Growth measurements of the Queen Conch Strombus gigas in The Turks and Caicos Islands: varying levels of reliability LUCY P. TOMB1, 2

1 Bowdoin College, Department of Biology, 2School for Field Studies, Center for Marine Resource Management

TOMB  2014   2  

ABSTRACT

Strombus gigas, the Queen Conch, is an extremely valuable fishing export in the Turks

and Caicos Islands. However, complex growth of the conch results in difficulties aging the

animals and subsequently poor enforcement of fishing restrictions. To investigate the reliability

of current commonly used measurements of lip thickness, siphonal length, and dirty meat weight,

as well as investigate new possible measurements, fished Queen Conch (n=99) were measured

for lip thickness, siphonal length, dirty meat weight, clean meat weight, operculum dimensions,

and nominal/live weight. Lip thickness and siphonal length showed no correlation to one

another, though both were correlated with meat weight. The lack of correlation between the two

may be attributed to the plasticity of these traits. In agreement with a previous study on

operculum dimensions, both length and width of the operculum were significantly positively

correlated with siphonal length, and operculum width with lip thickness. These findings paired

with previous and possible further research may suggest a re-evaluation of aging strategies for

conch and possibly new regulations for conch fisheries.

INTRODUCTION

The Queen Conch, Strombus gigas, is a vital

organism in and around the greater Caribbean area

(Figure 1) where it is more than just an icon of these

countries but also heavily exploited in the fishing

industry (Thiele 2001, Stoner, et al 2012). In 1992 it

was estimated that the conch fishery in the

Caribbean brought in revenue of $30 million

(McCarthy 2007). After declines in stock abundance were observed from 1985 to 1992, the

Convention on the International Trade in Endangered Species (CITES) listed S. gigas as an

Appendix II endangered species mandating CITES signatories to heavily monitor and regulate

their conch stocks (McCarthy 2007). In 2012, the United States was petitioned to list S. gigas as

endangered on the Endangered Species Act (Mueller and Stoner 2013). However, because of the

Figure  1.    Distribution  of  Strombus  gigas.    From  Oceana.org  

TOMB  2014   3  

complicated growth of S. gigas, size limitations are hard to implement, making regulation of this

fishery much more difficult.

Conch grow in a variety of different

ways depending on which life stage they are

in, so a single measurement of growth does

not suffice. Juvenile conch grow in siphonal

length, which is measured from the top of the

spire to the bottom of the lip of the conch

(Figure 2). As juveniles get older they get

longer, however the thickness of their lip stays

the same (Appeldoorn 1988). The siphonal length at sexual maturity has been estimated to be

249 mm for females and 234 mm for males (Avila-Poveda and Baquiero-Cardenas 2006). Once

the conch reaches adulthood siphonal length stops increasing, the lip beings to flare and the lip

thickness beings to increase (Figure 2). This happens when the conch is about 3-4 years old

(Avila-Poveda and Baquiero-Cardenas 2006, Appeldoorn 1990, Mueller and Stoner 2013).

Sometimes sexual maturity can lag up to 2 years after the lip begins to flare (Stoner, et al 2012).

Usually, an individual is thought to be mature when the lip thickness has grown to 5 mm and

cannot be sexually mature before the lip thickness is 2 mm (Avila-Poveda and Baquiero-

Cardenas 2006). As conch get very old, erosion can cause their siphonal length to decrease

(Thiele 2001). Because of this, juveniles are most accurately aged by measuring siphonal length,

while adults are most accurately aged using lip thickness (Cardenas and Aranda 2013).

Appeldoorn determined two equations to calculate age, the first using length-frequency analysis

and the second using growth-increment data (1990):

Appeldoorn also created an equation to determine age using lip thickness (1988):

Using these equations, it is possible to calculate the age of conch when they are still in their

shell, but it is important to consider cases in which the shell is not available.

TOMB  2014   4  

The growth of the shell is currently the most widely used way to measure age and

maturity of conch but this presents an array of problems for fisheries managers and biologists

alike. Most conch fishers knock the conch (remove the meat from the shell) on the boats,

leaving the shells in the ocean and only landing the conch meat and tailings. Without the shell

the conch is hard to age and any shell-based size or age restrictions cannot be enforced unless

there is an obvious mismatch (Avila-Poveda and Baquiero-Cardenas 2006).

The shell growth of the conch is also extremely plastic (Thiele 2001, Delgado, et al 2002,

McCarthy 2007). Conch that grow slower have thicker shells and shorter siphonal lengths, and it

is seen that conch that are heavily predated on, for example, grow slower so that their shells will

be thicker and more protective (Delgado, et al 2002). A conch exposed often to predation will

have a much different siphonal length and lip thickness than one of the same age that is exposed

to less predation. It is thought that this is due to the fact that conch exposed to predation spend

less energy moving because they bury themselves in the sand and protect their meat by not lifting

their shells to move. By remaining sedentary these conch have their mantel away from the

growing opening of the shell and instead deeper in the shell so calcium carbonate is deposited on

this inside of the shell making it thicker instead of longer (Delgado 2002). Other abiotic factors

can have an effect on growth rate, for example, conch have slower growth as depth increases

(McCarthy 2007). Habitat and food can also affect growth rates (Stoner, et al 2012).

Consequently shell thickness and siphonal growth can vary significantly over very small spatial

scales (Cardenas and Aranda 2013, McCarthy 2007, Clerveaux, et al 2005). Shell length at

maturity can also change with fishing pressure (Stoner, et al 2012). Essentially, shell growth is

so variable and plastic that it can be very misleading when aging conch.

Meat weight can also be used to calculate age, which is helpful when the shell is

unavailable, but this measurement comes with its own set of problems (Cardenas and Aranda

2013). Though juvenile conch have predictable meat growth as they develop, adult conch have

relatively constant body weight because as the shell starts to thicken, there is no room for them to

grow any larger (McCarthy 2007, Stoner, et al 2012). The oldest conch tend to have smaller

body sizes than their younger counterparts as the shell has reduced interior volume because of

their thick shells (Figure 2)(McCarthy 2007, Stoner, et al 2012). Meat weight, like shell growth

is a highly variable characteristic and therefore does offer a very accurate calculation of age.

TOMB  2014   5  

A relatively unexplored method of aging conch uses the

operculum, which has been found to be very accurate. Operculum striae

are used in Japan to age large gastropods, but the concentric circles that

show growth are not visible in tropical gastropods as they are in temperate

regions like Japan (Mueller and Stoner 2013). In a 2007 study, Uneputty

used operculum length to successfully age the tropical marine gastropod

Nerita undata. The operculum stops growth at sexual maturity as the shell

does, though can begin to erode as the animal ages (Mueller and Stoner

2013). Mueller and Stoner (2013) were able to correctly predict the sexual

maturity of 86% of the queen conch they sampled using two operculum

measurements: the length (OL) and the width (OW) (Figure 3).

Conch is a particularly important export in the Turks and Caicos Islands (TCI), bringing

in an estimate of $3.8 - $5 million a year and has been fished since the Lucayans arrived almost

1,000 years ago. The current yearly quota for conch meat in the TCI is 800,000 pounds with

500,000 for export and 300,000 for local use. With a significant decline in conch stock from

2001 to 2008, it may be crucial for the Department of Environmental and Maritime Affairs

(DEMA) to set new limitations on the conch fishery (DEMA 2012/2013). Limitations on size of

conch paired with enforcement could be important to a compromise between fishermen and

DEMA, but as the conch in the TCI are knocked before landing, understanding measurements

other than shell dimensions to estimate age is imperative. Asking fishers to land the conch with

their shells is unreasonable because they make significantly more money without the extra

weight and space that shells would take up, and it may actually be safer in the small boats they

don’t carry the shells with them (Thiele 2001). This research attempts to compare methods of

aging and analyze their accuracy in the Turks and Caicos Islands, specifically off the coast of

South Caicos.

TOMB  2014   6  

METHODS

Queen Conch (n=99) were collected off the coast of South Caicos, Turks and Caicos in

October and November 2014. The conch were all collected by two fishermen from all around

the island. Eight measurements were taken for each conch landed; measurements were lip

thickness, siphonal length, nominal/live weight, dirty meat weight, clean meat weight, length of

operculum, and width of operculum.

Measurements were taken on seven days within a 28-day period. Each of the conch were

taken from their shell and cleaned by either a worker at the processing plant Caicos Fisheries

Ltd. or by the fisher who caught them.

Siphonal length for each conch was measured in centimeters using a yardstick from the

tip of the spire to the end of the lip of the conch (Figure 2). Lip thickness was measured using

Vernier calipers at the middle of the lip, avoiding ridges (Figure 2). Nominal/live weight was

measured in kilograms by weighing the conch before it was knocked. Meat weights were

measured in grams on an electronic balance. Dirty meat weight included the entire animal out of

the shell. Clean meat weight was measured after the conch was cleaned for sale. Operculum

measurements were taken with a small Vernier caliper, measurements were taken in millimeters.

Operculum length was measured from the tip of the operculum to the base of the operculum.

Operculum width was the greatest width where the operculum was attached to the meat (Figure

3).

Statistical analyses for linear regressions were conducted in GraphPad Prism 6.

Relationships between measurements were all analyzed with linear regression. Normality of

frequency distribution of siphonal length, lip thickness, nominal and dirty weight, operculum

length and width were all tested with a Shapiro-Wilk test using JMP software.

Simultaneous research was done on the conch by Libby Humpal and Shayna Cohen with

which I assisted. Research was done to investigate conch responses to the shells of knocked

conch in the East Harbor Conch and Lobster reserve. Cleaning data was also taken for conch

brought into the processing plant.

TOMB  2014   7  

RESULTS

Correlation was found between

operculum measurements and shell

measurements. Operculum width (OW) and

Operculum Length (OL) were both

significantly positively correlated with

siphonal length (OW: R2 = 0.2576, P <

0.0001, df = 97; OL: R2 = 0.1651, P < 0.0001,

df = 97) (Figure 4). OW was positively

correlated with lip thickness (R2 = 0.2039, P <

0.0001, df = 97) (Figure 5), but the correlation

between OL and lip thickness was not

significant and therefore those results are not

shown (R2 = 0.02671, P = 0.1060, df = 97).

There was no significant correlation

between siphonal length and lip thickness (R2

= 0.005070, P =0.4837, df = 97) (Figure 6).

To determine whether lip thickness and

siphonal length would correlate when

separated by location caught, the two fishing

locations were separated and analyzed

separately, one at the North end of the island

and one at the south. Neither location showed

significant correlation between lip thickness and siphonal length (North: R2 = 0.03640, P =

0.1011, df = 73; South: R2 = 0.009082, P = 0.6578, df = 22) (Figure 6).

There was also correlation found between dirty meat weight and physical measurements.

Dirty meat weight was significantly positively correlated with both OL (R2 = 0.2072, P < 0.0001)

and OW (R2 = 0.5084, P < 0.0001, df = 97) (Figure 7). Dirty meat weight was also significantly

positively correlated to siphonal length (R2 = 0.2885, P < 0.0001, df = 97) and lip thickness (R2 =

0.09665, P=0.0017, df = 97) (Figure 8).

TOMB  2014   8  

Frequency distributions were

found to be normal for siphonal length

(P = 0.9592), operculum width (P =

0.1851), and nominal/live weight (P =

0.2055), and dirty meat weight (P =

0.0723). Distribution was not normal in

the case of lip thickness (P < 0.0001) or

operculum length (P = 0.0007). Clean

meat weight was not used in this study,

it was used instead in a simultaneous

study done by Libby Humpal.

TOMB  2014   9  

DISCUSSION  

  In  this  study,  relationships  and  correlations  between  Queen  Conch  measurements  

were  examined,  comparing  both  current  standard  aging  measurements  and  less  researched  

measurements.  The  most  widely  used  measurements  for  aging  conch:  lip  thickness  and  

siphonal  length,  were  not  found  to  be  correlated  to  one  another,  which  was  unexpected.    

Even  when  the  siphonal  length  and  lip  thickness  were  separated  by  two  general  fishing  

locations,  there  was  no  significant  trend.    Meat  weight,  on  the  other  hand,  was  significantly  

positively  correlated  with  both  siphonal  length  and  lip  thickness.  Operculum  dimensions  of  

TOMB  2014   10  

length  and  width,  which  recently  have  been  suggested  to  be  accurate  aging  measurements,  

were  also  found  to  be  significantly  positively  correlated  to  accepted  aging  measurements  of  

dirty  meat  weight,  siphonal  length,  and  lip  thickness,  with  the  exception  of  operculum  

length  and  lip  thickness  which  were  not  significantly  correlated.  These  findings  support  

previous  data  that  links  operculum  dimensions  to  shell  dimensions  and  goes  further  to  

suggest  that  operculum  growth  is  also  a  good  indicator  of  meat  weight  of  the  conch.    The  

lack  of  correlation  between  lip  thickness  and  siphonal  length  support  the  plasticity  of  these  

traits  due  to  environment,  and  may  suggest  that  these  are  less  accurate  measurements  of  

growth  than  other  characteristics  of  the  conch.      

  The  accepted  theory  of  conch  growth  is  that  siphonal  length  grows  in  juvenile  conch,  

while  lip  thickness  remains  constant,  and  then  when  the  conch  reaches  maturity  siphonal  

length  stops  growing  and  the  lip  starts  to  thicken  (Avila-­‐Poveda  and  Baquiero-­‐Cardenas  

2006,  Appeldoorn  1990,  Mueller  and  Stoner  2013).  However,  if  conch  matured  at  relatively  

the  same  siphonal  length,  a  significant  correlation  between  siphonal  length  and  lip  

thickness  would  be  seen.    Once  siphonal  length  reached  a  certain  point  lip  thickness  would  

start  to  increase.    This  study  found  no  such  correlation  and  even  a  brief  look  at  the  figure  

shows  an  extreme  range  of  lip  thicknesses  to  siphonal  length  (Figure  6).    As  of  now,  the  

accepted  siphonal  length  of  an  adult  conch  is  249  mm  for  female  conch  and  239  for  male  

conch,  and  the  minimum  accepted  lip  thickness  for  adult  conch  is  2mm  (Avila-­‐Poveda  and  

Baquiero-­‐Cardenas  2006).    Our  data  suggests  that  these  numbers  may  not  be  very  reliable,  

with  conch  under  200  mm  (20  cm)  having  lip  thicknesses  over  15  mm,  safely  suggesting  

they  are  mature,  and  conch  over  250mm  having  lip  thicknesses  under  2  mm  suggesting  

they  are  immature.    Even  when  the  conch  are  separated  by  where  they  are  caught  to  try  

and  account  for  large  differences  in  habitat,  no  correlation  is  found,  suggesting  that  these  

traits  are  so  plastic  that  conch  within  a  small  rage  of  each  other  can  grow  quite  differently.    

Factors  like  predation  and  depth  can  have  a  large  effect  on  the  growth  rate  of  conch  shells  

and  can  leave  two  conch  of  the  same  age  with  two  very  different  shell  morphologies  

depending  on  environment  (Delagdo,  et  al  2002,  McCarthy  2007,  Stoner,  et  al  2012).    The  

2012/2013  Turks  and  Caicos  minimum  siphonal  length  for  the  conch  fishery  was  17.8  cm  

(7  inches),  all  the  conch  used  in  this  study  exceeded  this  siphonal  length  but  more  than  

16%  had  lip  thicknesses  of  under  2  mm  safely  suggesting  that  they  were  juveniles  

TOMB  2014   11  

(Department  of  Environment  and  Maritime  Affairs  2012/2013).    Other  studies  put  

adulthood  at  a  lip  thickness  of  13.5,  which  only  28%  of  the  conch  in  this  study  meet  (Avila-­‐

Poveda  and  Baquiero-­‐Cardenas  2006).    Ultimately,  these  measurements  may  not  be  a  

stable  enough  measurement  of  maturity.      

  This  data  suggests  that  meat  weight,  the  alternative  measure  of  maturity  in  The  

Turks  and  Caicos  fisheries,  is  significantly  correlated  to  siphonal  length  and  lip  thickness.    

This  may  be  attributed  to  the  fact  that  siphonal  length  and  lip  thickness  both  contribute  

directly  to  the  area  within  the  shell  that  the  conch  can  grow.    Lip  thickness,  though  

correlated  with  meat  weight,  had  a  relatively  low  confidence  level  compared  to  siphonal  

length.    This  may  be  due  to  the  fact  that  as  the  lip  thickens  the  space  for  the  conch  to  grow  

within  decreases  even  though  the  conch  is  aging  (McCarthy  2007,  Stoner,  et  al  2012).      

  The  positive  correlation  of  operculum  length  and  width  to  siphonal  length  found  in  

this  study  supports  Mueller  and  Stoner’s  findings  from  The  Bahamas  (2013).        Mueller  and  

Stoner  did  not  find  correlation  between  operculum  dimensions  and  lip  thickness,  while  this  

study  found  a  significant  positive  correlation  between  operculum  width  and  lip  thickness,  

though  this  correlation  had  a  lower  confidence  level  than  when  operculum  dimensions  

were  compared  to  siphonal  lengths.    In  addition,  a  significant  positive  correlation  between  

operculum  length  and  width  and  dirty  meat  weight  furthers  the  idea  that  operculum  are  a  

reliable  measure  of  age.  These  findings  offer  more  evidence  to  the  suggestion  that  

operculum  dimensions  could  be  an  accurate  measure  of  conch  growth,  and  could  possibly  

reduce  or  remove  the  need  for  fishermen  to  land  conch  in  their  shells  in  order  to  age  them.    

It  is  important  to  consider,  however,  that  operculum  length  erodes  as  the  conch  ages,  so  

correlations  including  OL  may  break  down  at  older  ages  (Mueller  and  Stoner  2013).    Many  

of  the  operculum  measured  for  this  study  were  visibly  eroded  so  this  may  be  a  less  reliable  

measurement  than  operculum  width.  

  In  summary,  the  data  in  this  study  suggests  that  lip  thickness  and  siphonal  length  of  

Queen  conch  may  be  less  correlated  than  previously  assumed  and  supports  and  furthers  

data  that  suggests  that  operculum  dimensions  could  be  a  reliable  and  accurate  measure  of  

conch  growth.    Using  the  operculum  to  measure  conch  growth  could  have  huge  

implications  in  the  Queen  conch  fisheries  all  over  the  Caribbean.    While  not  only  

eliminating  the  need  for  shells  to  be  landed  with  conch  meat  in  order  to  enforce  

TOMB  2014   12  

regulations,  setting  operculum  limits  could  allow  fishers  to  check  maturity  before  the  conch  

is  removed  from  the  shell,  and  allow  undersized  conch  to  be  thrown  back  and  continue  

growing.    Restrictions  that  require  the  conch  to  be  dead  or  out  of  the  shellfor  

measurements  to  be  taken,  like  dirty  meat  weight  limits,  are  less  effective  because  

undersized  conch  cannot  be  thrown  back  and  survive.    Further  research  needs  to  be  done  

to  expand  the  amount  of  data  and  increase  confidence  level  in  these  correlations  and  also  

diversify  the  locations  in  which  these  measurements  are  examined.    Also,  this  research,  

since  it  was  done  with  fishermen,  did  not  include  very  small  conch,  which  could  have  an  

effect  on  data.  Research  including  a  wider  range  of  conch  sizes  may  be  important.    With  

further  research,  the  correlation  between  operculum  and  growth  could  lead  to  an  easier  to  

regulate  and  subsequently  more  sustainable  conch  fishery,  which  could  have  important  

implications  on  the  future  policies  of  this  currently  unstable  fishery.  

 

 

ACKNOWLEDGEMENTS  

  Funding  and  vehicles  for  this  project  were  provided  by  the  School  for  Field  Studies.    

I  am  grateful  to  Kathy  Lockhart  who  suggested  this  project  idea,  and  was  immensely  

helpful  in  writing  and  editing  this  paper.    In  addition,  I  am  grateful  to  Shayna  Cohen  and  

Libby  Humpal  who  assisted  in  field  research,    and  were  valuable  proof  readers.      

 

   

TOMB  2014   13  

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