetry data (Hawkes et al. 2007, 2011) and has been indicated by
the long-term consistency in SI signatures of scute layers (Vander Zanden et al. 2010).
Relation Between Body Size and 15
N Signatures
A surprising decrease of RBC 15
N with body size was revealed in adult male turtles that
stayed in waters off Cape Canaveral (Figure 2-5). Although the RBC 13
C varied from -15.42 to
-16.91‰, the lack of a relationship between body size and 13
C suggests that males were feeding
on prey utilizing similar carbon sources. Values of 15
N commonly increase with body size due
to diet shift to higher trophic level (Reñones et al. 2002). Among individuals feeding on the same
33
diet, low values of 15
N can provide evidence of a lower nitrogen discrimination value in smaller
juveniles with fast growth rates (Martínez del Rio and Wolf 2005; Reich et al. 2008). Unlike
juveniles, adult turtles grow very slowly after reaching sexual maturity (Bjorndal et al. 1983),
thus no growth effect on 15
N may be evident in adult individuals (Martínez del Rio and Wolf
2005). In our study, smaller adult males may preferentially venture into inshore waters of the
east central Florida where anthropogenic influence has been evidenced in primary producers,
which show elevated 15
N values (Barile 2004). If this pattern is consistent when a larger sample
size is analyzed, the cause for the pattern should be explored.
Implications of the Geographic Variation of Isotopic Signatures
The geographic variation in the isotopic signatures of male loggerheads can potentially
help me understand patterns of migratory connectivity between loggerhead foraging grounds and
breeding areas. In particular, by identifying and differentiating foraging subpopulations within
breeding areas in the NWA, I can assess how breeding populations are structured. This has
important implications in the long term. Changes through time in the relative composition of
individuals from a particular foraging ground observed in a breeding area may provide evidence
of threats to which turtles are exposed (Hatase et al. 2002; Zbinden et al. 2011). For example, in
the NWA, turtles using northern foraging grounds are at a higher risk of sublethal toxic effects
from high concentrations of organic pollutants than are southern foragers (Ragland et al. 2011).
Environmental forces acting on resource availability have been reported to drive the life history
of conspecifics, including leatherback (Dermochelys coriacea) and green (Chelonia mydas) sea
turtles in marine regions (Suryan et al. 2009). Recently, Hatase et al. (2010) found differences in
body size in adult female loggerheads (N = 149) related to differential foraging habitat use.
Similarly, Zbinden et al. (2011) found that nesting loggerheads in the Mediterranean (N = 58)
34
exhibited differences in body size and clutch size associated with geographically separated
foraging areas. In the NWA, however, Hawkes et al. (2007), based on a limited sample size (N
=12), did not find any difference in fecundity measures (clutch frequency, clutch size, body size,
remigration intervals, and inter-nesting intervals) between adult females using northern (N = 9)
versus southern areas (N = 3). In this study, I found no differences in body size between turtles
migrating to cool and highly productive waters of the MAB (Group 3, N = 8) and turtles
migrating south to warm waters of the subtropical NWA (Group 4, N = 3), but my samples size
for southern turtles was small. Further systematic assessment of how differences in nutrient
sources and environmental factors acting on nutrient availability, as well as differences in the
concentration of pollutants, may shape the life history and health of loggerheads is crucial, if we
want to understand changes in loggerhead population abundance in the Atlantic (Witherington et
al. 2009).
Studies using satellite telemetry have shown that female NWA loggerhead turtles follow
different migration patterns (Plotkin and Spotila 2002; Dodd and Byles 2003; Hawkes et al.
2007, 2011; Foley et al. 2008; Turtle Expert Working Group 2009) and show fidelity to foraging
areas with unique environmental characteristics after the mating season (Hawkes et al. 2007,
2011). NWA adult males in this study use foraging grounds (Figure 2-2) similar to those of
NWA females (Hawkes et al. 2011), and the agreement found in my study between SI and
satellite data suggest that males show site fidelity to these foraging areas. The similar patterns in
the use of foraging areas and in the SI values observed between females and males in the NWA,
may indicate that adult males have similar foraging strategiessimilar habitat use, foraging
areas, and movement patternsas those of adult female loggerheads in the NWA.
35
Conclusions
In the present study, adult male loggerheads breeding in Florida revealed a geographic
pattern in the SI values, which indicates that males use isotopically distinct geographical areas
after the mating season. Therefore, SI may help identify foraging subpopulations within a
breeding area and elucidate residency and migration patterns in sea turtles in the NWA.
Moreover, by linking foraging grounds to breeding areas through SI analysis, we can begin to
understand how distinct environmental factors in different foraging grounds affect the biology
and ecology of loggerheads. The agreement between the isotopic signatures and post-mating
movement patterns suggests a foraging site fidelity in male loggerheads that has been observed
in adult females. Also, adult male loggerheads revealed a variation in 13
C and 15
N values
similar to that observed in adult females, suggesting that males and females have similar
foraging strategies. The use of additional markers in combination with isotopic signatures may
help differentiate geographically separated areas with similar isotopic signatures. Understanding
the temporal and spatial distribution of sea turtle populations is essential for the development of
effective conservation and management strategies.
36
Table 2-1. Foraging area, sample size (N), and body size (SCL) of the five groups determined
according to the migration that satellite-tracked male loggerheads followed after the
mating season. SCL: straight carapace length.
Group Foraging area N SCL (cm)
mean ± SD min – max
1 Off Cape Canaveral 10 89.0 ± 2.1 86.6 – 96. 2
2 South Atlantic Bight* 2 88.5 ± 1.5 87.4 – 89.5
3 Mid-Atlantic Bight 8 97.8 ± 5.3 89.0 – 102.8
4 Subtropical Northwest
Atlantic
3 94.1 ± 7.6 86.9 – 102
5 Northeast Gulf of
Mexico
2 98.3 ± 12.4 89.5 – 107.0
* Refers to foraging areas in the South Atlantic Bight not including waters off Cape Canaveral.
37
Table 2-2. Adult male loggerhead population range (maximum – minimum) and variance (Var)
of 13
C and 15
N values in red blood cells (RBC), epidermis (EPI), and plasma (PLA)
samples.
RBC (N = 37) EPI (N = 26) PLA (N = 37)
Range (‰) Var Range (‰) Var Range (‰) Var
13
C 8.15 3.22 7.53 3.93 8.19 3.54
15
N 8.97 5.33 9.68 7.15 8.96 5.39
38
Figure 2-1. Stable isotope ratios (13
C and 15
N) of adult female (N = 310) and male (N = 37)
loggerheads in Florida. (a) Adult female loggerhead signatures show the 2 clusters
identified using 13
C (denoted by open triangles and open circles) by Reich et al.
(2010). (b) Comparison of 13
C and 15
N values of adult female (open symbols) and
male (filled symbols) loggerheads in Florida. Male loggerhead samples were
collected during mating seasons at Cape Canaveral, FL in 2006 and 2007. Female
samples from Florida were collected during nesting seasons in 2003 and 2004.
39
Labels indicate the foraging locations to which satellite-tracked male loggerheads (N
= 25) migrated after the mating season. FL Panhandle refers to the northeast Gulf of
Mexico area in Florida and FL Keys refers to the Florida Keys. Unknown turtles are
males without transmitters and satellite-tracked males for which foraging location
could not be determined. Samples are red blood cells (RBC) for males and epidermis
samples converted to RBC values for females (see Results for regression equation).
Other male samples (epidermis and plasma) show the same pattern as RBC (see
Appendix, Figure A-1).
40
Figure 2-2. Spatial distribution of satellite-tracked male loggerheads from Cape Canaveral,
Florida (filled star) after mating season in April 2006 and 2007. Turtles stayed in
waters near Cape Canaveral (11 - 20; numbered circle) or migrated and remained in
various continental shelf locations (1 through 10 and 21 through 25; numbered
circles). FL Panhandle refers to the northeast Gulf of Mexico area in Florida and FL
Keys refers to the Florida Keys. Dark line denotes the 200 m bathymetry. Dotted
lines separate coastal regions: Mid-Atlantic Bight (MAB) and South Atlantic Bight
(SAB). The Subtropical Northwest Atlantic (SNWA) is also shown. Adapted from
Arendt et al. (2012b).
41
Figure 2-3. Linear relationships between epidermis (EPI, N = 26) and red blood cells (RBC, N =
26) for (a) 13
C and (b) 15
N of adult male loggerheads. The relationship between
these tissues that reflect similar temporal resource assimilation is significant for both
13
C and 15
N (see Results). The solid line is the best-fit line, the dashed lines denote
the 95% confidence interval for the linear regression, and the dotted lines denote the
95% prediction interval (the range in which future observations will fall).
42
Figure 2-4. Stable isotope ratios of carbon (13
C; column A) and nitrogen (15
N; column B) of
red blood cells (RBC; N = 25; open circle), epidermis (EPI; N = 17; open square),
and plasma (PLA; N = 25; open triangle) from adult male loggerheads collected at
Cape Canaveral, FL, versus the latitude to which turtles migrated after the mating
season.
43
Figure 2-5. Red blood cell 15
N versus straight carapace length in male loggerhead turtles that
remained off Cape Canaveral (r2 = 0.64, F1,8 = 15.7, P = 0.004).
44
Figure 2-6. Comparison of stable isotope ratios of 13
C (left) and 15
N (right) of food web
organisms at the different foraging locations visited by male loggerheads after the
mating season (represented by latitude). Mean values are given. For scientific names,
sample sizes, and references see Table 2-3.
45
CHAPTER 3
ASSIGNMENT OF NESTING LOGGERHEAD TURTLES TO THEIR FORAGING AREAS
IN THE NORTHWEST ATLANTIC USING STABLE ISOTOPES2
Background
For endangered migratory fauna, knowledge of demographic parameters is key to
accurately assess the status and trends of populations (Esler 2000; National Research Council
2010; Wallace et al. 2010). Because values of demographic parameters can vary with
environmental conditions at the foraging habitats (Cooch et al. 2001; Chaloupka et al. 2008;
Saba et al. 2008), information on movement patterns and foraging locations of populations are of
crucial importance.
Populations of the loggerhead sea turtle (Caretta caretta) nesting in the Northwest
Atlantic (NWA) represent one of the major nesting aggregations for the species in the world
(Ehrhart et al. 2003). NWA loggerhead nesting aggregations are composed of genetically and
demographically distinct populations (Encalada et al. 1998; Shamblin et al. 2012). Loggerheads
swim hundreds of kilometers from a wide range of foraging grounds to their nesting beaches.
Concern was raised when nesting activity in one of these NWA populations declined markedly
from 1998 to 2007 (Witherington et al. 2009); however, an increase in nesting numbers has been
reported in recent years (Van Houtan and Halley 2011). Anthropogenic threats (Jackson et al.
2001; Witherington et al. 2009; Finkbeiner et al. 2011) and changing oceanographic conditions
(Chaloupka et al. 2008; Saba et al. 2008; Van Houtan and Halley 2011) have been proposed as
the main drivers of fluctuations in sea turtle abundance. Because these factors may change
depending on geographic location (Kot et al. 2010 and references therein), efforts to identify
Reprinted with permission from the authors. Original publication: Pajuelo M, Bjorndal KA, Reich KJ, Vander
Zanden HB, Hawkes LA, Bolten AB (2012b) Assignment of nesting loggerhead turtles to their foraging areas in the
Northwest Atlantic using stable isotopes. Ecosphere 3:art89.
46
foraging grounds of sea turtles are vital to understand spatial and temporal fluctuations in nesting
numbers.
To better understand how environmental changes and human threats at different foraging
grounds affect the various nesting populations in the NWA, it is important to evaluate not only
the demographic parameters of each breeding population, but also the proportions of females in
each breeding population located in different foraging areas. Initial efforts have been undertaken
to understand how differential foraging locations and oceanographic conditions affect
demographic parameters such as clutch size, number of clutches per nesting season, clutch sex
ratio, and female body size in loggerhead populations (Hatase et al. 2002; Hawkes et al. 2007a,
b; Zbinden et al. 2011; Bailey et al. 2012).
Satellite telemetry studies have revealed that NWA adult female loggerheads have at least
two different migration patterns (seasonal shuttling migration and year-round residency) when
they leave the nesting beaches and return to their foraging areas. Females may travel up to
hundreds of kilometers and forage in coastal areas along the U.S. Atlantic coast, Gulf of Mexico,
Cuba, and the Bahamas (Plotkin and Spotila 2002; Dodd and Byles 2003; Foley et al. 2008;
Hawkes et al. 2007a, 2011). They also show site fidelity to their foraging areas, characterized by
different environmental features (Hawkes et al. 2007a, 2011), thus revealing patterns of
migratory connectivity between nesting sites, foraging areas, and wintering areas.
Stable isotope analysis, a technique that uses ratios of stable isotopes of naturally occurring
elements (e.g., carbon, nitrogen), can complement information from satellite telemetry on
population connectivity (Webster et al. 2002). Because stable isotopes in the environment are
incorporated into primary producers and then transferred up the food chain (DeNiro and Epstein
1978; Minagawa and Wada 1984), the isotopic values of tissues of higher trophic level
47
organisms reflect differences in the stable isotope values of primary producers of the
environment in which these organisms foraged (Schell et al. 1989; Minami and Ogi 1997; Burton
and Koch 1999; Kurle and Worthy 2002; Cherel et al. 2007; Pajuelo et al. 2010). These spatial
isotopic differences in primary producers create isotopically distinct regions that can be used to
infer residency and movement patterns of organisms migrating among them (Rubenstein and
Hobson 2004; Graham et al. 2010). Within an organism, different tissues incorporate and
turnover stable isotopes at different rates. In sea turtles, epidermis, keratin, and red blood cells
reflect a longer-term foraging history (Reich et al. 2008). Therefore, such tissues collected for
turtles at breeding areas reflect their dietary history at foraging grounds prior to migration to the
breeding area (Wallace et al. 2006; Caut et al. 2008; Reich et al. 2010; Vander Zanden et al.
2010; Zbinden et al. 2011; Pajuelo et al. 2012; Seminoff et al. 2012).
Stable isotope values of animals can be used to identify their foraging areas if (1)
different foraging areas are isotopically distinct and (2) sampled tissues reflect the isotopic
signatures of the foraging grounds (Rubenstein and Hobson 2004). These requirements are met
for adult loggerheads in the NWA and have been demonstrated for adult males (Pajuelo et al.
2012). Male loggerheads show differences in their stable isotope values reflecting the use of
three geographic areas: the Mid-Atlantic Bight (MAB), the South Atlantic Bight (SAB), and the
subtropical NWA (SNWA) (Pajuelo et al. 2012; Figure 3-1), which represent well-established
biogeographic regions with distinctive biotic and abiotic features (Hutchins 1947; Wilkinson et
al. 2009). Moreover, based on satellite telemetry, adult males appear to use foraging grounds
similar to those of adult females in the NWA (Arendt et al. 2012). The large variation in δ13
C
and δ15
N values from nesting loggerheads in Florida, USA (Reich et al. 2010) is similar to that
48
observed in the satellite-tracked male loggerheads (Figure 3-1), and probably represents a
gradient of north to south foraging locations used by adult female loggerheads in the NWA.
The main objective of my study was to assign loggerhead sea turtles nesting along the
U.S. Atlantic coast to their foraging locations in the NWA using stable isotope analysis. First, I
evaluated whether satellite-tracked adult female loggerheads have the same relationship between
geographic areas and stable isotope values as adult males. Second, I characterized the geographic
areas used by adult loggerheads with isotopic values of satellite-tracked adult loggerheads and
additional turtles with known foraging locations. Then, I compared the isotopic values of nesting
turtles not fitted with satellite transmitters with those of adult loggerheads with known foraging
location to determine their foraging areas based on assignments from discriminant analysis.
Finally, I estimated the proportion of each nesting population foraging in each geographic area.
By combining stable isotope analysis and satellite telemetry to identify foraging locations of
breeding populations, I can then rely on stable isotope analysis alone to assign large numbers of
female loggerheads to their foraging grounds rapidly and at low cost. This knowledge will allow
us to assess with robust sample sizes how different environmental factors and threats at the
different foraging grounds affect the demography of adult loggerheads in the NWA and focus
management and conservation efforts appropriately.
Material and Methods
Data and Sample Collection
Epidermis samples were collected from 87 adult female loggerhead turtles during the
2004 and 2005 nesting seasons (May – Jul) at six nesting areas (Table 3-1; Figure 3-2): Bald
Head Island in North Carolina (BHI; 33.86° N, 77.99° W), and Wassaw (WAS; 31.84° N, 80.98°
W), Blackbeard (BLA; 31.61° N, 81.14° W), Sapelo (SAP; 31.40° N, 81.28° W), Jekyll (JEK;
31.07° N, 81.42° W), and Cumberland (CUM; 30.85° N, 81.45° W) Islands in Georgia.
49
Additionally, 15 adult-size turtles (curved carapace length, CCL ≥ 84 cm) were sampled at one
foraging area in Florida Bay, Florida (24.08° N, 81.03° W) in March and June 2011 (Table 3-1;
Figure 3-2). Previously published isotopic data from epidermis samples of adult female
loggerheads collected at four nesting areas in Florida (N = 310; Reich et al. 2010) and adult male
loggerheads collected at one breeding area in Florida (N = 23; Pajuelo et al. 2012) were also
included in this study (Table 3-1; Figure 3-2).
All epidermis samples were collected using a 6 mm biopsy punch and stored in 70%
ethanol at room temperature until dried at 60C prior to sample preparation and analysis.
Epidermis samples reflect the turtle’s dietary history over a long period of time (i.e., up to 4
months) based on studies conducted on juvenile loggerheads (Reich et al. 2008). An even longer
foraging history is probable in adult loggerheads because rates of isotopic incorporation slow
with reduced growth rates (Reich et al. 2008) and increasing body mass (Carleton and Martínez
del Rio 2005).
Twenty-two female turtles were fitted with satellite transmitters after clutch deposition in
Georgia (N = 18) and North Carolina (N = 4) beaches (Table 1). Hawkes et al. (2007a, 2011)
characterized the distinct movement patterns of these adult female loggerheads and classified
them into two groups, (1) turtles with seasonal migration between summer and winter coastal
areas and (2) turtles with migration to year-round foraging areas. I grouped turtles into three
groups according to the coastal region to where they migrated: the first group is the MAB turtles,
with seasonal migration between summer foraging areas in the MAB and wintering areas in the
SAB; the second and third groups are the SAB and SNWA turtles, with migration to year-round
foraging areas in waters of the SAB and SNWA, respectively. Turtles were tracked for 344.2 ±
148.4 days (mean ± SD) (individuals 5-8, 40-43, 49-52, and 54-63; Hawkes et al. 2011: Table
50
S1). Most of the turtles (N = 19) were tracked for the entire foraging period for turtles using
MAB waters, and for 6 months or more at the foraging ground for turtles using SAB and SNWA
waters. The remaining turtles were tracked for a period of 80 days or more after reaching the
foraging ground. Telemetry data were filtered following Hawkes et al. (2011) to retain location
classes 3, 2, 1 and A, and turning angles greater than 25°, and the latitude of the centroid of the
foraging ground (arithmetic mean of all the filtered location points from the foraging ground)
was used to evaluate the relationship between stable isotope values and geographic location.
Stable isotope values of epidermis samples from satellite-tracked female turtles (N = 22) were
used to characterize the three geographic areas used by adult loggerheads, assuming that isotopic
values from foraging areas used prior to nesting reflected those used post-nesting (identified by
satellite telemetry), as adult female loggerheads are known to exhibit site fidelity to foraging
areas (Hawkes et al. 2007a, 2011; Vander Zanden et al. 2010). Also, because the isotopic values
of satellite-tracked male loggerheads have shown a geographic pattern consistent with the use of
the three geographic areas mentioned above (Pajuelo et al. 2012), male data (N = 23) were
incorporated in the isotopic characterization of the three geographic areas used by adult
loggerheads. Few satellite tracked turtles migrated to the SNWA, so I included additional
epidermis samples from adult-size loggerheads collected at one foraging ground in the SNWA
(Florida Bay; N = 15).
Finally, I used the isotopic values from epidermis samples of nesting turtles not fitted
with satellite transmitters from BHI in North Carolina (N = 18) and WAS in Georgia (N = 47), in
combination with published isotopic data from nesting turtles at four nesting areas in Florida (N
= 310; Reich et al. 2010) (Table 3-1), to compare to those of turtles with known foraging ground
to assign them to one of the three geographic areas used by adult loggerheads in the NWA.
51
Sample Preparation and Analysis
Epidermis samples were washed with deionized water and wiped with isopropyl alcohol
to remove epibionts and extraneous particles. The outermost layer of the turtle epidermis was
separated from the underlying tissue, finely diced with a scalpel blade, and dried at 60C for 24
h. Lipids were extracted from samples with petroleum ether using an ASE300 accelerated
solvent extractor (Dionex).
For stable isotope analysis, 0.5-0.6 mg of each sample was weighed and sealed in a tin
capsule. Samples were analyzed for δ13
C and δ15
N ratios by combustion in a COSTECH ECS
4010 elemental analyzer interfaced via a ConFlo III device to a DeltaPlus XL isotope ratio mass
spectrometer (ThermoFisher Scientific) in the Stable Isotope Geochemistry Lab at the University
of Florida, Gainesville. Results are presented as stable isotope ratios of a sample relative to an
international standard and reported in the conventional notation:
X = [(Rsample/Rstandard) –1] x 1000, where X is the relative abundance of 13
C or 15
N in the
sample expressed in parts per thousand (‰); Rsample and Rstandard are the ratios of heavy to light
isotope (13
C/12
C and 15
N/14
N) in the sample and international standard, respectively. The standard
used for 13
C was Vienna Pee Dee Belemnite and for 15
N was atmospheric N2. The reference
material USGS40 (L-glutamic acid) (N = 22) was used to normalize all results, SD = 0.05‰ and
0.13‰ for 13
C and 15
N, respectively.
Statistical Analyses
The effect of geographic location on 13
C and 15
N values was evaluated with a
Spearman rank correlation test between isotope values and the latitudes of the foraging grounds
of the adult female turtles.
52
To determine the similarity of the isotopic values of samples from turtles of unknown
foraging ground (hereafter referred to as unknown turtles) to those of samples from turtles of
known foraging location (hereafter referred to as known turtles), I classified the isotopic values
of known turtles into three groups: MAB, SAB, and SNWA. Multivariate analysis of variance
(MANOVA) was used to test for variation in 13
C and 15
N values among groups to test if they
were quantitatively discrete. Then, these three isotopically defined groups were combined with
the unknown turtles in a quadratic discriminant analysis, due to unequal variance among groups.
The discriminant analysis assigned each unknown turtle to the geographic area for which it had
the highest probability of membership. To test the accuracy of assignment, I applied the leave-
one-out cross validation method to the reference groups, where a single turtle is removed from
the total and classified to a foraging region by the functions derived from all turtles other than
the excluded turtle, with the process being repeated for each remaining turtle.
Following the determination of geographic area for unknown turtles, I evaluated the
population structure of all breeding populations using only turtles assigned to one of the three
groups with ≥ 80% probability of group membership (Rocque et al. 2006; Seminoff et al. 2012)
(340 out of 375 turtles). Finally, a chi-square test was performed to test for inter-annual variation
in the proportion of turtles using different foraging grounds for nesting beaches that were
sampled in two consecutive years, whenever sample size allowed (i.e., Cape Canaveral-CNS,
Melbourne-MEL, and Juno-JUN beaches; Table 3-1). All data were analyzed using program R
(R Development Core Team 2011) with an level of 0.05.
Results
Epidermis isotopic values of adult females in the NWA ranged from 3.5 to 18.7‰
and -6.9 to -17.6‰ for 15
N and 13
C, respectively (Figure 3-3).
53
The epidermal isotope values from satellite-tracked female loggerheads (tracked long
enough to identify their foraging areas) revealed a geographic pattern: females that migrated
north to seasonal foraging grounds in the MAB (e.g., New Jersey, Virginia, and Delaware) after
the nesting season had high 15
N values and low 13
C values (Figure 3-3). The lowest 15
N value
and highest 13
C value were found in a female that migrated south to a year-round foraging
ground in the SNWA (The Bahamas) (Figure 3-3). Intermediate 15
N and 13
C values were
found in turtles migrating to coastal waters of the SAB (e.g., Georgia and northern Florida)
(Figure 3-3). A significant negative correlation was found between 13
C values and the latitude
to which females migrated after the nesting season (Spearman’s rank correlation rs = -0.64, N =
22, P = 0.001; Figure 4A), while a significant positive correlation was found between 15
N
values and latitude (Spearman’s rank correlation rs = 0.46, N = 22, P = 0.029; Figure 3-4B).
Stable isotope values of epidermis from adult loggerhead males and females using the
same geographic areas were similar. Even though three satellite-tracked turtles had isotopic
values that were not consistent with the geographic area to which they migrated (turtles 1, 2, and
3; Figure 3-5A), I found significant differences in combined 13
C and 15
N values among
geographic areas used by adult loggerheads in the NWA (MANOVA, F = 29.32, P < 0.001).
The isotopic signatures of these three groups were used as a reference from which to compare the
isotopic values of unknown turtles. Discriminant analysis assigned all unknown turtles to one of
the three geographic areas with 91% (340 out 375) of those turtles assigned to a unique
geographic area with a probability ≥ 80% of group membership (Table 3-2; Figure 3-5B). The
percentages of turtles assigned at higher probabilities were lower but remained substantial: 85
and 79% with a probability ≥ 90 and 95%, respectively. Leave-one-out cross validation revealed
a 6% (N = 4) misclassification rate, which corresponded to the misclassification of turtles 1, 2, 3,
54
and an additional turtle. Unknown turtles assigned with ≥ 80% probability to a geographic area
were used to evaluate the foraging structure of breeding populations. A latitudinal trend in the
foraging area use by nesting loggerheads was revealed; the proportion of turtles using the MAB
increased from south to north and the proportion using the SNWA increased from north to south
(Figure 3-6). The majority of turtles (72-80%) nesting at higher latitudes (i.e., BHI and WI) used
foraging areas in the MAB and few turtles (6%) used the SNWA (Figure3-6). Most turtles (46-
81%) nesting at lower latitudes (i.e., Juno Beach and Broward County) used the SNWA and few
(2-21%) used the MAB (Figure 3-6). A large number (36-59%) of nesting turtles from CNS used
the SAB. The use of the SAB declined north and south of CNS (Figure 3-6).
The proportion of turtles using the different foraging areas varied between years in CNS
(Pearson’s chi-square test, df = 2, 2 = 11.51, P = 0.003), MEL (Pearson’s chi-square test, df = 2,
2 =11.39, P = 0.003), and JUN (Pearson’s chi-square test, df = 2,
2 = 22.70, P < 0.001) beaches
(Figure 3-6). A marked pattern in the reduction in the proportion of turtles using MAB waters
and the increase of turtles using SNWA waters was observed in MEL and JUN in 2004 (Figure
3-6).
Discussion
Isotopic Characterization of Geographic Areas Used by Adult Loggerheads in the NWA
In this study, a combination of stable isotope and satellite-telemetry data allowed me to
characterize three main geographic regions used by adult loggerheads in the NWA (Fig 3-5A).
Recent studies have integrated telemetry data to validate marine geographic patterns in 13
C or
15
N values of highly migratory animals such as seabirds (Jaeger et al. 2010) and sea turtles
(Seminoff et al. 2012) over broad spatial scales (e.g., within ocean basins). Here, I present the
55
combined 13
C and 15
N spatial characterization for a highly migratory animal at a regional scale
in the NWA.
Isotopic turnover in epidermis samples of adult loggerhead turtles is estimated to be at
least 4 months (see introduction), longer than the expected migration period between the
foraging area and breeding ground for a satellite-tracked turtle used in this study (~ 1 month,
based on mean travel duration between nesting area and foraging grounds and between foraging
and wintering grounds; Hawkes et al. 2011, L. Hawkes, unpubl. data). Thus, isotopic values of
epidermis tissues from nesting loggerheads should reflect that of the foraging grounds used prior
to migrating to the nesting beaches.
Satellite-tracked nesting loggerheads demonstrated a geographic pattern in the stable
isotope values (Figure 3-3) similar to the one previously observed in satellite-tracked male
loggerhead turtles (Figure 3-1; Pajuelo et al. 2012). Although the MAB turtles use waters of both
the MAB (summer) and SAB (winter), they maintain distinct stable isotope values from those of
turtles that use waters in the SAB year-round. The difference could result from very slow
turnover rates in epidermis of adult turtles. Also, Hawkes et al. (2007a) suggested that seasonal
turtles (i.e., MAB turtles) that migrate south into SAB areas during winter might undergo fasting
during part of the winter. It has been proposed that 15
N values might increase with fasting
duration (Martínez del Rio et al. 2009). Because the extent of the enrichment in 15
N appears to
be tissue-dependent, this hypothesis has received mixed support (Martínez del Rio et al. 2009)
and remains to be tested in sea turtles.
Nesting loggerhead turtles can be assigned to their coastal foraging areas in the NWA
using stable isotope values because NWA adult female turtles show fidelity to their foraging
grounds both within and between years (Hawkes et al. 2007a, 2011), as has been observed in
56
other loggerhead populations (Broderick et al. 2007; Schofield et al. 2010; Thomson et al.
2012). However, some NWA turtles have been found to occasionally use oceanic waters
(Hawkes et al. 2007a, 2011), which may result in unusual stable isotope values. The isotopic
values of three turtles (1, 2, and 3; Figure 3-5A) did not correspond to the geographic area to
which they migrated after the breeding season. The most likely explanation is that these turtles
did not return to the same geographic area from which they originally came. For example, if a
turtle used waters of the SAB prior to its capture in the nesting beach and later migrated to
waters in the MAB, its isotopic values would show lower 15
N and higher 13
C values than
would be expected for a turtle using MAB waters (e.g., turtle 2; Figure 3-5A). Although adult
turtles are generally site-fixed to their foraging grounds, occasional shifts can be expected.
The distinct biotic and abiotic characteristics of the three geographic areas used by
loggerheads—MAB, SAB, and SNWA—likely influence isotopic values of turtles using those
areas. High anthropogenic input appears to raise the 15
N values of primary producers in the
MAB (McKinney et al. 2010). Also, high rates of denitrification, which also raise baseline 15
N
values, have been reported in the MAB (Fennel et al. 2006) although its effect on the 15
N
coastal biota has not been assessed yet (McKinney et al. 2010). In addition, the rate of nitrogen
fixation, which lowers the 15
N values of primary producers, is highest in the SNWA (Montoya
et al. 2002). Available 15
N values of particulate organic matter (a proxy for primary producers)
along the latitudinal gradient used by loggerheads reveal this pattern: nitrogen stable isotope
ratios range from 7.2 to 7.7‰ in near-shore waters off of Virginia and Delaware in the MAB
(McKinney et al. 2010), from 4.0 to 6.4‰ in near-shore waters off of South Carolina and
Georgia in the SAB (M. Pajuelo and M. Arendt unpubl. data), and from -0.9 to 3.6 in Florida
Bay in the SNWA (Macko et al. 1984; Behringer and Butler 2006; Lamb and Swart 2008).
57
Water temperature can also affect 13
C values at the base of the food web by affecting
cell growth rate and dissolved carbonate concentration, which have a direct effect on the 13
C
values of primary producers (MacKenzie et al. 2011). Sea surface temperatures in the MAB
during summer—the season when adult loggerheads mainly use MAB waters—range from 15-
27°C, while water temperatures in the SNWA range from 22.5-28°C year round (Wilkinson et al.
2009). Waters in the SNWA are also characterized by the presence of extensive seagrass
communities (Wilkinson et al. 2009), whose contribution to benthic food webs may be evidenced
by relatively low 13
C values in food web organisms (Fry et al. 1982). Ultimately, variation in
baseline isotopic values will be reflected in higher trophic level organisms such as NWA adult
loggerheads, which prey mainly on benthic invertebrates in coastal waters (Hopkins-Murphy et
al. 2003). Stable isotope values of other food web organisms in the NWA exhibit a pattern
similar to that of adult loggerhead turtles (Pajuelo et al. 2012) and indicate that baseline
differences rather than trophic level differences are driving the large isotopic variation in adult
loggerhead turtles.
While turtles using the MAB and SNWA have distinct stable isotope values (Figure 3-
5A), Pajuelo et al. (2012) found that isotopic values from male turtles using the SAB were
similar to those of turtles using coastal waters in the Gulf of Mexico (which were not included in
our analysis) (Figure 3-1). Because I are interested in determining the foraging locations of
loggerheads nesting along the U.S. Atlantic coast, I need to consider that adult female turtles use
foraging areas in regions other than the NWA. Telemetry studies have revealed that adult female
loggerheads nesting in Florida beaches use coastal waters in both the NWA and the Gulf of
Mexico (Foley et al. 2008). Therefore, the isotopic values reflecting the use of waters in the SAB
for turtles from southern nesting beaches may be confounded with those of turtles using waters
58
in the Gulf of Mexico. Other markers, such as trace elements and lead stable isotopes, may help
differentiate these two foraging areas with similar 13
C and 15
N signatures (M. López-Castro
pers. comm.).
In this study, I chose to define the three loggerhead geographic foraging grounds in the
NWA based on the knowledge that these represent well-established biogeographic regions, each
of which shows distinct oceanographic conditions and faunal communities (Hutchins 1947;
Wilkinson et al. 2009). Because stable isotopes may be influenced by factors (see above) that
vary among the biogeographic areas, I was able to find significant differences in the stable
isotope values of turtles among these three areas. However, within a particular foraging ground,
stable isotope values of loggerheads can vary due to differences in habitat type and/or diet
(Rubenstein and Hobson 2004). Thus, to identify feeding areas at a finer scale than the one
presented here will likely require the use of additional biomarkers (e.g., trace elements).
Foraging Locations of Adult Female Loggerheads in the NWA
For many years, much of what was known about the foraging locations of adult female
loggerheads in the NWA relied on information from flipper tag returns (Bell and Richardson
1978; Meylan et al. 1983; Williams and Frick 2008). While informative, tag return data can be
biased because these data mainly rely on the capture of flipper tagged turtles by fisheries. In
recent years, satellite transmitters have been deployed on nesting loggerheads, which have
provided more accurate information on the post-nesting migratory routes, location of foraging
grounds, and feeding behavior of adult female loggerheads in the NWA (Godley et al. 2008).
However, the expense of satellite tags, which limits the number of individuals that can be
tracked, has prevented more widespread use. Thus, stable isotope analysis, which is low cost and
59
can yield results rapidly, can be useful in identifying foraging areas of a large number of
individuals.
Carbon and nitrogen stable isotopes allowed us to assign most nesting loggerheads to a
distinct geographic area in the NWA at a probability of > 80%. The remaining turtles assigned to
an area with a probability of < 80% do not suggest that they use unidentified foraging locations.
Because I found a latitudinal trend in both 13
C and 15
N along the NWA (Figure 3-4), I believe
that those foraging locations are found within one of the three geographic areas in the NWA. I
could not assign all turtles with a probability > 80% probably as a result of isotopic variation
within each of the three geographic areas that was not captured by the satellite-tracked turtles, or
because they travelled to the Gulf of Mexico.
I found that nesting loggerheads showed geographic segregation of foraging grounds;
northern nesting turtles preferred higher latitude foraging areas while the opposite was seen in
southern nesting turtles. Thus, my initial observations revealed that female loggerheads in the
NWA generally use foraging areas in the vicinity of their natal nesting beaches. These results are
consistent with satellite telemetry data from nesting loggerheads in the NWA. For turtles nesting
in North Carolina, South Carolina, and Georgia, Hawkes et al. (2011) revealed that most females
(N = 48) migrated north to seasonal foraging grounds in the MAB, while few (N = 18) move to
year-round waters of the SAB and SNWA after the nesting season. Similarly, based on smaller
sample sizes, Dodd and Byles (2003) and Foley et al. (2008) revealed that nesting turtles from
southern beaches in Florida migrated to waters in the SNWA and rarely migrated to northern
waters in the MAB.
The use of foraging grounds adjacent to natal nesting areas has been suggested previously
for large juvenile loggerheads in the NWA by mixed stock analysis of mitochondrial DNA
60
haplotypes for aggregations of juveniles along the U. S. east coast (Bowen et al. 2004). The
stable isotope approach used in my study allowed us to sample adult female loggerheads at
various nesting areas— where they are more easily accessible—without having to sample turtles
at the different foraging areas to reveal a similar pattern of foraging ground segregation. Mixed
stock analysis of mitochondrial DNA haplotypes has been widely used to assess the contribution
of various nesting areas to feeding grounds (Bolker et al. 2007). Because this technique relies on
differential haplotype frequencies at the various nesting areas, each nesting individual cannot be
assigned to its foraging ground. The existence of habitat-specific stable isotope signatures allows
stable isotope analysis to assign each individual to its foraging area (Rubenstein et al. 2004).
Thus, geographic assignment models in sea turtles may be improved by incorporating traditional
tools such as genetic analyses, mark-recapture data, and satellite telemetry along with stable
isotope analyses to understand the connection between nesting areas and foraging grounds.
The temporal variation in the proportion of turtles using different geographic areas within
three Florida nesting beaches, CNS, MEL and JUN, suggests differential remigration intervals
may exist (i.e., the period of time between reproductive seasons) among foraging subpopulations.
Given the greater distance that turtles foraging in MAB waters travel to reach southern beaches,
there may be differential remigration intervals for these turtles within a southern nesting
population. Adult female turtles that forage in highly productive waters of the MAB during
summer are known to migrate to the SAB during winter months (Hawkes et al. 2007a). Another
possibility is that turtles using MAB waters seasonally may spend more energy undergoing
seasonal migration (Hawkes et al. 2007a), which may be reflected in longer remigration
intervals. Hawkes et al. (2007a), based on a small sample size of females from a northern nesting
beach in North Carolina, did not find differences in remigration intervals (and other fecundity
61
measures) between females using seasonal foraging areas in the MAB (N = 9) versus year round
areas in the SAB and SNWA (N = 3), suggesting that neither differential foraging/migratory
strategies within this northern breeding population was more advantageous (Hawkes et al.
2007a). However, females using MAB waters in the Hawkes et al. (2007a) study were closer to
their northern nesting beach. Recently, variations in reproductive output and demography due to
inter-basin differences in feeding and movement behavior have been reported in leatherback
turtles (Bailey et al. 2012). Further research is needed to assess whether the pattern observed in
northern nesting loggerheads is consistent with a larger sample size and in different nesting
populations.
My study incorporated previously published stable isotope values of epidermis from adult
female loggerheads nesting in Florida beaches from Reich et al. (2010). Additionally, Vander
Zanden et al. (2010) collected scute (carapace keratin) from a subsample of these loggerheads to
investigate the long-term consistency in resource use through stable isotope analysis of scute
layers. Both studies suggested that large differences in 13
C and 15
N observed could be
accounted for by foraging location (Reich et al. 2010; Vander Zanden et al. 2010). In this study,
I confirmed that stable isotope values of female loggerheads in the NWA reflect their foraging
locations by ground-truthing stable isotope values with information obtained through satellite
telemetry. The much greater number of females that can be assigned to foraging grounds based
on stable isotope analysis than on satellite telemetry will allow robust analyses of foraging
ground effects on demographic parameters such as number of eggs per clutch, number of
clutches deposited during a nesting season, and remigration intervals, which are critical to
understand trends in sea turtle nesting populations (National Research Council 2010).
62
Conservation Implications
In order to effectively manage populations of highly migratory endangered species, an
understanding of spatio-temporal distribution is essential. In the particular case of adult
loggerhead populations in the NWA that use waters over a wide geographic range, knowing
which feeding areas a major nesting population primarily uses is important, because it allows
managers to focus conservation efforts where appropriate.
Adult female loggerhead populations segregate among foraging grounds, which is
promising for refining management strategies. I can identify, at a large scale, what areas are
more or less important for a particular nesting population in the NWA. For example, in this study
I identified that foraging areas in the SNWA are highly important for turtles nesting in Florida
beaches, followed by areas in the SAB; while areas in the MAB are used to a lesser degree.
Fisheries bycatch is one of the major threats for loggerhead turtles in the NWA (Bolten et al.
2010). Research on sea turtle bycatch has revealed spatial and temporal variations in loggerhead
bycatch in U.S. fisheries (Kot et al. 2010; Finkbeiner et al. 2011), with shrimp trawl fisheries in
the SAB, SNWA and Gulf of Mexico accounting for the most interactions with loggerhead
turtles in the U.S. (Finkbeiner et al. 2011). Thus, efforts can be focused in the SAB and SNWA
to assess how fisheries interaction, as well as other environmental factors such as changing
oceanographic conditions and prey distribution, impact fecundity measures of Florida nesting
populations. Additionally, I can further our understanding of how these threats and factors drive
the temporal fluctuations in the proportion of individuals within each nesting population that use
the different foraging areas. However, more northerly and lesser used foraging areas may
currently be important, regarding conservation efforts, because they are used by smaller or more
at-risk nesting populations. These foraging areas may become even more important in the future,
63
if southern turtle populations were to shift northward, as suggested under future climate
scenarios (i.e., if southern beaches become too hot; Hawkes et al. 2007b).
Organic pollutants are another anthropogenic threat to which turtles are exposed in the
NWA (Alava et al. 2011 and references therein). Recent research revealed that adult loggerheads
using northern foraging grounds in the MAB have higher concentrations of organic pollutants
than turtles that use waters off central Florida, and my results support the hypothesis that this
may be due to spatial structuring of foraging grounds by population (Ragland et al. 2011).
Similarly, a recent study found that loggerhead eggs laid in a northern nesting beach in North
Carolina had higher organic concentration of pollutants than eggs laid in southern nesting
beaches in Florida (Alava et al. 2011). These differential threats can also affect demographic
parameters and health of the foraging subpopulations and should also be considered in
management plans.
Conclusions
My study demonstrates that stable isotope analysis can be used parsimoniously to identify
foraging areas of adult loggerheads in the NWA at a regional scale. Future research is needed to
assess if stable isotope analyses, perhaps integrated with other biomarkers such as trace
elements, could identify foraging areas at a finer scale. Additionally, I found that adult female
loggerheads nesting along the U.S. Atlantic coast tend to use foraging areas closer to their natal
nesting beaches; a smaller proportion of individuals undertake migrations to distant foraging
grounds. These results are useful for the design of management strategies for the conservation of
loggerhead turtle populations in the NWA. Assignment of large numbers of nesting females to
foraging grounds with stable isotope analysis will allow future research to explore the effects of
foraging ground location on demographic parameters. The conclusions and methods developed
64
in this study are also relevant for other populations of sea turtles and for other highly migratory
species.
65
Table 3-1. Location (state, breeding/foraging area, and latitude), year of collection, and sample
size of epidermis samples from adult loggerhead turtles with known and unknown
foraging grounds used in this study. State Area Lat (°N) Year n Foraging area Isotope data
source
Breeding area Known§ Unknown
NC Bald Head Island 33.9 2004 10 4 6 This study
2005 12 12 This study
GA Wassaw Island 31.8 2005 47 47 This study
Blackbeard Island 31.6 2005 4 4 This study
Sapelo Island 31.4 2004 2 2 This study
2005 8 8 This study
Jekyll Island 31.1 2004 3 3 This study
Cumberland Island 30.9 2004 1 1 This study
FL Canaveral National
Seashore
28.8 2003 44 44 Reich et al. 2010
2004 31 31 Reich et al. 2010
Melbourne Beach 28.1 2003 60 60 Reich et al. 2010
2004 46 46 Reich et al. 2010
Juno Beach 26.9 2003 41 41 Reich et al. 2010
2004 41 41 Reich et al. 2010
Broward County 26.2 2003 47 47 Reich et al. 2010
Port Canaveral 28.4 2006-07 23 23 Pajuelo et al. 2012
Foraging area
FL Florida Bay 25.0 2011 15 15 This study
Total 435 60 375
Notes: NC: North Carolina, GA: Georgia, FL: Florida; Lat: latitude; n: sample size. §Turtles
fitted with satellite transmitters except for turtles sampled at Florida Bay. NC: North Carolina,
GA: Georgia, FL: Florida; Lat: latitude; n: sample size.
66
Table 3-2. Assignment of adult female loggerheads of unknown foraging location to a
geographic area with ≥ 80% probability of group membership. Values in parentheses
are additional turtles assigned with a probability < 80% of group membership.
Geographic area
State Nesting area Lat (°N) n MAB SAB SNWA
NC Bald Head Island, BHI 33.9 18 10 (2) 3 (2) 1
GA Wassaw Island, WAS 31.8 47 31 (3) 6 (5) 2
FL Canaveral National Seashore,
CNS 28.8 75 10 34 (5) 25 (1)
Melbourne Beach, MEL 28.1 106 13 (2) 37 (2) 50 (2)
Juno Beach, JUN 26.9 82 9 16 (6) 50 (1)
Broward County, BRO 26.2 47 1 14 (3) 28 (1)
Total 375 74 (7) 110 (23) 156 (5)
Notes: NC: North Carolina, GA: Georgia, FL: Florida; Lat: latitude; n: sample size. MAB: Mid-
Atlantic Bight, SAB: South Atlantic Bight, SNWA: Subtropical Northwest Atlantic.
67
Figure 3-1. Distribution of stable isotope ratios (13
C and 15
N) of adult male (N = 25) and
female (N = 310) loggerheads in the Northwest Atlantic (NWA). Male loggerheads
(black symbols) are coded by the foraging area used based on satellite telemetry:
Mid-Atlantic Bight (MAB), South Atlantic Bight (SAB), Subtropical Northwest
Atlantic (SNWA). Two male turtles that used coastal waters of the Gulf of Mexico
(GoM) are also shown. Female data (open circles) are from Reich et al. (2010) and
were sampled at various nesting beaches in Florida; male data are from Pajuelo et al.
(2012a). The right panel depicts the main geographic areas used by adult male
loggerheads in the NWA and the Gulf of Mexico.
68
Figure 3-2. Map showing the locations of the six nesting areas and single foraging ground (filled
symbols) sampled in this study: Bald Head Island (BHI), Wassaw Island (WAS),
Sapelo Island (SAP), Blackbeard Island (BLA), Jekyll Island (JEK), Cumberland
Island (CUM), and Florida Bay (FL Bay). Nesting areas sampled in the Reich et al.
(2010) study (open symbols) are also shown: Canaveral National Seashore (CNS),
Melbourne Beach (MEL), Juno Beach (JUN), and Broward County (BRO).
69
Figure 3-3. Stable isotope ratios (13
C and 15
N) of adult female loggerheads in the Northwest
Atlantic. Females tracked with satellite telemetry (N = 22; black filled symbols) were
sampled during nesting seasons in 2004 and 2005 at Bald Head Island, Sapelo Island,
Blackbeard Island, Jekyll Island, and Cumberland Island, and are grouped based on
the geographic area to which they migrated after the nesting season: Mid-Atlantic
Bight (MAB), South Atlantic Bight (SAB), or Subtropical Northwest Atlantic
(SNWA). Additional females sampled (grey filled circles) in this study and females
sampled in the Reich et al. (2010) study (open circles) of unknown foraging location
are also shown.
70
Figure 3-4. Relationship between (A) carbon (13
C) and (B) nitrogen (15
N) stable isotope values
of adult female loggerheads (N = 22) and the latitude (°N) to which they migrated
after the nesting season. Spearman rank correlation is significant for both 13
C (rs = -
0.64, P = 0.002) and 15
N values (rs = 0.46, P = 0.029). Geographic regions in the
Northwest Atlantic: Mid-Atlantic Bight (MAB), South Atlantic Bight (SAB), and
Subtropical Northwest Atlantic (SNWA) are shown under the x-axis separated by
dotted lines. Dashed lines denote trend lines.
71
Figure 3-5. (A) Stable isotope ratios of carbon (13
C) and nitrogen (15
N) of adult loggerhead
turtles with known foraging location (N = 60) showing three groups representing the
three geographic areas used by adult loggerheads in the Northwest Atlantic: Mid-
Atlantic Bight (MAB) South Atlantic Bight (SAB), and Subtropical Northwest
Atlantic (SNWA).The isotopic values of turtles 1, 2, and 3 fell within a group that did
not correspond to the foraging location as observed through satellite telemetry.
Combined 13
C and 15
N values were significantly different among groups
(MANOVA, F = 29.62, P < 0.001). (B) Stable isotope ratios of carbon (13
C) and
nitrogen (15
N) of 375 adult female loggerhead turtles of unknown foraging ground.
Symbols (same as above) indicate the geographic area to which each individual
unknown female turtle was assigned by the discriminant analysis. Turtles assigned
with a probability ≥ 80% of group membership are shown as filled symbols (N =
340). Open symbols represent additional turtles assigned with a probability < 80% of
group membership (N = 35).
72
Figure 3-6. Breeding population structure according to foraging area used by loggerheads nesting
along the U.S. Atlantic coast as determined through discriminant analysis using
carbon and nitrogen stable isotope values of adult loggerhead turtles with known
foraging grounds as reference data. Foraging areas are Mid-Atlantic Bight (MAB;
white fill), South Atlantic Bight (SAB; grey fill), and the Subtropical Northwest
Atlantic (SNWA; black fill). Nesting turtles from Bald Head Island (BHI) and
Wassaw Island (WAS) were sampled in 2004-2005 and 2005 nesting seasons,
respectively. Florida nesting turtles were sampled in 2003 and 2004 nesting seasons;
results from nesting season 2003 are shown in the main map. Inset shows results
from 2004 for Canaveral National Seashore (CNS), Melbourne (MEL), and Juno
(JUN) beaches. BRO is Broward County beaches. The proportion of turtles using the
different geographic areas varied between years for CNS, MEL and JUN beaches (see
text for statistics). The boundaries of the three geographic areas: MAB, SAB, and
SNWA are depicted by dotted lines.
73
CHAPTER 4
LONG-TERM RESOURCE USE AND FORAGING SPECIALIZATION IN ADULT MALE
AND FEMALE LOGGERHEAD TURTLES
Background
Adult sea turtles migrate from distant foraging areas to breeding areas to mate and
reproduce. Females crawl up onto nesting beaches to lay eggs, but males remain in the marine
environment throughout their lives. As such, encountering male turtles for study is challenging.
Because female sea turtles are easily accessible on the nesting beaches, researchers have
gathered substantial information on their biology and ecology. For instance, studies revealed that
some aggregations of female turtles exhibit variations in foraging strategies (Hatase et al. 2002a;
Hawkes et al. 2006; Zbinden et al. 2011) and show long-term fidelity to their foraging areas
(Broderick et al. 2007; Marcovaldi et al. 2010; Seminoff et al. 2012; Tucker et al. 2014).
A few studies on loggerhead sea turtles, Caretta caretta, have addressed the foraging
behavior and migration patterns of male turtles, and found that male loggerheads appear to use
foraging strategies similar to those of female loggerheads. For instance, male loggerhead turtles
from a population in the North Pacific exhibit the body size related foraging strategy dichotomy,
in which larger individuals forage in coastal waters and smaller turtles use offshore oceanic
waters (Hatase et al. 2002b; Saito et al. 2015), that was previously reported for adult female
loggerheads (Hatase et al. 2002a). In the Northwest Atlantic (NWA), recent studies showed that
male loggerheads are almost exclusively confined to coastal waters (Arendt et al. 2012a,b), just
like adult female loggerheads (Hawkes et al. 2011; Ceriani et al. 2012; Foley et al. 2014; Griffin
et al. 2014), and use foraging areas similar to those of female loggerheads (Arendt et al.
2012a,b). Moreover, a recent study comparing stable isotope values between adult male and
female loggerheads in the NWA suggested that male and female loggerheads use not only similar
foraging areas, but also similar habitats and prey items (Pajuelo et al. 2012b). Thus, loggerhead
74
turtles may not exhibit intra-specific sex-based differences in foraging area use. Additionally, the
foraging site fidelity that was widely reported for females in sea turtle populations was first
reported for an aggregation of male loggerheads in the Mediterranean (Schofield et al. 2010). It
is not certain, however, whether all male loggerhead aggregations exhibit the same pattern of
foraging site fidelity.
Accurate parameter estimates are needed for models of population dynamics to predict
how sea turtles will respond not only to climatic changes, but also to conservation and
management strategies. Most data on sea turtle biology and ecology have focused on results at
the population level, but studies on a wide range of organisms revealed that individual
differences in resource use can strongly influence a population’s ecological and evolutionary
dynamics (see reviews by Bolnick et al. 2003 and Araújo et al. 2011).
Individual specialization in resource use within generalist populations has been widely
reported in natural populations (Bolnick et al. 2003), but only recently have studies quantified
the degree of individual specialization within natural populations (Araújo et al. 2011; Vander
Zanden et al. 2013a; Newsome et al. 2015; Rosenblatt et al. 2015). Among the factors that can
affect the degree of individual specialization are intra-specific competition (Svänback and
Bolnick 2007; Tinker et al. 2008), inter-specific competition (Bolnick et al. 2010), predation
(Peacor and Pfister 2006) and ecological opportunity, i.e. diversity of available resources (Araújo
et al. 2011). Even though ecological opportunity is one of the main conditions for individual
specialization, few studies have assessed how diversity of available resources affects the
magnitude of such specialization among individuals (Herrera et al. 2008; Darimont et al. 2009;
Rosenblatt et al. 2015).
75
Sea turtles record their chronological foraging histories in their scutes, the keratinized
inert tissue covering the carapace. Much like continually growing otoliths in fish (Rooker et al.
2008), baleen in whales (Schell et al. 1989) and the gladius in squids (Lorrain et al. 2012), sea
turtle scutes provide a sequential, long-term record because biomarkers in foraging areas are
incorporated and retained in the inert scute tissue (Vander Zanden et al. 2010). Nitrogen and
carbon stable isotope values, 15
N and 13
C, respectively, have been analyzed in serially sampled
scutes of NWA female loggerhead turtles and revealed that, although generalists at the
population level, nesting turtles exhibit individual specialization in resource use that is
maintained for up to a decade (Vander Zanden et al. 2010, 2013). It is not known whether male
loggerhead turtles exhibit a similar pattern of resource use.
In this study, I had four objectives. I serially sampled scutes of adult male loggerheads to:
1) investigate whether individual patterns of resource use (diet and habitat) within individual
male loggerhead turtles are maintained through time and 2) evaluate the degree of individual
foraging specialization among male loggerhead turtles. Most samples were collected at one
foraging area in South Carolina/Georgia, USA, but a few male and female loggerheads were
sampled at a foraging area in Florida Bay, Florida, USA. I report initial results from these
samples with respect to: 3) the effect of resource availability on the degree of individual
specialization between two loggerhead aggregations that use distinct foraging areas, and 4) inter-
sex differences in resource use in loggerhead turtles.
Methods
Data Collection
Scute samples were collected from 18 male and six female adult-size loggerheads
(straight carapace length, SCL>78 cm) at two foraging areas (Table 4-1). SCL was measured
from the anterior nuchal scute to the posterior notch (Bolten 1999). Samples were taken from the
76
posterior medial region of the third lateral scute, using a 6-mm biopsy punch. Sampling took
place during boreal summers of 2011 through 2013 off South Carolina and Georgia, USA
(SC/GA) in the South Atlantic Bight (SAB), and from March to June 2011 from Florida Bay,
Florida, USA (FLB) in the Subtropical Northwest Atlantic (SNWA) (Figure 4-1). The SAB and
SNWA are well-established biogeographic areas with distinct oceanographic and biological
characteristics (Wilkinson et al. 2009). The three males sampled at FLB in 2011 were also fitted
with satellite transmitters (Wildlife Computers SPLASH10) and their movements were tracked
from 136 to 790 days after release (A. Foley, B. Schroeder, B. Witherington unpubl. data). One
of these turtles had its satellite transmitter replaced in 2013 and was tracked for a total of 851
days. These three turtles remained within the western FLB waters throughout the duration of
tracking and only one made a short excursion to offshore waters.
Scute Preparation and Analysis
Prior to micro-sampling, all scutes were rinsed with deionized water and dried at 60°C
for 24 hr. Each scute sample was glued to a glass slide with the ventral side down and the dorsal
surface (oldest tissue) exposed, and was then micro-sampled in 50-m increments using a
carbide end mill. A previous study determined that each 50-m scute layer, the minimum
amount necessary for stable isotope analysis represents ~0.6 years of resource use in adult
loggerhead turtles (Vander Zanden et al. 2010). In cases for which single layers did not provide
enough material for stable isotope analysis, consecutive 50-m scute layers were combined.
Samples were analyzed for 15
N and 13
C by combustion in a Carlo Erba NA 1500 CNS
elemental analyzer interfaced via a ConFlo II device to a DeltaV Advantage isotope ratio mass
spectrometer in the Stable Isotope Geochemistry Lab at the University of Florida, Gainesville,
USA. Results are presented as stable isotope ratios of a sample relative to an international
77
standard and reported in the conventional notation: X = [(Rsample
/Rstandard
) –1] x 1000, where
X is the relative abundance of 13
C or 15
N in the sample expressed in parts per thousand (‰);
Rsample
and Rstandard
are the ratios of heavy to light isotope (13
C/12
C and 15
N/14
N) in the sample and
international standard, respectively. The standard used for 13
C was Vienna Pee Dee Belemnite
and for 15
N was atmospheric N2. Working standards, L-glutamic acid USGS40 (13
C = -26.39 ‰
and 15
N = -4.52 ‰), and L-glutamic acid USGS41 (13
C = 37.63 ‰ and 15
N = 47.57 ‰)
were used to calibrate results. In addition, a reference laboratory standard, homogenized
loggerhead scute (13
C = -18.36 ‰ and 15
N = 7.68 ‰), was used to examine consistency in
isotopic values in a sample similar to the samples used in this study. The analytical accuracy of
my measurements—calculated as the SD of replicates of standards—was 0.10 and 0.17 ‰ for
13
C and 15
N of L-glutamic acid USGS40 (N = 53), and 0.28 for 13
C and 0.23 for 15
N of L-
glutamic acid USGS41 (N = 10), respectively, and 0.17 and 0.29 ‰ for 13
C and 15
N of scute
standards (N = 27), respectively.
Data Analysis
The total niche width (TNW) of a population is determined by the sum of the within
individual component (WIC), which is the mean variability in resource use within individuals,
and the between individuals component (BIC), which is the variability of resource use among
individuals, such that TNW = WIC + BIC, and the ratio WIC/TNW is used as a measure of the
degree of individual specialization (Bolnick et al. 2003). Values close to 0 indicate that
individuals are specialists or use a narrow range of resources, and values close to 1 indicate that
individuals are generalists or use a wider range of resources (Bolnick et al. 2002). WIC has been
used as a proxy for temporal consistency, as it measures how variable an individual’s resource
use is over time (Matich et al. 2011; Vander Zanden et al. 2013). Thus, following the methods of
78
Matich et al. (2011) and Vander Zanden et al. (2013), I used the variance in 15
N and 13
C,
estimated using the ANOVA framework, to calculate the temporal consistency in resource use
and the degree of individual specialization among male loggerheads. The mean sum of squares
within individuals (MSW) was used as a proxy for WIC:
MSW =∑ ∑ (𝑥𝑖𝑗−𝑥𝑖)
2𝑗𝑖
(𝑁−𝑘)
The mean sum of squares between individuals (MSB) was used as a proxy for BIC:
MSB =∑ ∑ (𝑥𝑖−𝑥)2
𝑗𝑖
(𝑘−1)
where i represents an individual, j represents a single scute layer, N is the total number of
observations, and k is the number of individuals. The sum of MSW and MSB was used as a
proxy for TNW and the degree of specialization was calculated as WIC/TNW.
Body size difference between foraging areas was assessed using a two-tailed t-test, and a
Pearson correlation test was used to evaluate the correlation between thickness of scute and turtle
body size. Comparisons of WIC and WIC/TNW within and between foraging areas were
calculated through non-parametric bootstrapping with 1000 replications. All statistics were
conducted in R (R Development Core Team 2014) with an level of 0.05.
Results
Thickness of scutes ranged from 600 to 1450 m, representing approximately 7.2 to 17.4
years of foraging history (Figure 4-2). The longest records were obtained from scutes of turtles
using the foraging area in the SNWA. However, body size did not differ between turtles sampled
in SC/GA and FLB (t = -0.5431, df = 19.576, P = 0.5932) and scute thickness and body size
were not correlated among all turtles combined (Pearson’s r = 0.068, N = 24, P = 0.752) or
within foraging areas (SC/GA, rs = 0.178, N = 15, P = 0.526, and FLB, rs = -0.345, N = 9, P =
79
0.363) (Figure 4-3). I cannot rule out the possibility that scute growth may be affected by distinct
environmental conditions in the two foraging areas (López-Castro et al. 2014).
Temporal consistency within individuals (as represented by WIC) varied with foraging
area for 15
N, but not for 13
C (Figure 4-4a) for male loggerheads. SC/GA turtles were
significantly more consistent in resource use through time than FLB male turtles (15
N WIC, P <
0.01). Within the FLB foraging area, mean 15
N was significantly higher for males (P = 0.040),
but mean 13
C WIC was not significantly different between males and females (P = 0.346).
Because my male sample size was low in FLB, I combined FLB male and female turtles and
revealed that both mean 15
N and 13
C WIC in combined FLB turtles were significantly higher
(15
N WIC, P = 0.022 and 13
C, P = 0.033) than in SC/GA males, revealing an overall greater
consistency in resource use in SC/GA turtles.
WIC/TNW, representing the degree of individual specialization, had mean values close to
0 (mean WIC/TNW < 0.1, Figure 4-4b), indicating that male loggerheads at both foraging areas
exhibit a high degree of individual specialization. Mean WIC/TNW values were not significantly
different for 15
N (P = 0.193) or 13
C (P = 0.443) among SC/GA and FLB males. However, the
smaller isotopic mean range within individuals compared to that of the population (Table 4-1)
suggests that SC/GA turtles could be more individually specialized. Within a foraging area,
WIC/TNW values were not significantly different between FLB males and females for 15
N (P =
0.649) or 13
C (P = 0.863). Thus, combined FLB male and female turtles exhibited a
significantly lower WIC/TNW for 13
C (P < 0.05) than SC/GA turtles, while WIC/TNW values
were significantly lower for 15
N (P < 0.05) in SC/GA loggerheads than in FLB turtles.
80
Discussion
Temporal Consistency in Resource Use
I found a similar pattern of long-term consistency in resource use in male loggerheads
that has been previously observed in adult female loggerheads from a nesting beach in Florida,
USA (Vander Zanden et al. 2010) and that has been quantified in adult female green turtles,
Chelonia mydas, from a nesting beach in Costa Rica (Vander Zanden et al. 2013a). Unlike the
two previous studies, which sampled turtles at nesting beaches, I sampled adult turtles at their
foraging areas. Thus, I assume that variations in 15
N and 13
C represent dietary and habitat
variations within the foraging area.
Male loggerhead turtles were highly consistent in the use of resources through time in
SC/GA, but not in FLB, in particular with respect to 15
N. Even though my male sample size in
FLB was small, females sampled in this foraging area also exhibited lower temporal consistency
than SC/GA male loggerheads. Temporal consistency in diet and habitat use in SC/GA males is
similar to that of nesting green turtles (WIC < 0.5, Vander Zanden et al. 2013a). FLB males (and
females) were not very consistent in resource use, so that 15
N WIC (>1.0) was higher than that
of oceanic juvenile green turtles (Vander Zanden et al. 2013a), which most likely feed
opportunistically in oceanic waters (Bolten 2003), and were expected to show lower temporal
consistency than adult green turtles (Vander Zanden et al. 2013a). However, comparisons with
other turtle populations, without knowledge of how isotopic values vary at the foraging area,
should be made with caution, because organisms may forage on similar prey items and use
similar habitats, but still exhibit different isotopic values (Cummings et al. 2012). A study that
used both bulk and compound-specific stable isotope analyses showed that the lower SC/GA
loggerhead isotopic variation is caused mainly by base of the food web variation, whereas the
81
greater FLB loggerhead isotopic variation is driven by both baseline variation and dietary
variation among loggerheads (M. Pajuelo et al. unpubl. data).
Loggerhead turtles using waters of the NWA have been shown to exhibit geographic
variation in 15
N and 13
C (Pajuelo et al. 2012b; Ceriani et al. 2014) that can be traced to isotopic
differences at the base of the food web (Pajuelo et al. 2012a). SC/GA male loggerheads exhibited
long-term consistency in 15
N and 13
C, with values in agreement with those found within this
foraging area (Pajuelo et al. 2012a, b). Even though FLB males exhibited a high degree of
isotopic variation within individuals, the overall isotopic values found within (and between)
individuals corresponds to isotopic values expected in turtles using this foraging area (Pajuelo et
al. 2012b; Ceriani et al. 2014; Vander Zanden et al. 2015). These concordant stable isotope
values and the satellite tracks of male loggerheads, which revealed a high fidelity to the west side
of the FLB, indicate that male loggerheads exhibit high foraging-site fidelity.
Foraging site fidelity was also revealed in another aggregation of male loggerheads in the
Mediterranean, using long-term satellite telemetry data (Schofield et al. 2010). Thus, foraging
site fidelity may be characteristic of male loggerhead turtles in general. Consistent use of a
known foraging ground that provides sufficient resources is considered a more beneficial
strategy than wandering through unexplored new areas (Schofield et al. 2010). However, this
behavior can prove detrimental if foraging areas become heavily impacted by anthropogenic
activities. A recent study showed that loggerhead turtles in the Gulf of Mexico continued to
forage near the site of the disastrous 2010 oil spill, thus risking exposure to the effects of oil and
chemical dispersants (Vander Zanden et al. submitted).
The low isotopic variation within SC/GA male loggerheads indicates that turtles may be
consistently feeding on similar prey items or on prey items with similar isotopic composition.
82
Stomach content analysis in loggerheads at this foraging area has revealed that turtles rely on
prey items with similar trophic level such as crabs and that some turtles were selectively
consuming particular prey items (Youngkin 2001). Only one SC/GA male loggerhead showed a
large decrease in both 15
N and 13
C values, followed by a return to isotopic values similar to
those before the decrease (Figure 4-2). These values, however, were within the isotopic values
expected for turtles within this foraging area. Such great changes in both 15
N and 13
C values
suggest that the turtle moved to another area within its foraging area where both isotope values
were different at the base of the food web. It could also indicate that when the turtle moved to a
new habitat (13
C), it utilized prey items of lower trophic position, which would have lowered
the 15
N values.
FLB is located in the SNWA, whose warm waters are characterized by great biotic and
habitat diversity. Benthic invertebrate diversity along the US east coast is greater in the FLB
(Roy et al. 1998) as are the available habitats such as seagrass beds, mangroves, and coral reefs
(Wilkinson et al. 2009). And because isotopic variations in FLB loggerheads are also linked to
dietary variations (M. Pajuelo et al. unpubl. data), the larger 15
N variation observed within male
loggerheads using FLB waters suggests that these turtles exploit a variety of resources with
isotopic values that are different from those of turtles using SC/GA waters. However, visual
inspection of Figure 4-2a reveals shorter intervals with less isotopic variation within FLB turtles.
Thus, FLB turtles appear to be consistent in the use of resources for shorter periods of time.
Although there was great variation in 15
N values of FLB males, there was not a
corresponding variation in 13
C. Consistency in 13
C values was similar for male loggerheads at
both foraging areas. However, when combining FLB male and females (justified because WIC
values for both 15
N and 13
C are not significantly different) and comparing them to SC/GA
83
males, I find that SC/GA turtles are also more consistent in 13
C. This is not surprising, as carbon
sources (e.g., seagrass, macroalgae, and phytoplankton) in the FLB have wide-ranging 13
C
values, from -18‰ to -6‰ (Behringer and Butler 2006), which suggests that FLB turtles may
consume prey items that use different carbon sources. Overall, loggerhead turtles show 13
C
values over time that are strikingly more consistent than their 15
N values. This indicates that
turtles are consistently using the same foraging habitat (reflected by 13
C), while the amount of
dietary variation (reflected by 15
N) may depend on the availability of resources within a
particular foraging area.
Individual Specialization
Exhibiting temporal consistency in resource use alone does not necessarily indicate the
degree of individual foraging specialization in a population, as the latter will depend upon how
individuals are partitioning the range of available resources (Bolnick 2003). However, it is
essential to obtain information on individual resource use through time to understand whether the
pattern of resource use (i.e., individualization or generalization) is maintained over time.
In this study, SC/GA male loggerheads exhibited both consistency in resource use
through time and a high degree of individual specialization, similar to those observed in nesting
loggerheads (Vander Zanden et al. 2010) and green turtles (Vander Zanden et al. 2013a). The
isotopic range within male turtles is smaller than the total isotopic range found across all
individuals at each foraging area, revealing that male turtles are part of a generalist population
with specialized individuals. However, Vander Zanden et al. (2010, 2013a) reported on
individual specialization in a nesting population, which was composed of individuals that used
different foraging areas. Because geographic area can account for the large isotopic variation
found in nesting sea turtles (Pajuelo et al. 2012a, b; Vander Zanden et al. 2013b), I cannot
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compare the degree of individual specialization found in nesting turtles to that of turtles from one
foraging ground.
The first observations of dietary specialization in long-assumed opportunistic loggerhead
turtles, were reported by Ruckdeschel and Shoop (1998). They analyzed stomach contents of
hundreds of loggerhead turtles and showed that some individuals selectively consumed the same
prey types. However, the dietary information obtained from stomach contents reveals what an
organism consumed within the past few days, not whether the organism’s dietary preferences are
maintained through time. Even though I do not have information on the specific prey items or
groups of prey items the SC/GA male loggerheads consume in this foraging area, the isotopic
evidence reveals that males forage consistently on the same prey items or groups of prey items
with similar isotopic values over a long period of time. This also suggests that availability of
resources in SC/GA waters remained consistent or that environmental variability that affects
resource abundance was low, so that turtles could maintain individualized foraging behaviors for
long periods of time.
Male individuals in SC/GA waters appeared to be more specialized in 15
N than FLB
loggerheads males (both males and females), whereas FLB turtles were more specialized in 13
C,
suggesting that foraging area may also affect the magnitude of specialization exhibited by
individuals. The lower individual specialization for 15
N found among FLB turtles could be a
consequence of the larger variation in individual resource use over time and that this variation in
resource use, for some individuals, almost encompasses all available resources in the foraging
area. Although FLB individuals are more variable in their use of 13
C than SC/GA turtles, FLB
turtles still appear to be using a narrower range of carbon sources than what is available. Thus,
while a wider range of resources (as reflected by the wider isotopic variation in both 13
C and
85
15
N) is available for FLB turtles, they appear to be more specialized in their habitat use
(reflected by 13
C) than in their dietary preferences (reflected by 15
N). However, some FLB
individuals appear to specialize their diet and habitat use for shorter periods of time, as
evidenced by shorter intervals with less isotopic variation within FLB turtles. Thus, even though
individual loggerheads in general do specialize their diet and habitat use, this behavior is not
consistent in the long term for turtles in FLB waters. These results differ from those of previous
studies in which an increase in resource diversity increased individual specialization. Darimont
et al. (2009) found that insular populations of wolves (Canis lupus) had higher among-individual
dietary variation than wolves inhabiting mainland areas. Island wolves are apparently exposed to
more diverse food sources and habitats (terrestrial and marine) than mainland wolves, which rely
on terrestrial prey items in a more homogeneous habitat (Darimont et al. 2009). Similarly,
Rosenblatt et al. (2015) revealed that American alligators (Alligator mississippiensis) exhibited
higher individual specialization in coastal areas, where alligators had access to a wider variety of
aquatic habitats and prey items, than in freshwater lakes. Common in both studies was the
expansion of the population niche width (TNW), a consequence of the increase in available
resources, which affects individual specialization (WIC/TNW) (Bolnick et al. 2003). Population
niche width also increased with resource diversity in loggerhead turtles (this study). However,
unlike the above studies that based results on short-term data, analysis of isotope values in turtle
scutes (an archival tissue) enabled me to examine the degree of individual foraging specialization
over a longer period of time (~7 to 17 years).
Why are FLB loggerhead turtles less consistent in their foraging behavior over a long
period of time? FLB is a diverse and highly dynamic ecosystem (Fourqurean and Robblee 1999).
86
FLB has experienced dramatic ecological changes since the late 1980s, including seagrass die-
offs and phytoplankton and cyanobacteria blooms, which have had repercussions on the diversity
and abundance of food web organisms such as fish, crustaceans and sponges (Fourqurean and
Robblee 1999; Matheson Jr. et al. 1999; Peterson et al. 2006). Given that loggerhead turtles are
considered generalist carnivores (Hopkins-Murphy et al. 2003), I hypothesize that changes in
abundance of benthic invertebrates in the FLB may have led loggerhead turtles to diverge from
their preferred diet items. Preliminary data from FLB loggerhead gut contents and feces reveal
that the turtles often rely on sponges (B. Stacy pers. comm. and B. Witherington unpubl. data).
Large sponge die-offs within the FLB have been attributed to persistent cyanobacteria blooms
(Peterson et al. 2006). Thus, it is likely that those turtles that rely on sponges may switch to other
available prey items when sponges are not available. A similar diet switch was found in a coastal
loggerhead population, where a long-term study of stomach contents revealed that the diet of the
turtles shifted from benthic invertebrates to fish over the course of 20 years because of declines
in commercially important invertebrate prey (blue crabs, Callinectes sapidus, and horseshoe
crabs, Limulus polyphemus, Seney and Musick 2007). Thus, in the short-term, FLB turtles may
exhibit a preference for a particular prey type, but their diet may change over a long period of
time (more than a decade) because of environmental change.
Intra- and inter-specific competition may also influence foraging decisions of individuals
over time. Experimental and observational studies have found that high intra-specific
competition leads to increased individual specialization (Araújo et al. 2011), whereas inter-
specific competition weakens individual specialization (Bolnick et al. 2010). Data on loggerhead
population density, prey abundance and possible competitors needed to assess intra-specific and
87
inter-specific competition, are not available for each foraging area and thus, I cannot address
these topics at the moment.
My initial results showed a sex-based difference in temporal consistency of resource use,
but not in the degree of individual specialization among FLB loggerhead turtles. These results
suggest that individual female loggerhead diets are more restricted than those of males. Male and
female individuals within a population face different evolutionary and energetic pressures, in
particular related to reproduction (Elliot Smith et al. 2015). Thus, sex-based differences in
resource use are not uncommon. Because my FLB male sample size was small, the differences in
individual specialization between sexes should be interpreted with caution. More studies to
replicate the extensive work conducted on female sea turtle movement patterns and feeding
behavior will allow us to evaluate any sex-based differences in resource use.
Conclusions
My study shows that SC/GA male loggerheads exhibit consistency in resource use over
time and a high degree of specialization of resource use similar to that reported for female
loggerhead and green turtles. The long-term consistency in resource use found among SC/GA
male loggerheads revealed that, similar to female sea turtles, males also exhibit fidelity to their
foraging areas. This long-term consistency also suggests that resource abundance may have been
consistent in waters of the SC/GA, leading to a consistency in male loggerhead foraging
behavior. The initial results reported here on population-level variation in temporal consistency
and individual specialization in resource use among loggerhead turtles that use distinct foraging
areas, provide insights into how resource diversity and abundance may affect the foraging
behavior of loggerheads. Moreover, initial results showed sex-based differences in individual
specialization, suggesting that males may exploit a greater range of resources than females.
Future research should examine this pattern with larger sample sizes and in other populations to
88
test the validity of my results. As higher trophic level organisms, loggerhead sea turtles play an
important role in their ecosystems. Understanding how sea turtles utilize resources will allow us
to predict how they will respond and whether they will be able to adapt to changing climate and
environmental conditions.
89
Table 4-1. Mean and range of carapace length for individual loggerhead turtles, and mean and
total range of 15N and 13C values of loggerhead turtle scute tissue sampled at two
foraging areas, South Carolina/Georgia (SC/GA) and Florida Bay (FLB). For isotope
values, mean is the mean range of isotopic values within individual turtles and total is
the range of isotopic values across all individuals. Foraging area Sex N Layers SCL mean, range (cm)
15N mean, total (‰)
13C mean, total (‰)
SC/GA M 15 6 – 21 83.5, 10.9 1.55, 4.59 1.47, 4.08
FLB M 3 12 – 22 84.2, 2.4 3.40, 6.85 3.06, 4.69
F 6 18 – 29 84.4, 9.0 3.72, 6.42 2.42, 6.01
N is number of individuals, Layers indicate the range of scute samples analyzed per turtle at each foraging area and
per sex, and SCL is straight carapace length.
90
Figure 4-1. Foraging locations, South Carolina/Georgia and Florida Bay, where male
loggerheads turtles were sampled for scute tissue. Foraging areas are found within
distinct biogeographic areas (delimited with dotted lines): South Atlantic Bight (SAB)
and Subtropical Northwest Atlantic (SNWA).
91
Figure 4-2. Resource use of individual male loggerhead turtles (N = 18) as indicated by (a) 15
N
and (b) 13
C values of successive scute layers. Foraging areas are South
Carolina/Georgia (SC/GA) and Florida Bay (FLB). Arrow indicates the FLB male
loggerhead that made a short excursion to oceanic waters. Resource use of individual
female loggerheads (N = 6, dashed lines) is also included for comparison in the FLB.
92
Figure 4-3. Thickness of scute in relation to straight carapace length in loggerhead turtles from
South Carolina/Georgia (open circles) and Florida Bay (filled circles).
93
Figure 4-4. Comparison of (a) temporal consistency (WIC) and (b) degree of individual
specialization (WIC/TNW) between loggerhead turtles from two foraging areas,
South Carolina/Georgia (SC/GA) and Florida Bay (FLB). Circles represent mean and
error bars represent the standard error. Results for SC/GA are for male turtles only
while both male and females are shown for FLB. Groups that do not share letters in
common are significantly different ( = 0.05).
94
CHAPTER 5
SUMMARY AND FUTURE RESEARCH
Summary
The aim of this study was to further the understanding of the foraging ecology of
loggerhead turtles in the Northwest Atlantic using stable isotope analysis. Results show that on a
broad scale, highly elusive male loggerhead turtles exhibit similar foraging strategies to those of
widely studied female loggerheads, as male stable isotope values indicate they feed on similar
prey items and use similar foraging areas as females (Chapter 2). At a more regional scale,
results show that male loggerheads also exhibit individual foraging specialization that is
maintained for up to 17 years (Chapter 4). Consistency in temporal resource use, as reflected in
stable isotope values, suggests that male loggerheads consistently forage on similar prey items or
groups of prey items and use the same foraging habitats through time. These results reaffirm that
although loggerhead turtles are generalists at the population level, individual turtles partition the
resources available at their foraging areas and become specialists at the individual level. Previous
observations from stomach content analysis showed that some loggerhead turtles selectively
consumed particular prey or groups of prey items (Ruckdeschel and Shoop 1998). However, by
using scute samples (an archival tissue) from male loggerheads, it was possible to obtain both
dietary and location information over many years, which shows that male turtles also exhibit
specialized foraging behavior, previously reported only in female sea turtles.
Initial results of the effect of resource availability on the degree of individual
specialization among loggerhead aggregations suggest that loggerheads that use areas with a
greater diversity of resources may exhibit a lower degree of individual resource specialization
(Chapter 4). This is not in agreement with recent findings that high diversity of resources
increases the degree of individualization (Rosenblatt et al. 2015). I suggest that the degree of
95
individual specialization is context-dependent. If a foraging area with high diversity of resources
experiences changing environmental conditions, then individual turtles will be able to specialize,
but for shorter periods of time compared to turtles using more stable, yet less diverse
environments. Limited environmental change in a foraging area will allow individuals to
specialize on reliable food sources over long periods of time.
By combining stable isotope data with satellite telemetry data, I was able to validate the
use of stable isotope data alone to assign loggerhead turtles to their foraging areas in the NWA
(Chapter 3). I successfully used stable isotope data to identify foraging areas of loggerheads,
because the different areas studied were isotopically distinct. The foraging areas used by
loggerheads in the NWA are found within three major biogeographic regions, whose distinct
biogeochemical processes set the characteristic isotopic values found in particulate organic
matter (a proxy for primary producers), which were also observed in loggerhead turtles. Previous
work reported a wide range of isotopic values in nesting loggerhead turtles, but researchers were
not able to distinguish between location or diet as the cause of the wide range in isotopic values
(Reich et al. 2008; Vander Zanden et al. 2010). This study shows that the large isotopic variation
found among loggerheads is mainly driven by location differences.
The stable isotope technique developed in this study allowed the assignment of more than
300 nesting turtles from five nesting beaches along the U.S. east coast to their foraging areas,
and found that nesting loggerhead turtles tend to use foraging areas closer to their natal nesting
beaches. Additionally, initial findings revealed temporal variation in the contribution of foraging
areas to various nesting beach populations, suggesting differences in remigration intervals among
turtles with different foraging area origins. A later study that expanded on these results found
similar results in a northern loggerhead nesting aggregation, in which the proportion of turtles
96
originating from different foraging areas varied over seven years, indicating that the recent
increase in nesting population numbers had been driven by an increased contribution of turtles
from one foraging area in particular (Vander Zanden et al. 2014). Thus, the technique developed
in the present study (Chapter 3) allowed for the monitoring of the inter-annual variability in
nesting population abundance (Vander Zanden et al. 2014).
In summary, this study expanded our knowledge of loggerhead foraging strategies and
demonstrated that 13
C and 15
N of loggerhead turtles are effective biochemical tags to link
loggerhead foraging grounds and breeding areas.
Significance and Implications
Findings of this study have several implications for the use of stable isotopes in marine
environments and for the conservation and management of loggerhead turtles. First, I reaffirmed
the importance of knowledge of baseline isotopic variation when comparing the resource use of
populations using distant foraging areas. Thus, future studies should take into account the
isotopic values at the base of the food web when interpreting stable isotope data. The use of
compound-specific stable isotope analysis of amino acids (CSIA-AA), which has expanded in
recent years, allows effective differentiation between trophic/dietary effects and baseline isotopic
shifts (Popp et al. 2007). Thus, CSIA-AA should be incorporated in sea turtle research using
stable isotopes for dietary evaluation and to identify baseline isotopic values.
The wide range of foraging areas that the loggerhead turtles use in the NWA exposes
them to a variety of distinct environmental conditions and anthropogenic threats, which have
been proposed as drivers of loggerhead nesting population numbers (Witherington et al. 2009;
Arendt et al. 2013). Thus, identifying major loggerhead foraging areas is key to understanding
spatial and temporal fluctuations in nesting population numbers. This study relied on both
97
satellite telemetry and stable isotopes to link regional foraging grounds to breeding areas, which
will enable assessment of how anthropogenic threats such as fisheries interactions affect nesting
population numbers. This approach can also be useful to determine the origin of stranded turtles,
as it can reveal whether turtles were residents or migrants to the area where they were stranded,
and assess unusual stranding/mortality events.
The high degree of individual foraging specialization found in both male and female
loggerheads reveals that loggerheads in general are able to partition a wide range of available
resources. This foraging specialization can be maintained for many years, but can also change,
possibly in response to unfavorable environmental conditions that affect prey
distribution/abundance. If indeed loggerhead populations are able to exploit resources, even
under changing environmental conditions, it could imply that they will have greater potential to
respond to climate change.
The long-term consistency in resource use indicated that male loggerheads also exhibit
site fidelity to their foraging areas. This behavior can prove harmful for loggerhead populations
that are exposed to anthropogenic threats. For instance, Vander Zanden et al. (submitted)
revealed that loggerheads in the Gulf of Mexico continued to use areas that were impacted by the
2010 oil spill and were exposed to harmful oil contaminants and chemical dispersants used in
cleanup efforts.
Future Research
The stable isotope approach developed in this study allows the identification of foraging
areas at the regional level in the NWA, as turtles using the same biogeographic area shared
similar isotopic values. However, some turtles that use waters in the Gulf of Mexico had isotopic
values similar to turtles in one area of the NWA. The use of stable isotope values, in combination
with other biochemical tags, could allow for finer-scale identification of loggerhead foraging
98
areas in the NWA and help differentiate geographically separated areas with similar isotopic
values. Trace elements, in combination with stable isotopes, have been used successfully to
determine the origin of fish to estuaries along the US east coast (Anstead et al. 2015). Trace
elements have been analyzed in scute of sea turtles to link the contribution of oceanic habitats to
neritic foraging grounds (López-Castro et al. 2013) at a very large scale. Future studies should
implement this approach in the NWA and Gulf of Mexico, where the presence of estuaries that
are known to influence coastal waters should present distinct trace element ratios that can be
reflected in the loggerheads scute tissue. A finer-scale resolution in the identification of sea turtle
foraging areas could allow managers to focus efforts on areas of high conservation value. Thus,
the stable isotope approach implemented in this study is valuable, because it allows identification
of the foraging area of large number of turtles at a low cost, but it can still be refined.
Another approach to improving the assignment of sea turtles to their foraging areas was
validated by Vander Zanden et al. (2015). A continuous surface of stable isotope values was
created to assign the foraging origin of nesting loggerheads in the Gulf of Mexico. Because
loggerheads from multiple areas in the Gulf of Mexico had similar isotopic values, it was
difficult to assign turtles to their foraging areas accurately. However, the advantages of this
approach are that all individuals can be assigned to the continuous surface and individual results
can be aggregated to identify hotspots of foraging area use (Vander Zanden et al. 2015). This
approach can be implemented in the NWA to identify the foraging areas of nesting populations
that are known to use the NWA almost exclusively.
Because of the opportunistic nature of loggerhead turtles that can specialize on a wide
range of resources, as I have reaffirmed in this study, I caution against the use of bulk stable
isotope analyses alone to address trophic structure questions. CSIA-AA has proven useful to
99
obtain information from both the base of the food web and the trophic level of the consumer.
However, this approach has still not been validated in sea turtles. Only relative trophic level
estimations have been obtained for sea turtles because discrimination factors have not been
obtained for any sea turtle species, which prevents trophic comparisons between sea turtles and
other species using the same foraging grounds.
Initial results evaluating the sex-based differences in loggerhead turtles suggest that
female loggerheads are more restricted in resource use than are males. Further studies with larger
sample sizes will allow me to evaluate any sex-based differences in resource use. Future studies
should address how varying degrees of individual variation in resource use within females can
have demographic consequences on the population.
The recent increase in loggerhead numbers poses new challenges. I do not know what the
effect of increased numbers of loggerheads will be on their ecosystems. What will the effect be
on the relationship between loggerheads and ecologically similar species? For instance, in the
NWA, Kemp’s ridleys, Lepidochelys kempii, are found within the same coastal foraging areas as
loggerheads and their diet may include similar prey items as those of loggerhead turtles (Seney
2002; Witzell and Schmid 2005). However, the degree of ecological interaction between these
two sea turtle species has not yet been assessed. Future studies should evaluate how these two
species partition resources to further our understanding of how these two species coexist in the
same environment.
100
APPENDIX
EPIDERMIS AND PLASMA ISOTOPIC VALUES
Figure A-1. Stable isotope ratios (13
C and 15
N) of epidermis (EPI; N = 26; a) and plasma
(PLA; N = 37; b) samples of adult male loggerheads in Florida. Samples were
collected during mating season at Cape Canaveral, FL. EPI and PLA values show the
same pattern as the one observed in red blood cells (see Figure 2-1b). Labels indicate
the geographic location to which satellite-tracked males migrated after the mating
season. Unknown turtles are males without transmitters and satellite-tracked males
for which foraging area could not be determined.
101
LIST OF REFERENCES
Alava JJ, Keller JM, Wyneken J, Crowder L, Scott G, Kucklick JR (2011) Geographical
variation of persistent organic pollutants in eggs of threatened loggerhead sea turtles
(Caretta caretta) from southeastern United States. Environ Toxicol Chem 30:1677–1688
Anstead KA, Schaffler JJ, Jones CM (2015) Coastwide Otolith Signatures of Juvenile Atlantic
Menhaden, 2009–2011. T Am Fish Soc 144:96-106
Araújo M, Bolnick DI, Layman CA (2011) The ecological causes of individual specialisation.
Ecol Lett 14:948–958
Arendt MD, Schwenter JA, Witherington BE, Meylan AB, Saba VS (2013) Historical versus
Contemporary Climate Forcing on the Annual Nesting Variability of Loggerhead Sea
Turtles in the Northwest Atlantic Ocean. PLoS ONE 8(12): e81097.
doi:10.1371/journal.pone.0081097
Arendt MD, Segars AL, Byrd JI, Boynton JI, Schwenter JA, Whitaker JD, Parker L (2012b)
Migration, distribution, and diving behavior of adult male loggerhead sea turtles (Caretta
caretta) following dispersal from a major breeding aggregation in the Western North
Atlantic. Mar Biol 159:113–125
Arendt MD, Segars AL, Byrd JI, Boynton JI, Whitaker JD, Parker L, Owens DW, Blanvillain G,
Quattro JM, Roberts MA (2012a) Distributional patterns of adult male loggerhead sea
turtles (Caretta caretta) in the vicinity of Cape Canaveral, Florida during and after a
major annual breeding aggregation. Mar Biol 159:101–112
Avens L, Goshe LR, Pajuelo M, Bjorndal KA et al. (2013) Complementary skeletochronology
and stable isotope analyses offer new insight into juvenile loggerhead sea turtle oceanic
stage duration and growth dynamics. Mar Ecol Prog Ser 491:235-251
Bailey H, Fossette S, Bograd SJ, Shillinger GL, Swithenbank AM et al. (2012) Movement
patterns for a critically endangered species, the leatherback turtle (Dermochelys
coriacea), linked to foraging success and population status. PLoS ONE 7:e36401
Barile PJ (2004) Evidence of anthropogenic nitrogen enrichment of the littoral waters of east
central Florida. J Coast Res 20:1237–1245
Behringer DC, Butler MJ (2006) Stable isotope analysis of production and trophic relationships
in a tropical marine hard-bottom community. Oecologia 148:334–341
Bell R, Richardson JI (1978) An analysis of tag recoveries from loggerhead sea turtles (Caretta
caretta) nesting on Little Cumberland Island, Georgia. Florida Mar Res Pub 33:1–66
Bjorndal, K.A. 2003. Roles of loggerhead sea turtles in marine ecosystems. In: Bolten AB,
Witherington BE (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington, DC,
pp 235-254
102
Blanvillain G, Pease AP, Segars AL, Rostal DC, Richards AJ, Owens DW (2008) Comparing
methods for the assessment of reproductive activity in adult male loggerhead sea turtles
Caretta caretta at Cape Canaveral, Florida. Endang Species Res 6:75–85
Block BA, Dewar H, Blackwell SB, Williams TD, Prince ED, Farwell CJ, Boustany A, Teo
SLH, Seitz A, Walli A, Fudge D (2001) Migratory movements, depth preferences, and
thermal biology of Atlantic bluefin tuna. Science 293:1310–1314
Block BA, Teo SLH, Walli A, Boustany A, Stokesbury MJW, Farwell CJ, Weng KC, Dewar H,
Williams TD (2005) Electronic tagging and population structure of Atlantic bluefin tuna.
Nature 434:1121–1127
Bolker BM, Okuyama T, Bjorndal KA, Bolten AB (2007) Incorporating multiple mixed stocks in
mixed stock analysis: ‘many-to-many’ analyses. Molecular Ecology 16:685–695.
Doi:10.1111/J.1365-294x.2006.03161.X
Bolnick D, Yang LH, Fordyce JA, Davis JM, Svanback R (2002) Measuring individual-level
resource specialization. Ecology 83:2936–2941
Bolnick DI, Svanbäck R, Fordyce JA, Yang LH, Davis JM, Hulsey CD, Forister ML (2003) The
ecology of individuals: incidence and implications of individual specialization. Am Nat
161:1–28. doi:10.1086/343878
Bolnick D, Ingram T, Stutz WE, Snowberg LK, Lau OL, Paull JS (2010) Ecological release from
interspecific competition leads to decoupled changes in population and individual niche
width. Proc R Soc Lond B 277:1789–1797
Bolten AB, Crowder LB, Dodd MG, MacPherson SL, Musick JA, Schroeder BA, Witherington
BE, Long KJ, Snover ML (2011) Quantifying multiple threats to endangered species: An
example from loggerhead sea turtles. Front Ecol Environ 9:295–301
Bolten AB (2003) Active swimmers–passive drifters: the oceanic juvenile stage of loggerheads
in the Atlantic system. In: Bolten AB, Witherington BE (eds), Loggerhead Sea Turtles.
Smithsonian Books, Washington, DC, pp 63–78
Bolten AB (1999) Techniques for measuring sea turtles. In: Eckert KL, Bjorndal KA, Abreu-
Grobois A, Donnelly M (eds) Research and management techniques for the conservation
of sea turtles. IUCN Marine Turtle Specialist Group, pp 110–114
Bowen BW et al. (2004) Natal homing in juvenile loggerhead turtles (Caretta caretta). Mol Ecol
13:3797–3808
Broderick AC, Coyne MC, Fuller WJ, Glen F, Godley BJ (2007) Fidelity and overwintering of
sea turtles. Proc Biol Sci 274:1533−1538
Buchheister A, Latour RJ (2011) Trophic Ecology of Summer Flounder in Lower Chesapeake
Bay Inferred from Stomach Content and Stable Isotope Analyses. T Am Fish Soc
140:1240–1254
103
Burton R, Koch P (1999) Isotopic tracking of foraging and long-distance migration in
northeastern Pacific pinnipeds. Oecologia 119:578–585
Carleton SA, Martínez del Rio C (2005) The effect of cold-induced increased metabolic rate on
the rate of 13
C and 15
N incorporation in house sparrows (Passer domesticus). Oecologia
144:226–232
Caut S, Fossette S, Guirlet E, Angulo E, Das K, Girondot M, Georges YG (2008) Isotope
analysis reveals foraging area dichotomy for Atlantic leatherback turtles. PLoS ONE
3:e1845
Ceriani SA, Roth JD, Evans DR, Weishampel JF, Ehrhart LM (2012) Inferring foraging areas of
nesting loggerhead turtles using satellite telemetry and stable isotopes. PLoS ONE
7:e45335
Ceriani SA, Roth JD, Sasso CR, McClellan CM et al (2014) Modeling and mapping isotopic
patterns in the Northwest Atlantic derived from loggerhead sea turtles. Ecosphere
5(9):122. http://dx.doi.org/10.1890/ ES14-00230.1
Chaloupka M, Kamezaki N, Limpus C(2008) Is climate change affecting the population
dynamics of the endangered Pacific loggerhead sea turtle? J Exp Mar Biol Ecol 356:136–
143
Cherel Y, Hobson KA (2007) Geographical variation in carbon stable isotope signatures of
marine predators: a tool to investigate their foraging areas in the Southern Ocean. Mar
Ecol-Prog Ser 329:281–287
Cooch E, Rockwell RF, Brault S (2001) Retrospective analysis of demographic responses to
environmental change: A Lesser Snow Goose example. Ecol Monograph 71:377–400
Cummings DO, Buhl J, Lee RW, Simpson SJ, Holmes SP (2012) Estimating Niche Width Using
Stable Isotopes in the Face of Habitat Variability: A Modelling Case Study in the Marine
Environment. PLoS ONE 7(8): e40539.
Darimont CT, Paquet PC, Reimchen TE (2009) Landscape heterogeneity and marine subsidy
generate extensive intrapopulation niche diversity in a large terrestrial vertebrate. J Anim
Ecol 78: 126–133
DeNiro MJ, Epstein S (1978) Influence of diet on distribution of carbon isotopes in animals.
Geochim Cosmochim 42:495–506
Dodd CK Jr, Byles R (2003) Post-nesting movements and behavior of loggerhead sea turtles
(Caretta caretta) departing from east-central Florida nesting beaches. Chelonian Conserv
Biol 4:530–536
104
Ehrhart LM, Bagley DA, Redfoot WE (2003) Loggerhead turtles in the Atlantic Ocean:
Geographic distribution, abundance, and population status. In: Bolten AB, Witherington
BE (eds), Loggerhead sea turtles. Smithsonian Institution Press, Washington, DC, pp
157–174
Elliot Smith EA, Newsome SD, Estes JA, Tinket MT (2015) The cost of reproduction:
differential resource specialization in female and male California sea otters. Oecologia
DOI 10.1007/s00442-014-3206-1
Encalada SE, Bjorndal KA, Bolten AB, Zurita JC, Schroeder B, Possardt E, Sears CJ, Bowen
BW (1998) Population structure of loggerhead turtle (Caretta caretta) nesting colonies in
the Atlantic and Mediterranean regions as inferred from mtDNA control region
sequences. Mar Biol 130:567–575
Esler D (2000) Applying metapopulation theory to conservation of migratory birds. Conserv Biol
14:366–372
Fantle MS, Dittel AI, Schwalm SM, Epifanio CE, Fogel ML (1999) A food web analysis of the
juvenile blue crab, Callinectes sapidus, using stable isotopes in whole animals and
individual amino acids. Oecologia 120:416–426
Fennel K, Wilkin J, Levin J, Moisan J, O’Reilly J, Haidvogel D (2006) Nitrogen cycling in the
Middle Atlantic Bight: Results from a three-dimensional model and implications for the
North Atlantic nitrogen budget. Global Biogeochem Cyc 20: -
doi:10.1029/2005gb002456
Finkbeiner EM, Wallace BP, Moore JE, Lewison RL, Crowder LB, Read AJ (2011) Cumulative
estimates of sea turtle bycatch and mortality in USA fisheries between 1990 and 2007.
Biol Conserv 144:2719–2727
Foley AM, Schroeder BA, Hardy R, MacPherson SL, Nicholas N (2014) Long-term behavior at
foraging sites of adult female loggerhead sea turtles (Caretta caretta) from three Florida
rookeries. Mar Biol 161:1251–1262
Foley AM, Schroeder BA, MacPherson SL (2008) Post-nesting migrations and resident areas of
Florida loggerheads. Pages 75–76 in Kalb H, Rohde A, Gayheart K and Shanker K,
compilers. Proceedings of the twenty-fifth annual symposium on sea turtle biology and
conservation. NOAA Tech Mem NMFS-SEFSC-582
Fourqurean JW, Robblee MB (1999) Florida Bay: a history of recent ecological changes.
Estuaries 22:345–357
Frick MG, Williams KL, Bolten AB, Bjorndal KA, Martins HR (2009) Foraging ecology of
oceanic-stage loggerhead turtles Caretta caretta. Endang Species Res 9:91–97
Fry B, Lutes R, Northan M, Parker PL, Ogden J (1982) A 13
C/12
C comparison of food webs in
Caribbean seagrass beds and coral reefs. Aquat Bot 14:389–398
105
Fry B, Mumford PL, Robblee MB (1999) Stable isotope studies of pink shrimp
(Farfantepenaeus duorarum Burkenroad) migrations on the southwestern Florida shelf.
Bull Mar Sci 65:419 – 430
Godley BJ, Blumenthal JM, Broderick AC, Coyne MS, Godfrey MH, Hawkes LA, Witt MJ
(2008) Satellite tracking of sea turtles. Where have we been and where do we go next?
Endang Species Res 4:3–22
Goericke R, Fry B (1994) Variations of marine plankton 13
C with latitude, temperature, and
dissolved CO2 in the World Ocean. Global Biogeochem Cy 8:85-90
Graham BS, Koch PL, Newsome SD, McMahon KW, Aurioles D (2010) Using isoscapes to
trace the movements and foraging behavior of top predators in oceanic ecosytems. In:
West JB, Bowen GJ, Dawson TE, Tu KP (eds), Isoscapes: understanding movement,
pattern and process on earth through isotope mapping. Springer-Verlag, New York, NY,
pp 299–318
Griffin DB, Murphy SR, Frick MG, Broderick AC et al. (2013) Foraging habitats and migration
corridors utilized by a recovering subpopulation of adult female loggerhead sea turtles:
implications for conservation. Mar Biol 160:3071–3086, doi:10.1007/s00227-013-2296-3
Harrigan P, Zieman JC, Macko SA (1989) The base of nutritional support for the gray snapper
(Lutjanus griseus): an evaluation based on combined stomach content and stable isotope
analysis. Bull Mar Sci 44:65–77
Hatase H, Takai N, Matsuzawa Y, Sakamoto W et al (2002a) Size-related differences in feeding
habitat use of adult female loggerhead turtles Caretta caretta around Japan determined by
stable isotope analysis and satellite telemetry. Mar Ecol Prog Ser 233:273–281
Hatase H, Matsuzawa Y, Sakamoto W, Baba N, Miyawaki I (2002b) Pelagic habitat use of an
adult Japanese male loggerhead turtle Caretta caretta examined by the Argos satellite
system. Fisheris Sci 68: 945–947
Hawkes LA, Broderick AC, Coyne MS, Godfrey MH, Godley BJ (2007a) Only some like it hot –
quantifying the environmental niche of the loggerhead sea turtle. Diversity Distrib
13:447–457. Doi:10.1111/J.1472-4642.2007.00354.X
Hawkes LA, Broderick AC, Coyne MS, Godfrey MH, Lopez-Jurado LF, Lopez-Suarex P,
Merino SE, Varo-Cruz N, Godley BJ (2006) Phenotypically linked dichotomy in sea
turtle foraging requires multiple conservation approaches. Curr Biol 15:990–995
Hawkes LA, Broderick AC, Godfrey MH, Godley BJ (2007b) Investigating the potential impacts
of climate change on a marine turtle population. Global Change Biolog 13:923–932
Hawkes LA, Witt MJ, Broderick AC, Coker JW, Coyne MS, Dodd M, Frick MG, Godfrey MH,
Griffin DB, Murphy SR, Murphy TM, Williams KL, Godley BJ (2011) Home on the
range: spatial ecology of loggerhead turtles in Atlantic waters of the USA. Diversity
Distrib 17:624–640. doi:10.1111/j.1472-4642.2011.00768.x
106
Hays GC, Broderick AC, Godley BJ, Luschi P, Nichois WJ (2003) Satellite telemetry suggests
high levels of fishing-induced mortality in marine turtles. Mar Ecol Prog Ser 262:305–
309
Herrera LGM, Korine MC, Fleming TH, Arad Z (2008). Dietary implications of intrapopulation
variation in nitrogen isotope composition of an old world fruit bat. J. Mammal 89:1184–
1190
Hobson KA (1999) Tracing origins and migration of wildlife using stables isotopes: a review.
Oecologia 120:314–326
Hobson KA, Piatt JF, Pitocchelli J (1994) Using stable isotopes to determine seabird trophic
relationships. J Anim Ecol 63:786–798
Hopkins-Murphy SR, Owens DW, Murphy TM (2003) Ecology of immature loggerheads on
foraging grounds and adults in internesting habitat in the eastern United States. In: Bolten
AB, Witherington BE (eds). Loggerhead Sea Turtles. Smithsonian Books, Washington,
DC, USA, pp 79–92
Hutchins LW (1947) The bases for temperature zonation in geographical distribution. Ecol
Monographs 17:325–335
Jaeger A, Lecomte V, Weimerskirch H, Richard P, Cherel Y (2010) Seabird satellite tracking
validates the use of latitudinal isoscapes to depict predators' foraging areas in the
Southern Ocean. Rapid Comm Mass Spec 24:3456–3460
Jackson JBC et al. (2001) Historical overfishing and the recent collapse of coastal ecosystems.
Science 293:629–638
Knoff AJ, Macko SA, Erwin RM (2001) Diets of nesting Laughing Gulls (Larus atricilla) at the
Virginia Coast Reserve: Observations from stable isotope analysis. Isot Environ Healt S
37:67–88
Kot CY, Boustany AM, Halpin PN (2010) Temporal patterns of target catch and sea turtle
bycatch in the US Atlantic pelagic longline fishing fleet. Can J Fish Aquat Sci 67:42–57
Kurle CM, Worthy GAJ (2002) Stable nitrogen and carbon isotope ratios in multiple tissues of
the northern fur seal Callorhinus ursinus: implications for dietary and migratory
reconstructions. Mar Ecol Prog Ser 236:289–300
Lamb K, Swart PE (2008) The carbon and nitrogen isotopic values of particulate organic
material from the Florida Keys: a temporal and spatial study. Coral Reefs 27:351–362
Lemons GE, Eguchi T, Lyon BN, LeRoux R, Seminoff JA (2012) Effects of blood
anticoagulants on stable isotope values (13
C and 15
N) of sea turtle blood tissue:
implications for studies at remote field sites. Aquatic Biol. doi:10.3354/ab00397
107
Limpus CJ, Miller JD, Parmenter CJ, Reimer D, McLachlan N, Webb R (1992) Migration of
green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern
Australian rookeries. Wildl Res 19:347–358
López-Castro M, Bjorndal KA, Bolten AB (2014) Evaluation of scute thickness to infer life
history records in the carapace of green and loggerhead turtles. Endang Species Res
24:191–196
López-Castro MC, Bjorndal KA, Kamenov GD, Zenil-Ferguson R, Bolten AB (2013) Sea turtle
population structure and connections between oceanic and neritic foraging areas in the
Atlantic revealed through trace elements. Mar Ecol Prog Ser 490:233–246
Lorrain A, Argüelles J, Alegre A, Bertrand A, Munaron JM, Richard P, Cherel Y (2011)
Sequential isotopic signature along gladius highlights contrasted individual foraging
strategies of jumbo squid (Dosidicus gigas). PLoS One, 6(7), e22194
Macko SA, Entzeroth L, Parker PL (1984) Regional differences in nitrogen and carbon isotopes
on the continental shelf of the Gulf of Mexico. Naturwissenschaften 71:374–375
MacKenzie KM, Palmer MR, Moore A, Ibbotson AT, Beaumont WRC, Poulter DJS, Trueman C
(2011) Locations of marine animals revealed by carbon isotopes. Sci Rep 1, 21;
DOI:10.1038/srep00021
Mansfield KL, Saba VS, Keinath JA, Musick JA (2009) Satellite tracking reveals a dichotomy in
migration strategies among juvenile loggerhead turtles in the Northwest Atlantic. Mar
Biol 156, 2555–2570. doi:10.1007/s00227-009-1279-x
Marcovaldi MA, Lopez GG, Soares LS, Lima EHSM, Thome JCA, Almeida AP (2010) Satellite-
tracking of female loggerhead turtles highlights fidelity behavior in northeastern Brazil.
Endang Species Res 12:263−272
Martínez del Rio C, Wolf N, Carleton SA, Gannes LZ (2009) Isotopic ecology ten years after a
call for more laboratory experiments. Biol Rev Cambridge Philosophical Society 84:91–
111
Matich P, Heithaus MR, Layman CA (2011) Contrasting patterns of individual specialization and
trophic coupling in two marine apex predators. J Anim Ecol 80:294–305.
doi:10.1111/j.1365- 2656.2010.01753.x
Matheson Jr. RE, Camp DK, Sogard SM, Bjorgo KA (1999) Changes in seagrass-associated fish
and crustacean communities on Florida Bay mud banks: The effects of recent ecosystem
changes? Estuaries 22:534-551
McClellan CM, Braun-McNeill J, Avens L, Wallace BP, Read AJ (2010) Stable isotopes confirm
a foraging dichotomy in juvenile loggerhead sea turtles. J Exp Mar Biol Ecol 387:44–51.
Doi:10.1016/J.Jembe.2010.02.020
108
McClellan CM, Read AJ (2007) Complexity and variation in loggerhead sea turtle life history.
Biol Lett 3:592–594
McKinney RA, Oczkowski AJ, Prezioso J, Hyde KJW (2010) Spatial variability of nitrogen
isotope ratios of particulate material from Northwest Atlantic continental shelf waters.
Estuar Coast Shelf S 89:287-293. Doi:10.1016/J.Ecss.2010.08.004
McMahon KW, Hamady L, Thorrold SR (2013) A review of ecogeochemistry approaches to
estimating movements of marine animals. Limnol Oceanogr 58:697-714
Meylan AB, Bjorndal KA, Turner BJ (1983) Sea turtles nesting at Melbourne Beach, Florida, II.
Post-nesting movements of Caretta caretta. Biol Conserv 26:79–90
Michener RH, Kaufman L (2007) Stable isotope ratios as tracers in marine food webs: an update.
In: Michener RH, Lajtha K (eds), Stable isotopes in ecology and environmental science,
2nd edn Blackwell Publishing, Malden, MA, pp 238–282
Minagawa W, Wada E (1984) Stepwise enrichment of 15
N along food chains: further evidence
and the relation between δ15
N and animal age. Geochim Cosmochim Acta 48:1135–1140
Minami H, Ogi H (1997) Determination of migratory dynamics of the sooty shearwater in the
Pacific using stable carbon and nitrogen isotope analysis. Mar Ecol Prog Ser 158:249–
256
Montoya JP (2007) Natural abundance of 15
N in the marine environment. In: Michener RH,
Lajtha K (eds) Stable isotopes in ecology and environmental science. Blackwell
Publishing, Malden, MA, pp 176–201
Montoya JP, Carpenter EJ, Capone DG (2002) Nitrogen fixation and nitrogen isotope
abundances in zooplankton of the oligotrophic North Atlantic. Limnol Oceanogr
47:1617–1628
National Research Council (2010) Assessment of sea-turtle status and trends: integrating
demography and abundance. Washington DC: National Academies Press
Newsome SD, Tinker MT, Monson DH, Oftedal OT, Ralls K, Staedler MM, Fogel ML, Estes JA
(2009) Using stable isotopes to investigate individual diet specialization in California sea
otters (Enhydra lutris nereis). Ecology 90:961–974
Newsome SD, Tinker MT, Gill VA, Hoyt ZN, Doroff A, Nichol L, Bodkin JL (2015) The
interaction of intraspecific competition and habitat on individual diet specialization: a
near range-wide examination of sea otters. Oecologia DOI 10.1007/s00442-015-3223-8
Owens DW, Ruiz GJ (1980) New methods of obtaining blood and cerebrospinal fluid from
marine turtles. Herpetologica 36:17–20
109
Pajuelo M, Bjorndal KA, Alfaro-Shigueto J, Seminoff JA, Mangel JC, Bolten AB (2010) Stable
isotope variation in loggerhead turtles reveals Pacific-Atlantic oceanographic differences.
Mar Ecol Prog Ser 417:277–285
Pajuelo M, Bjorndal KA, Reich KJ, Arendt MA, Bolten AB (2012a) Distribution of foraging
habitats of male loggerhead turtles (Caretta caretta) as revealed by stable isotopes and
satellite telemetry. Mar Biol 159:1255−1267
Pajuelo M, Bjorndal KA, Reich KJ, Vander Zanden HB, Hawkes LA, Bolten AB (2012b)
Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic
using stable isotopes. Ecosphere 3:art89. http://dx.doi.org/10.1890/ES12-00220.1
Peacor SD, Pfister CA (2006) Experimental and model analyses of the effects of competition on
individual size variation in wood frog (Rana sylvatica) tadpoles. J Anim Ecol 75:990–
999. doi:10.1111/j.1365-2656.2006.01119.x
Peterson BJ, Howarth RW (1987) Sulfur, carbon, and nitrogen isotopes used to trace organic
matter flow in the salt-marsh estuaries of Sapelo Island, Georgia. Limnol Oceanogr
32:1195–1213
Peterson BJ, Chester CM, Jochem FJ, Fourqurean JW (2006) Potential role of sponge
communities in controlling phytoplankton blooms in Florida Bay. Mar Ecol Prog Ser
328:93–103
Plotkin PT, Spotila JR (2002) Post-nesting migrations of loggerhead turtles Caretta caretta from
Georgia, USA: conservation implications for a genetically distinct subpopulation. Oryx
36:396–399. Doi:10.1017/S0030605302000753
Popp BN, Graham BS, Olson RJ, Hannides CCS, Lott MJ, Lopez-Ibarra GA, Galvan-Magana F,
Fry B (2007) Insight into the trophic ecology of yellowfin tuna, Thunnus albacares, from
compound-specific nitrogen isotope analysis of proteinaceous amino acids. In: Dawson
T, Siegwolf R (eds) Stable Isotopes as Indicators of Ecological Change, Elsevier
Academic Press pp 173-190
Post D (2002) Using stable isotopes to estimate trophic position: models, methods, and
assumptions. Ecology 83:703–718
Pruell RJ, Taplin BK, Cicchelli K (2003) Stable isotope ratios in archived striped bass scales
suggest changes in trophic structure. Fisheries Manag Ecol 10:329–336
R Development Core Team (2014) R: a language and environment for statistical computing. R
Foundation for Statistical Computing, Vienna
Ragland JM, Arendt MD, Kucklick JR, Keller JM (2011) Persistent organic pollutants in blood
plasma of satellite-tracked adult male loggerhead sea turtles (Caretta caretta). Environ
Toxicol Chem. Doi 10.1002/etc.540
110
Reich KJ, Bjorndal KA, Bolten AB (2007) The ‘lost years’ of green turtles: using stable isotopes
to study cryptic lifestages. Biol Lett 3:712–714
Reich KJ, Bjorndal KA, Frick MG, Witherington BE, Johnson C, Bolten AB (2010) Polymodal
foraging in adult female loggerheads (Caretta caretta). Mar Biol 157:113–121.
Doi:10.1007/S00227-009-1300-4
Reich KJ, Bjorndal KA, del Rio CM (2008) Effects of growth and tissue type on the kinetics of 13
C and 15
N incorporation in a rapidly growing ectotherm. Oecologia 155: 651–663.
Doi:10.1007/S00442-007-0949-Y
Rocque DA, Ben-David M, Barry RP, Winker K (2006) Assigning birds to wintering and
breeding grounds using stable isotopes: lessons from two feather generations among three
intercontinental migrants. J Ornithol 147:395–404
Rooker JR, Secor DH, De Metrio G, Schloesser R, Block BA, Neilson JD (2008). Natal homing
and connectivity in Atlantic bluefin tuna populations. Science, 322:742–744
Rosenblatt AE, Nifong JC, Heithaus MR, Mazzotti FJ, Cherkiss MS, Jeffery BM, Elsey RM,
Decker RA, Silliman BR, Louis J, Guillette LJ Jr, Lowers RH, Larson JC (2015) Factors
affecting individual foraging specialization and temporal diet stability across the range of
a large “generalist” apex predator. Oecologia. DOI 10.1007/s00442-014-3201-6
Roy K, Jablonski D, Valentine JW, Rosenberg G (1998) Marine latitudinal diversity gradients:
tests of causal hypotheses. Proc Natl Acad Sci USA 95:3699–3702
Rubenstein DR, Hobson KA (2004) From birds to butterflies: animal movement patterns and
stable isotopes. Trends Ecol Evol 19:256–263
Ruckdeschel C, Shoop CR (1988) Gut contents of loggerheads: Findings, problems, and new
questions. Proceedings of the Eighth Annual Workshop on Sea Turtle Biology and
Conservation. NOAA Tech Mem NMFS-SEFC-214:145 pp
Saba VS, Shillinger GL, Spotila JR, Chavez FP, Musick JA (2008) Bottom-up and climatic
forcing on the worldwide population of leatherback turtles. Ecology 89:1414–1427
Saito T, Kurita M, Okamoto H, Uchida I, Parker D, Balazs G (2015) Tracking Male Loggerhead
Turtle Migrations Around Southwestern Japan Using Satellite Telemetry. Chelonian
Conserv Biol 14:82–87
Schell DM, Saupe SM, Haubenstock N (1989). Natural isotope abundances in bowhead whale
(Balaena mysticetus) baleen: markers of aging and habitat usage. In: Stable isotopes in
ecological research, Springer, New York, pp 260–269
Schofield G, Hobson VJ, Fossette S, Lilley MKS, Katselidis KA, Hays GC (2010) Fidelity to
foraging sites, consistency of migration routes and habitat modulation of home range by
sea turtles. Divers Distrib 16:840–853. doi:10.1111/j.1472-4642.2010.00694.x
111
Seminoff JA, Benson SR, Arthur KE, Dutton PH, Tapilatu R, Popp BN (2012) Stable isotope
tracking of endangered sea turtles: validation with satellite telemetry and 15N analysis
of amino acids. PLoS ONE 7:e37403
Seminoff JA, Jones TT, Eguchi T, Hastings M, Jones DR (2009) Stable carbon and nitrogen
isotope discrimination in soft tissues of the leatherback turtle (Dermochelys coriacea):
Insights for trophic studies of marine turtles. J Exp Mar Biol Ecol 381:33–41.
Doi:10.1016/J.Jembe.2009.08.018
Seminoff JA, Jones TT, Eguchi T, Jones DR, Dutton PH (2006) Stable isotope discrimination
(delta C-13 and delta N-15) between soft tissues of the green sea turtle Chelonia mydas
and its diet. Mar Ecol Prog Ser 308:271–278
Schell DM, Saupe SM, Haubenstock N (1989) Bowhead whale (Balaena mysticetus) growth and
feeding as estimated by 13C techniques. Mar Biol 103:433–443
Schofield G, Hobson VJ, Fossette S, Lilley MKS, Katselidis K, Hays GC (2010) Fidelity to
foraging sites, consistency of migration routes and habitat modulation of home range by
sea turtles. Diver Dist 16:840–853
Seney EE, Musick JA (2007) Historical diet analysis of loggerhead sea turtles (Caretta caretta)
in Virginia. Copeia 2007:478–489
Shamblin BM et al. (2012) Expanded mitochondrial control region sequences increase resolution
of stock structure among North Atlantic loggerhead turtle rookeries. Mar Ecol Prog Ser
Snover ML, Hohn AA, Crowder LB, Macko SA (2010) Combining stable isotopes and skeletal
growth marks to detect habitat shifts in juvenile loggerhead sea turtles Caretta caretta.
Endang Species Res 13:25–31. Doi: 10.3354/esr00311
Stoner AW, Waite JM (1991) Trophic biology of Strombus gigas in nursery habitats: diets and
food sources in seagrass meadows. J Mollusk Stud 57:451–460
Suryan RM, Saba VS, Wallace BP, Hatch SA, Frederiksen M, Wanless S (2009) Environmental
forcing on life history strategies: Evidence for multi-trophic level responses at ocean
basin scales. Progr Oceanogr 81:214–222. Doi:10.1016/J.Pocean.2009.04.012
Svanbäck R, Bolnick DI (2007) Intraspecific competition drives increased resource use diversity
within a natural population. Proc R Soc Biol Sci 274: 839–844
Thomson JA, Heithaus MR, Burkholder DA, Vaudo JJ, Wirsing AJ, Dill LM (2012) Site
specialists, diet generalists? Isotopic variation, site fidelity, and foraging by loggerhead
turtles in Shark Bay, Western Australia. Mar Ecol Prog Ser 453: 213–226
Thorrold SR, Latkoczy C, Swart PK, Jones CM (2001) Natal homing in a marine fish
metapopulation. Science 291:297–299
112
Tinker MT, Bentall G, Estes JA (2008a) Food limitation leads to behavioral diversification and
dietary specialization in sea otters. PNAS 105:560–565, doi:10.1073/pnas.0709263105
Tucker AD, MacDonald BD, Seminoff JA (2014) Foraging site fidelity and stable isotope values
of loggerhead turtles tracked in the Gulf Mexico and Northwest Caribbean. Mar Ecol
Prog Ser 502: 267–279, doi:10.3354/meps10655
Turtle Expert Working Group (2009) An assessment of the loggerhead turtle population in the
western North Atlantic Ocean. NOAA Tech Mem NMFS-SEFSC-575
Van Houtan KS, Halley JM (2011) Long-term climate forcing in loggerhead sea turtle nesting.
PLoS ONE 6:e19043
Vander Zanden HB, Arthur KE, Bolten AB, Popp BN, Lagueux CJ, Harrison E, Campbell CL,
Bjorndal KA (2013b) Trophic ecology of a green turtle breeding population. Mar Ecol
Prog Ser 476:237–249
Vander Zanden HB, Bjorndal KA, Bolten AB (2013a) Temporal consistency and individual
specialization in resource use by green turtles in successive life stages. Oecologia
173:767–777, doi 10.1007/s00442-013-2655-2
Vander Zanden HB, Bjorndal KA, Reich KJ, Bolten AB (2010) Individual specialists in a
generalist population: results from a long- term stable isotope series. Biol Lett 6:711–714.
doi:10.1098/Rs- bl.2010.0124
Vander Zanden HB, Pfaller JB, Reich KJ, Pajuelo M, Bolten AB, Williams KL, Frick MG,
Shamblin BM, Nairn CJ, Bjorndal KA (2014) Foraging areas differentially affect
reproductive output and interpretation of trends in abundance of loggerhead turtles. Mar
Biol 161:585-598
Vander Zanden HB, Tucker AD, Hart KM, Lamont MM, Fujisaki I, Addison DS, Mansfield KL,
Phillips KF, Wunder MB, Bowen GI, Pajuelo M, Bolten AB, Bjorndal KA (2015)
Determining origin in a migratory marine vertebrate: a novel method to integrate stable
isotopes and satellite tracking. Ecol Appl 25:320–335 http://dx.doi.org/10.1890/14-
0581.1
Wallace BP et al. (2010) Regional management units for marine turtles: a novel framework for
prioritizing conservation and research across multiple scales. PLoS ONE 5:e15465.
Wallace BP, Avens L, Braun-McNeill J, McClellan CM (2009) The diet composition of
immature loggerheads: Insights on trophic niche, growth rates, and fisheries interactions.
J Exp Mar Biol Ecol 373:50–57. Doi:10.1016/J.Jembe.2009.03.006
Wallace BP, Seminoff JA, Kilham S, Spotila J, Dutton PH (2006) Leatherback turtles as
oceanographic indicators: stable isotope analyses reveal a trophic dichotomy between
ocean basins. Mar Biol 149:953–960
113
Wilkinson T, Wiken E, Bezaury-Creel J, Hourigan T et al. (2009) Marine ecoregions of North
America. Commission for Environmental Cooperation, Montreal, Quebec, Canada
Williams KL, Frick MG (2008) Tag returns from loggerhead turtles from Wassaw Island, GA.
Southeastern Nat 7:165–172
Witherington B, Kubilis P, Brost B, Meylan A (2009) Decreasing annual nest counts in a
globally important loggerhead sea turtle population. Ecol Appl 19:30–54
Witzell WN, Schmid JR (2005) Diet of immature Kemp’s ridley turtles (Lepidochelys kempii)
from Gullivan Bay, Ten Thousand Islands, southwest Florida. Bull Mar Sci 77:191–199
Woodland RJ, Secor DH, Wedge ME (2011) Trophic resource overlap between small
elasmobranchs and sympatric teleost in Mid-Atlantic Bight nearshore habitats. Estuar
Coast 34:391–404
Zbinden JA, Bearhop S, Bradshaw P, Gill B, Margaritoulis D, Newton J, Godley BJ (2011)
Migratory dichotomy and associated phenotypic variation in marine turtles revealed by
satellite tracking and stable isotope analysis. Mar Ecol Prog Ser 421:291−302
114
BIOGRAPHICAL SKETCH
Mariela E. Pajuelo was born in Lima, Perú. She attended María de la Providencia School,
where she graduated in 1993. She then attended Universidad Nacional Mayor de San Marcos in
Lima, Perú where she graduated in 2001 with a focus in Hydrobiology and Fisheries Science. In
2004, she joined Peruvian non-govermental organization Pro Delphinus to work as a research
associate conducting work on conservation of marine mega-fauna. In fall 2007, she started
master´s studies at the University of Florida (UF) with Dr. Karen A. Bjorndal, where she focused
on the trophic ecology of oceanic juvenile loggerhead sea turtles. In fall 2010, she began doctoral
work under guidance of her former master’s advisor, studying the foraging ecology of
loggerhead turtles in the North Atlantic Ocean. Mariela graduated in the fall of 2015 with a
degree in zoology.