Spatial distribution of juvenile

Queen conch (Lobatus gigas)

in Lac Bay, Bonaire

A Master Biology Thesis

Aquatic Ecology and Water Quality Management

Report no: 07/2013

Ineke Willemse

Student no. 890524 958050

MSc Biology, specialization Marine Biology

Thesis AEW 80430

February 2013

Supervision:

Dr. Rudi (R.M.M.) Roijackers

Aquatic Ecology and Water Quality Management, Wageningen University, Wageningen, The

Netherlands

Dr. Ir. Klaas (K.) Metselaar

Soil Physics, Ecohydrology and Groundwater Management, Wageningen University, Wageningen, The

Netherlands

Drs. Sabine (M.S.) Engel

Stichting Nationale Parken, Bonaire, Dutch Caribbean

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Contents

Preface ............................................................................................................................................................ 5

Summary ......................................................................................................................................................... 7

1. Introduction ................................................................................................................................................ 9

1.1 Background of the problem: the Queen conch population in Lac Bay, Bonaire .................................. 9

1.2 State of the art ...................................................................................................................................... 9

1.3 Research questions ............................................................................................................................. 13

2. Methodology ............................................................................................................................................. 15

2.1 Study area ........................................................................................................................................... 15

2.2 Site selection ....................................................................................................................................... 16

2.3 Site survey ........................................................................................................................................... 17

2.4 Data analysis ....................................................................................................................................... 18

3. Results ....................................................................................................................................................... 21

3.1 Juvenile distribution ........................................................................................................................... 21

3.2 Habitat characteristics ........................................................................................................................ 21

3.3 Day and night surveys ........................................................................................................................ 24

3.4 Predation ............................................................................................................................................ 24

3.5 Population structure ........................................................................................................................... 24

4. Discussion ................................................................................................................................................. 27

4.1 Habitat characteristics ........................................................................................................................ 27

4.2 Population structure ........................................................................................................................... 29

4.3 Predation ............................................................................................................................................ 30

5. Conclusion and recommendations ........................................................................................................... 31

Literature references .................................................................................................................................... 33

Annex I - Percentage cover for each plant taxa per quadrant ...................................................................... 37

Annex II - Additional figures .......................................................................................................................... 39

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Preface

Sometimes luck just crosses your path, and accompanies you along the whole road. In my case it

started while sitting in a lecture room when a fellow student next to me told about her thesis project.

Now, almost a year later, we have had an amazing time while performing our fieldwork on Bonaire,

This report contains the results of the research part I was responsible for.

The focus of this thesis was on the spatial distribution of the population of juvenile Queen conch in Lac

Bay, Bonaire, and was conducted as part of my MSc Biology at the Aquatic Ecology and Water Quality

Management group of Wageningen University. The study is performed in the context of the Queen

Conch Restoration Project of STINAPA, Bonaire, and closely guided by Sabine Engel. Fieldwork was

done together with Paulien Prent, who focussed on the distribution of veliger larvae of the Queen

conch.

This study gave me the opportunity to perform my thesis research within the broader context of a

restoration program, in which research by a variety of disciplines, awareness raising among locals and

social issues are involved. I gained experience in the challenges of, and limitations in carrying out

fieldwork, while fully enjoying my stay on a beautiful island and the fieldwork in a beautiful lagoon

together with amazing people.

Hereby, I would like to thank the people who made this all possible. First of all, thanks to Paulien Prent

- my favourite thesis buddy in so many ways! Then my supervisors; Rudi Roijackers (your tempering

but realistic view, your advice, time management and reviewing), Klaas Metselaar (your amazing

enthusiasm), and Sabine Engel (your knowledge, patience, hospitality and humour). I would like to

thank the STINAPA staff, especially Ramon de Léon for providing facilities and assistance, and Gevy

Soliana for his birdseye and indispensable assistance in and on the water (hopi piska!). Thanks to Rita

Peachey, who allowed us to use the CIEE Bonaire laboratory facilities, and to Graham Epstein and

Rachael Wright for their assistance in the lab. Edwin Peeters, thanks for your guidance in the statistical

analysis. Dolfi Debrot and Jeroen Goud, thank you for your advice in fieldwork preparations. And of

course I may not forget to mention all the rangers from WSNP (you made it our home!); the staff from

Dive Friends Bonaire, and Frank in particular; Fabian, Ruben and Fonsjie (for taking us out on the open

ocean); Funchi; Lotte, Iris, Tatiana and Vinni; Abi and Franziska; Edwin & Naomi (reviewing every word

and punctuation); André & Addie and the rest of the family; Boudewijn (your patience); and Ingrid

(thanks for tagging assistance).

Masha danki!

Ineke Willemse, Wageningen 2013

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Summary

The population of Queen conchs (Lobatus gigas) in the Caribbean region has declined largely in size

over the last three decades. In Lac Bay, a shallow lagoon on Bonaire, overfishing and habitat reduction

are important causal factors. As habitat requirements and life cycle characteristics of the Queen conch

differ largely between different areas, identification of the life cycle of this species in Lac Bay is crucial

for an effective protection and restoration of the population. In this study the spatial distribution of

the first year class of juveniles (with a maximum size of 150 mm) in Lac Bay was investigated.

To this end, 33 locations were surveyed to record conch densities in combination with several

environmental variables. A canonical correspondence analysis was used to get insight in the influence

of specific plant species, water depth and temperature, and sediment composition on the spatial

distribution of juveniles. Besides, conchs larger than 150 mm and empty conch shells were measured

in order to get an impression of the population structure and predation in Lac Bay.

A mean density of 0.0058/m2 of juveniles ≤150 mm was observed, with highest densities in the central

part of the lagoon. A positive trend was recognized with depth and a finer sediment composition.

Higher densities were associated with a combination of several plant taxa, including Halophila

stipulacea, Syringodium filiforme, Thalassia testudinum, turf-like algal species and a yet unknown

cyanobacteria. Juveniles were mainly observed in similar areas as where high conch densities were

found in previous studies in Lac Bay, so it appeared that juveniles ≤150 mm do no occupy specifically

different areas than larger conspecifics. Observed juveniles were not smaller than 70 mm, but

evidence was found that infaunal juveniles (<70 mm) must be present. To get insight in the

distribution of the infaunal conchs, an efficient night survey method needs to be developed. If infaunal

juveniles prefer areas with fine sediment, they could be found in the same region as epifaunal

juveniles. This implies that juveniles do not always move to a different area when changing the

infaunal for epifaunal state. This study showed that nearly 40% of the mortality is due to poaching, and

only conchs larger than 150 mm are subject to these activities. However, there is evidence that

juveniles ≤150 mm do not escape poaching. For the total population of Queen conch in Lac Bay, a

mean density of 0.0226/m2 was observed, indicating a significant increase of the population over the

last decade. This density is relatively low compared to other areas in the Caribbean region, but the

population growth shows that Lac Bay is increasingly important as a nursery area. Future studies

should focus on the distribution of the infaunal juveniles in Lac Bay as knowledge on their status is still

lacking. Continuing the protection and restoration of the Queen conch population in Lac Bay is strongly

recommended as this study shows that the positive effects become visible.

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1. Introduction

1.1 Background of the problem: the Queen conch population in Lac Bay, Bonaire

The population of the Queen conch (Lobatus gigas; Linnaeus, 1758), a valuable and vulnerable

gastropod in the Caribbean Region, has declined largely in number over the last three decades (Stoner,

2003). Overfishing and habitat reduction are important causal factors, also in Lac Bay, Bonaire (Lott,

2001). Lac Bay is a historically important fishing area, with the Queen conch as one of the main target

species. Fishing on this species is prohibited since 1985, but since then the population has only slightly

increased (Engel, 2008). Nowadays, because of its scarcity, the market price for conch meat is very

high and for locals it has become very attractive to catch and sell conch illegally (Engel, personal

comm.). Furthermore, the Queen conch population in Lac Bay is threatened by a reduction of the

habitat area as the mangroves surrounding the lagoon extend toward the open water, and this process

causes a decrease in the size of the central bay (Moorsel and Meijer, 1993; Hylkema and Vogelaar,

2012).

This study is part of the Queen Conch Restoration Project of STINAPA (National Parks Foundation,

Bonaire). In this project both research on the status of the Queen conch in Lac Bay and the creation of

awareness among locals are combined, with a healthy conch population in the future as the main goal.

A successful restoration program includes the conservation of important habitats for different life

stages of this species. Stoner (2003) states that the understanding of the habitat requirements of the

species is the key to a successful conservation. Whereas these habitat requirements and life cycle

characteristics of the Queen conch differ largely between different areas, identification of the life cycle

of the Queen conch in Lac Bay is crucial for an effective protection and restoration of its population.

Based on benthic percent coverage ranges, Lac Bay is considered as being a suitable nursery area (Lott,

2000). A focus on the earliest life stages of the Queen conch can therefore give a better insight in the

situation of this species in Lac Bay.

Several species of the Lobatus genus, formerly placed in the Strombus genus, are present in de

Caribbean region, with the subject species Queen conch (Lobatus gigas) being the largest (McCarthy,

2007). The Queen conch is in appearance easily distinguishable from other species of the Lobatus

genus present in Lac Bay. Milk conchs (Lobatus costatus) were found in Lac Bay by Moorsel and Meijer

(1993) and Engel (2008), but in lower numbers. Incidentally, a Rooster conch (Lobatus gallus) has been

observed (Engel, personal comm.).

Potential natural predators on the Queen conch that are observed in Lac Bay are turtles, crabs,

octopuses, lobsters, porcupine fishes and rays. Incidentally, a shark, triton or cushion star (also known

as reticulated seastar) has been seen in the lagoon (Moorsel and Meijer, 1993; Engel, 2008; personal

observ.).

1.2 State of the art

Morphology and growth

The life cycle of the Queen conch, as displayed in figure 1.1, starts with a planktonic stage (Randall,

1964). The veliger larvae stay in the surface layer of the water column for 3 to 5 weeks (Davis et al.,

1993). When a size of about 1.2 mm is reached, larvae settle in the bottom where metamorphosis into

a so-called postlarvae occurs (Stoner et al., 1998). Postlarvae develop into juveniles, however little

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information is available about the recently settled infaunal juveniles since observations are rare (Ray

and Stoner, 1995). Juveniles from 35 to 54 mm were found buried in the substrate at a maximum

depth of 30 to 40 mm (Iversen et al., 1986; Sandt and Stoner, 1993). Infaunal juveniles show a daily

migration pattern from benthos to surface. Juveniles move to the upper water column before sunset

and bury themselves again into the sediment before sunrise. When a shell length of about 50 mm is

reached, the sediment is left for settlement on the bottom (Sandt and Stoner, 1993). For juveniles in

Lac Bay it is thought that the infaunal stage is left at a length of about 70 mm (Engel, personal comm.).

The epifaunal juveniles live in shallow waters till the age of 3.5 to 4.0 years, at which sexual maturity is

reached. At the shell a flared lip of about 8 mm has been formed then (Engel, personal comm.).

Deeper waters up to 35 or 40 m can be occupied and further growth (to a maximum of 300 mm) and

reproduction takes place (Stoner, 2003). For many conch populations the reproduction season lasts 8

months, but highest activity is observed between July and September (Aranda et al., 2003; Davis,

2005).

Habitat choice

In the life cycle of the Queen conch several ontogenetic shifts can be recognized. The first shift is from

larvae to infaunal juvenile, the second is from infaunal to epifaunal juvenile. A third remarkable shift

occurs when sexual maturity is reached and the conch moves to deeper waters. Every shift implies a

change in habitat type. Ray and Stoner (1995) hypothesized that predation is one of the most

important factors in habitat choice. Juveniles leave behind the infaunal life once they have survived

their stage of highest vulnerability (Sandt and Stoner, 1993). Also food availability is crucial (Ray and

Stoner, 1995; Stoner, 2003). Whereas the veliger larvae feed on phytoplankton, juvenile conch feed on

Figure 1.1 Schematic overview of the life cycle of the Queen conch (after Bonny Bower-Dennis [1])

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macroalgae, epiphytes and detritus. Also adult conch are herbivorous and can have seagrass added to

their diet (Stoner and Ray, 1993; Stoner, 2003). The occurrence in specific habitat types is not only

influenced by predation risk and food resources but lies in a combination of multiple biotic and abiotic

factors (Sandt and Stoner, 1993). Competition and active habitat choice can play a major role, and

depth, water velocity and sediment type influence the quality of the habitat (Sandt and Stoner, 1993;

Ray and Stoner, 1995; McCarthy, 2007).

Aggregations are characteristic for the distribution of juvenile conch. Although this might increase

competition for resources in some parts of the year, the greatly reduced risk of predation is more

advantageous (Sandt and Stoner, 1993; Stoner and Ray, 1993; Stoner, 2003). Juvenile conch were most

often found in densities of 0.1 to 0.2 per m2 (Stoner and Ray, 1993; Stoner et al., 1996a), but higher

densities have also been observed (Phillips et al., 2010).

A specific area can function as nursery ground for a long time. Investigation of shell remains of the

Queen conch showed that the distribution of the conch population near Lee Stocking Island in the

Exuma Cays, Bahamas, was constant over decades (Stoner and Ray, 1996). Stoner and Ray (1993) state

that a combination of mechanisms may underlie this phenomenon, such as (1) the unique habitat

characteristics of the area, (2) contagious settlement of larvae, (3) reduced predation risk in an

aggregation, and/or (4) the gregarious nature of juvenile conchs. The supply of larvae which are ready

for settlement also influences the spatial distribution of conch juveniles. The presence and number of

juvenile Queen conch are positively correlated with the supply of larvae (Stoner and Ray, 1993; Stoner

and Ray, 1996; Stoner, 2003; de Jesus-Navarrete and Valencia-Beltran, 2003). However, Stoner (2003)

mentioned that the settlement of these larvae is not so much associated to the habitat type, but more

to specific locations.

Stoner (2003) noticed that, although information on the preferred habitat of juvenile conch increases,

a prediction of the sites where Queen conch juveniles can be found remains very difficult. The

complexity of all variables is restricting, but more important is that juveniles at different sites prefer

different habitats. Furthermore, optimal conditions for juvenile growth are dependent on a complex

set of variables, and it is observed that not all locations with optimal conditions are also occupied by

conch (Stoner et al., 1996a).

Habitat characteristics

Infaunal juveniles are often found in shallow waters close to the shoreline or on sandbanks (Iversen et

al., 1986; Sandt and Stoner, 1993). De Jesus-Navarrete and Valencia-Beltran (2003) found that the

presence of infaunal juveniles was not associated with observed plant taxa such as Laurencia sp. or

Thalassia testudinum, but with coarser sediment type. Higher densities of juveniles with ≤60 mm shell

length were present in areas with medium sand and high organic matter. Juveniles were also observed

on plains and rubble reefs close to tidal channels and with strong water currents (Stoner et al., 1996b;

de Jesus-Navarrete and Valencia-Beltran, 2003). After approximately one year they migrate from sand

to a vegetation covered bottom (Sandt and Stoner, 1993). Epifaunal juveniles are primarily associated

with seagrass meadows of T. testudinum with an intermediate biomass of about 700 shoots/m2, and

also Syringodium filiforme is often present (Stoner and Waite, 1990; Ray and Stoner, 1995; Stoner et

al., 1996a; Stoner, 2003). Juveniles are also found on sites with a high percentage of algae, like reefs or

coral rubble (Stoner, 2003). Phillips et al. (2010) observed Dictyota sp. and turf algae as common flora

in a conch nursery area. Stoner et al. (1996a) reported a preferred depth ranging from 1.5 to 4 m. The

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supply of clean water from the ocean by tidal movements or water circulations seems to be another

important factor (Stoner and Ray, 1993).

Predation

The predation risk that the conch is exposed to varies with e.g. age (and related size) and habitat.

According to McCarthy (2007), the predation mortality of juvenile Queen conch is high. Older conch

are better protected than young ones because of their increased size (Iversen et al., 1986). The risk of

being crushed is for example reduced by becoming larger, with a most effective size of 90 mm (Ray

and Stoner, 1995). Also habitat can play a role in being protected, by providing a hiding place. Close to

the southern Berry Islands, Bahamas, conch with a size of 70 to 130 mm were found at one meter

depth or less, on flats or close to the shore. It was hypothesized that the conchs large size protected it

for shore crabs, and the habitat served as hiding place against larger swimming predators (Iversen et

al., 1986). In Lac Bay juveniles conchs are mainly observed in the deeper parts of the lagoon and Lott

(2000) suggested that this is an area where they are probably less vulnerable for predation.

Besides, not all attacks necessarily lead to death of the juvenile conch, as scars and damages were

found on shells of individuals from 75 mm and larger (Iversen et al., 1986).

Empty conch shells or shell remains

can give an insight in the types of

predation (Debrot, personal comm.),

because certain predators leave the

shells in a characteristic way. Key

predator of larger juvenile conchs is

the tulip shell (Fasciolaria tulipa),

which actively attacks the conch and

leaves an empty, undamaged conch

shell (Randall, 1964; Iversen et al.,

1986; Sandt and Stoner, 1993;

Debrot, personal comm.). However,

this species has never been observed

in Lac Bay. A number of empty,

undamaged shells near a hole in the

ground indicates predation by an

octopus (Engel, personal comm.). Human fishing or poaching activities can be recognized by a hole in

the shell (Engel, personal comm.) (fig. 1.2). A damaged empty shell can be the work of a spiny lobster

(Panulirus argus) or a hermit crab. These crustaceans bite off pieces of the shell in order to reach the

animal inside (Randall, 1964; Iversen et al., 1986). Empty shells can be occupied by hermit crabs

(Iversen et al., 1986). Drilling of the conch shells is characteristic for the predation by moon snails

(Natica sp. and Polinices sp.), as observed in the Bahamas (Iversen et al., 1986). Blue crabs (Callinectes

sapidus) and box crabs (Calappa gallus) peel open the shell in a specific pattern, but are known as well

for crushing the shell (Ray-Culp et al., 1999). Crushing is also done by xanthid crabs, which are a thread

for early juveniles in particular (Ray-Culp et al., 1999). Other, less observed predators that crush the

conch shell are turtle species, like the loggerhead turtle (Caretta caretta), which is active during the

night, and other species like the southern stingray (Dasyatis americana), spotted eagle rays (Aetobatis

narinari) and large Atlantic permit (Trachinotus falcatus) might predate on conch in this way (Randall,

1964; Iversen et al., 1986). According to Glazer (unpubl. data) the porcupine fish (Diodon hystrix) is a

voracious conch predator. A scavenging role is defined for cushion stars (Oreaster reticulatus); this

Figure 1.2 A hole in the shell of a juvenile conch as sign of poaching activities (picture: I. Willemse, Oct. 2012).

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species is only reported as feeding on conchs, but no active predation has been observed (Iversen et

al., 1986; Sandt and Stoner, 1993).

Population structure

Defining the structure of a conch population is a complex process. The morphology of the conch can

differ greatly between different habitats, even on small spatial scales, under influence of different

growth rates and food resources (Martinmora et al., 1995; Ray and Stoner, 1995; McCarthy, 2007).

The variation between different locations makes it difficult to age the conch directly (McCarthy, 2007),

and a long term process of tagging and measuring is necessary to determine the relation between size

and age for a specific population. An estimation of the mean shell lengths per year class of a juvenile

population in St. John, U.S. Virgin Islands, is 108 mm for one, 170 mm for two and 205 mm for three

year old conch (Berg, 1976). This is consistent with size ranges estimated for the conch population in

Lac Bay, namely 90 to 150 mm (1 year) and 150 to 240 mm (2 year) (Engel, 2008). Only the third year

class is different, as the minimum size for three year and older conchs in Lac Bay is suggested to be 240

mm.

Length-frequency distributions can give an insight in the occurrence of different sizes for a specific

location, whereas repetitions over time can provide knowledge on growth per year class or change in

population structure (Stoner and Ray, 1993; Engel, 2008). In a nursery area in Carlisle Bay, Barbados,

Phillips et al. (2010) found a juvenile population with a size range from 170 to 210 mm. As a result of

repeated measurements a mean growth rate of 0.3 mm/day for this size class could be calculated. In

Lac Bay, an increased number of conchs in the second year class was found in 2007 by Engel (2008)

and she also observed sexual mature conch, which were not found in 1999 by Lott (2000).

1.3 Research questions

In this study the spatial distribution of the population of juvenile Queen conch in Lac Bay, Bonaire was

investigated, to enable a characterization of specific important areas for this species in the lagoon. The

focus was on the first year class of the Queen conch population, which includes all juveniles with a

maximum length of 150 mm. To get insight in the spatial distribution of juveniles in Lac Bay, three

research questions were formulated:

1) What are the habitat requirements of juvenile Queen conch?

According to the literature, infaunal juveniles are thought to live buried in the coarse sediment of

shallow areas with a bare bottom, close to seagrass beds of T. testudinum. Epifaunal juveniles have

migrated to seagrass meadows of T. testudinum. The Queen conch is mainly associated with this

seagrass species, but also other plant species are mentioned. For Lac Bay it was investigated in which

areas juveniles occur and whether the presence of juveniles can be associated with other plant species

than T. testudinum, or with a combination of several species.

2) What is the length-frequency distribution of the population?

Previous studies showed a slight increase in the population of Queen conchs in Lac Bay. In this study

we have created a length-frequency distribution of the population of 2012, to see whether growth of

the population has continued in the last few years, the time in which the Queen Conch Restoration

Project was started.

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3) What are the main types of predation on juvenile Queen conch?

The third aim of this study was to get an impression of the ratio between natural predation and

poaching. Juveniles ≤150 mm were expected to be subject to predators as octopuses or crabs, as

larger juveniles would also be attractive for poaching.

With the statement of Stoner and Ray (1993) that ‘juvenile aggregations and nursery habitats must be

identified and protected because of vital importance for conch population dynamics and healthy

fisheries’ all the elements of this study come together. A better insight in the habitat requirements of

juvenile Queen conch in Lac Bay is a step further in the development of a more specific conservation

and protection program.

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2. Methodology

2.1 Study area

Lac Bay is a lagoon in the southeast of Bonaire (fig. 2.1), which covers an area of 7.5 km2. Coordinates

of Lac’s borders are at 12⁰ 07’ 35.6” (north), 12⁰ 05’ 30.3” (south), 68⁰ 14’ 30.3” (west), and 68⁰ 12’

51.1” (east). It is a legally protected Ramsar site and part of the Bonaire Marine National Park,

managed by the National Parks Foundation of Bonaire, STINAPA Bonaire. The shallow water of Lac Bay

is clear and is influenced by a strong, most often northeast wind (Moorsel and Meijer, 1993). The

depth varies between 0.5 and 6 meters. Maximum height difference in water level due to tidal

movements is 0.3 m (Wagenaar Hummelinck and Roos, 1969; Moorsel and Meijer, 1993). The water

temperature in the central bay slightly fluctuates around 29 ⁰C. The salinity varies between 35 and 37

ppt and the amount of dissolved oxygen between 4 and 8 mg/L (Hylkema and Vogelaar, 2012; personal

observ.).

Figure 2.1 Overview of Lac Bay, Bonaire (after Hylkema and Vogelaar, 2012).

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Different habitat types which can be recognized in Lac are algal beds, seagrass beds, bare sand, rubble,

fringing reef, mangrove pools, mangrove fringes and backwaters (Hylkema and Vogelaar, 2012). The

bay is surrounded by mangroves and this area forms a hypersaline environment which serves as a

nursery area for many fish species. Older life stages of these fishes may migrate to the reef, located at

the shallow sandy plateau (Awa Blanku) between the basin and the Caribbean Sea (Wagenaar

Hummelinck and Roos, 1969; Hylkema and Vogelaar, 2012). Between Awa Blanku and Lac Cai, a

former peninsula in the north, is an open connection with the sea, called Boca di Lac (Wagenaar

Hummelinck and Roos, 1969).

On Awa Blanku there is hardly any vegetation, except for some small seagrass beds in the southern

part of the plateau. The total biotic cover of the central bay is about 52% (Hylkema and Vogelaar,

2012), mainly consisting of patchy seagrassbeds with T. testudinum and S. filiforme (Moorsel and

Meijer, 1993; Hylkema and Vogelaar, 2012). Hylkema and Vogelaar (2012) also reported the presence

of the invasive seagrass species Halophila stipulacea.

2.2 Site selection

In Lac Bay 33 quadrants were surveyed, of which 24 quadrants were located in the central bay and 9

on the shallow plateau at the oceanside of the lagoon (fig. 2.2). Locations were chosen randomly as

well as based on recent observations of juvenile conch by Engel (in preparation). As the present study

was a reconnaissance survey priority was given to the areas where higher densities of juveniles were

expected, such as the central bay and the shallow plateau. Quadrants were as much as possible spread

over these areas. Quadrant locations were selected either in advance using Google Earth, or during the

fieldwork.

Figure 2.2 Quadrants surveyed in Lac Bay, Bonaire with all data obtained on the exact location (white) and water and sediment data obtained on locations nearby. Quadrants 5, 9, 18 and 31 were also surveyed at night. (Google earth © 2009, Image © 2013 TerraMetrics, Image © 2013 GeoEye)

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Four quadrants (5, 9, 18, 31) were also surveyed at night. These quadrants were selected during the

time of research, based on observations of a high number of juveniles ≤150 mm and on being located

in different areas of Lac Bay (fig. 2.2).

All quadrants were visited once (except for the quadrants which were visited a second time during the

night), between July and October 2012.

2.3 Site survey

At each location a quadrant of approximately 30x30m was put in place. Locations were reached with a

boat, using a Garmin GPS foretrex 301 device. At location, the first corner was marked with a buoy and

from there four transect lines forming a square were marked on the bottom with 30 m measuring

tapes. Each corner was then marked with a buoy. Coordinates of each corner were stored in the GPS

device, and subsequently used to calculate the surface area of each quadrant in Google Earth Pro. Two

to four persons performed a visual survey of each quadrant, using SCUBA or snorkelling gear. Night

surveys were done between 6:30 and 9:00 PM, using SCUBA, except for the first time when the survey

was conducted in shallow water and snorkelling gear was used.

For 14 quadrants the conch density together with vegetation, water and sediment data was obtained

at the exact location. For the other 19 quadrants only conch and vegetation data were from the site

itself, as the data on water variables and sediment were taken at a site from a sample grid covering Lac

Bay. Due to limited available time, for these quadrants the data of nearest grid points were used.

Distances from quadrant to grid point ranged from 16 to 168 m, with a mean of 85 (±51) m.

Conch measurements

For each quadrant all conchs, both alive and

dead, were taken into the boat and were

released back into the water after

completion of all measurements. Only very

old poached shells were left in the water.

The total length (TL), body length (BL) and

body depth (BD) (as shown in fig. 2.3) of

each juvenile conch ≤150 mm were

recorded. For all larger conchs only the TL

was recorded. Also the TL of empty shells

was measured, and the state of these shells

was noted. Categories of these shells were:

poached, (occupied by) hermit crab, (close

to shelter of) octopus, crushed, slightly damaged, and undamaged. Conchs that were collected during

the nightdives were not measured precisely, as this was difficult to perform during the night. Instead

of exact measurements, the conchs were categorized in the size classes ≤100 mm, >100 to ≤150 mm,

>150 to ≤200 mm and >200 mm.

Identification of plant species

The vegetation composition for each quadrant was determined through sampling 4 m2 of each

quadrant, a method which was also used by Hylkema and Vogelaar (2012). A PVC grid of 1 m2 was

placed four contiguous times in the northern or north-eastern corner of the quadrant. The PVC grid

Figure 2.3 Shell measurements: total length, body length and body depth. (Picture: Eddy Hardy [2], adapted)

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was divided into 100 squares, so for each algal and seagrass species the percentage cover could be

estimated in order to calculate the mean percentage cover. Only the species on the bottom and in

approximately 0.2 m up in the water column were taken into account. Identification of epiphytes on

seagrass was done as good as possible. Samples of unknown species were taken to the lab and

identified further with the help of identification guides (Littler et al., 1989; Littler and Littler, 2000).

Water variables

The following parameters were determined: salinity, dissolved oxygen, temperature and depth.

Salinity, dissolved oxygen and temperature were measured with a YSI Professional Plus handheld

multiparameter meter, approximately 0.3 m above the bottom. Depth was measured with a

Speedtech SM-5 Depthmate Portable Depth Sounder.

Depth and temperature were measured twice: on the day of the water measurements, and on the day

of conch and vegetation sampling. The latter were used in the further analysis, because these were

taken at the same date as the conch measurements. These measurements were done with a dive

computer (Suunto Gekko) to within one decimal (depth) and one degree (temperature) precision.

Sediment samples

Sediment samples were taken by using a PVC tube (diameter 119 mm, height 79 mm, volume 650 ml),

to collect a sample of the top layer of the sediment. Samples were stored in plastic cups. The sediment

was dried for 24-36 hours at 60˚C in a Precision Scientific 31619 oven. Sediment particle size analysis

was conducted using a Retsch AS200 sediment shaker (15 min, amplitude 60) with sieves of 2 mm, 1

mm, 500 µm, 250 µm, 125 µm and 63 µm (table 2.1). Each fraction was weighed on a Scout Pro Ohaus

SPE scale to within two decimals.

2.4 Data analysis

Microsoft Excel 2007 was used to store all data. To correct for the differences in total area per

quadrant, the number of living juveniles ≤150 mm was converted into the density of juveniles per m2

per quadrant.

In case of several unknown algal species or turf-like algae per quadrant, no information was available

about possible overlap in cover (species might be observed in the same squares of the sampling grid).

To avoid an overestimation of the percentage cover of unknown algal species, the individual highest

percentage cover of all unknown species was used in the analysis.

Sediment fraction Terminology

(based on Blott and Pye, 2012)

Abbreviation

(as used in this report)

≥2 mm gravel grav

1 - 2 mm very coarse sand vcsand

1 mm - 500 µm coarse sand csand

250 µm - 500 µm medium sand msand

125 µm - 250 µm fine sand fsand

63 µm - 125 µm very fine sand vfsand

≤63 µm silt silt

Table 2.1 Classification of sediment samples into particle sizes with corresponding terminology and abbreviations.

19

A multivariate analysis was conducted in Canoco for Windows 4.5 to gain insight in which abiotic

variables influence the distribution of plant species and juvenile conchs over the different areas of Lac.

The density of juveniles consisted of only one value per quadrant and could therefore not be used as

the dependent variable in the multivariate analysis. To enable a multivariate analysis with all the

different abiotic factors as explanatory variables, the distribution of all plant species was used as

dependent variable and the analysis was split up in two steps. First, a canonical correspondence

analysis (CCA) (Ter Braak, 1986) was conducted to summarize the effects of the abiotic variables on

the distribution of all plant species over the different quadrants. A direct analysis was required

because of the use of abiotic variables. Together with the existence of a long gradient in variances and

no necessity of detrending, CCA was chosen as direct method. The option ‘downweighting of rare

species’ was used to reduce the influence of rare plant species. The relevance of including each abiotic

variable was checked with the option ‘automatic forward selection’. Variables with a variance inflation

factor score higher than 20 (indicating a correlation with another variable already included in the

analysis) were excluded, which was the case with very fine sand (indicated with vfsand). The second

step was the addition of the density of juveniles in the ordination diagram displaying the distribution

of all quadrants. Finally, the distribution of juveniles could be compared with the distribution of plant

species and the influence of abiotic variables.

A paired samples t-test was conducted in SPSS 19.0 in order to test for a significant difference in

observed juveniles ≤150 mm between the day and night surveys. The test was performed on number

of living, dead and all juveniles.

The graph of the Queen conch population structure in Lac Bay is based on the total number of living

conchs, divided into size classes of 10 mm.

Predation was categorized into six causes of death, which are shown in percentage of the total number

of empty shells observed in this study.

20

21

3. Results

3.1 Juvenile distribution

In this study a total of 683 living conchs were found, of which were 175 juveniles ≤150 mm, in a

surveyed area of 30181 m2. The average total length was 122 (±25) mm, the total length of the

smallest individual was 74 mm. The mean juvenile density over the surveyed area is 0.0058/m2. The

density of juveniles ≤150 mm for each quadrant is shown in figure 3.1 (for exact values see appendix II

fig. A).

3.2 Habitat characteristics

Environmental variables

Sampling depths ranged between 0.5 and 5.0 m, and juveniles were found over the whole range of

depths. The water temperature during the study was nearly constant with a mean temperature of 29.7

⁰C. The mean salinity was 35.3 ppt. The amount of dissolved oxygen in the water layer just above the

bottom ranged between 4.1 and 7.8 mg/L, with a mean of 6.1 mg/L. It appears that higher oxygen

levels are associated with shallower water (appendix II fig. B). A summary of the abiotic variables is

displayed in table 3.1.

The partitioning of each sediment sample into seven fractions is shown in figure 3.2. A difference can

be seen between the bottom of different areas surveyed in Lac Bay. The bottom of the shallow reef

plateau consists of coarser sediment than the bottom of the central bay.

Figure 3.1 Calculated densities of juveniles ≤150 mm per quadrant. (Google earth © 2009, Image © 2013 TerraMetrics, Image © 2013 GeoEye)

22

In the 33 quadrants a number of 30 known plant species were found. Algal taxa that were most

abundant are Acantophora specifera, Dictyota sp., Halimeda decipiens, Halophila incrassata, H.

stipulacea, S. filiforme and T. testudinum. A yet unknown form of red cyanobacterial growth on the

sand (in Hylkema and Vogelaar, 2012 referred to as ‘Cyano brown’) was another locally abundant

taxon. Approximately 10 observed algal species could not be identified, as well as several so-called

turf-like algae which were too small to be identified further. The percentage cover of each taxon per

quadrant is shown in appendix I.

this study

mean (±SE)

min

max

Lott, 2000

mean

Hylkema and

Vogelaar, 2012

mean (±SE)

depth (m) 2.7 ±1.4 0.5 5.0 1.9 3.7±0.7

temp (⁰C) 29.7 ±0.9 28.0 31.0 29.5 28.9±0.4

salinity (ppt) 35.26 ±1.11 32.76 37.30 36.6 36.9±0.4

dissolved oxygen (mg/L) 6.07 ±1.07 4.07 7.84 - -

Patterns in plant taxa and juvenile densities

The explained variance of the multivariate analysis is 31.0% (sum of all canonical eigenvalues/sum of

all eigenvalues = 1.079/3.486). Figure 3.3 shows the diagram of the multivariate analysis summarizing

the distribution of plant taxa as explained by the abiotic variables. Figure 3.4 shows the corresponding

distribution of (a) all quadrants and (b) the density of juveniles ≤150 mm for each quadrant. The

lengths of the arrows indicate that depth and the different sediment fractions are important factors in

the explanation of the variation, whereas temperature is less important. Furthermore, an increasing

depth is associated with finer sediment.

Table 3.1 Mean, minimum and maximum values of the measurements of depth, temperature, salinity and dissolved oxygen on all sites, compared to the results of earlier studies. Values of Lott (2000) are based on surveys from June to September 2009 covering the same study area, values of Hylkema and Vogelaar (2012) are based on surveys from September to December 2011 (only data from central bay, none from shallow reef plateau available).

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

sed

ime

nt

par

ticl

e s

ize

fra

ctio

ns

quadrant

≤ 63 µm

63-125 µm

125-250 µm

250-500 µm

500 µm - 1 mm

1 - 2 mm

≥ 2 mm

central bay shallow plateau

Figure 3.2 The partitioning of the sediment sample of each quadrant into seven fractions.

23

Figure 3.3 Ordination diagram displaying the first two ordination axes of a canonical correspondence analysis summarizing influences of sediment particle size, temperature and depth on the distribution of plant species.

-1.0 1.0

-1.0

1.0

A spic

A cren

A tenu

A nigr

C cupr

C mexi

C sert

Cera sp

C cruc

C nite

Cham sp

Clad sp

Dasy sp Dyct sp

Ente sp

F indi

H deci

H incr

H stip

Laur sp

M boer

Padi sp

P capi

Poly sp

Rami

S fili

T abbo

T test

T turb

U flab

V vent

red sand

turf

unknowngrav

vcsand

csand

msand

fsand

siltdepth

temp

Figure 3.4 Ordination diagrams displaying the first two ordination axes of a canonical correspondence analysis summarizing (a) the distribution of quadrants based on the distribution of plant species explained by sediment particle size, temperature and depth, and (b) the density of juveniles ≤150 mm for each quadrant, displayed in a limited number of decimals (a value of 026 means 0.0026 juveniles per m

2).

-1.0 1.0

-1.0

1.0

grav

vcsand

csand

msand

fsand

siltdepth

temp

1

23

4

5

6

7

8

9

10

11

12

13

14

15 16

17

18 19

20

21

22

23

24

25

26

27

28

29

30 31

3233

a b -1.0 1.0

-1.0

1.0

grav

vcsand

csand

msand

fsand

siltdepth

temp

116

027

081

026

027

085

207

009

267

117

061

160

024

000

024012

000

115 123

198

020

015

007

000

020

098

000

000

008

000 000

000051

24

3.3 Day and night surveys

Four night surveys were performed in this study. For two night surveys the number of juveniles was

higher than during the day, whereas this number was lower for the other two night surveys (see fig.

3.5). A paired samples t-test showed that there was no significant difference in number of juveniles

≤150 mm observed at day and night, not for alive (t=-0.382, df=3, p=0.782), dead (t=1.530, df=3,

p=0.223) and all individuals (t=0.120, df=3, p=0.912).

3.4 Predation

A total number of 62 empty conch shells or shell remains were found (size range: 23-264 mm). Of

these 34 shells were ≤150 mm (size range: 23-147 mm). Shells were subject to different categories of

predation, namely poaching, predation by a hermit crab or octopus, or other (fig. 3.6). This study

shows that nearly 40% of the mortality is due to poaching, where only conchs larger than 150 mm are

subject to these activities. The remaining 60% show another, probably natural cause of death.

Potential predators that have been observed during the study period are spotted eagle rays (on Awa

Blanku), several crab species, octopuses, porcupine fishes and sea turtles (including loggerhead turtles,

Caretta caretta), and once a reef shark was seen.

3.5 Population structure

In this study a total number of 683 conchs was found in 33 quadrants. The mean conch density was

0.0226/m2, which is 5 times higher than the density as calculated in 2007 (table 3.2). The methodology

of data collection in 1999 and 2007 differs from the one used in 2012. This makes the comparison with

the data of 2012 difficult, as these data are an overestimation, due to the fact that only those areas

were investigated where juvenile conch were expected to be. The average size of all individuals was

172 (±39) mm. The Queen conch population structure in Lac Bay as seen in figure 3.7 shows a bell-

shaped curve, with a remarkable low number of conchs between 100 and 130 mm.

Figure 3.5 The number of living and dead juveniles ≤150 mm per quadrant found during the day and night surveys.

0

5

10

15

20

25

30

35

day night day night day night day night

5 9 18 31

nu

mb

er

of

juve

nile

s ≤1

50

mm

quadrant

dead

alive

25

year survey period density

(n/m2)

total number

of conchs

survey area

(m2)

1999 April - Nov 0.0021 111 51000

2007 July - Sept 0.0044 223 51000

2012 July - Oct 0.0226 683 30181

0

10

20

30

40

50

60

70

80

90

70

- 80

80

- 90

90

- 10

0

10

0 - 11

0

11

0 - 12

0

12

0 - 13

0

13

0 - 14

0

14

0 - 15

0

15

0 - 16

0

16

0 - 17

0

17

0 - 18

0

18

0 - 19

0

19

0 - 20

0

20

0 - 21

0

21

0 - 22

0

22

0 - 23

0

23

0 - 24

0

24

0 - 25

0

25

0 - 26

0

26

0 - 27

0

27

0 - 28

0

28

0 - 29

0

29

0 - 30

0n

um

be

r o

f in

div

idu

als

size category (mm)

1999

2007

2012

Figure 3.7 The length-frequency distribution of the Queen conch population in Lac Bay in 1999, 2007 and 2012. The population is divided into size categories of 10 mm. The methodology of data collection in 1999 and 2007 differs from the one used in 2012.

0%

5%

10%

15%

20%

25%

30%

35%

40%

poached hermit crab octopus crushed slightlydamaged

undamaged

pe

rce

nta

ge o

f p

red

ate

d c

on

chs

> 150 mm

≤ 150 mm

Figure 3.6 Percentage of shells per cause of death, both from total number of conchs and the juveniles ≤ 150 mm. Categories of predation are: poaching, predation by hermit crab or octopus, or otherwise. The last category is divided into: crushed, slightly damaged and undamaged shells.

Table 3.2 Summarized results of conch density surveys from Lott (2000), Engel (2008) and this study. The methodology of data collection in 1999 and 2007 differs from the one used in 2012.

26

27

4. Discussion

The results of this study give insight in the spatial distribution of the population of juvenile Queen

conch in Lac Bay. In the first paragraph the habitat characteristics for juveniles are discussed, followed

by paragraphs on respectively the length-frequency distribution of the population and main types of

predation.

4.1 Habitat characteristics

In this study we focussed on the identification of habitat requirements of juvenile Queen conch in Lac

Bay. The observed values of the environmental variables in the studied area of Lac Bay do not deviate

much from earlier measurements by Engel (2008) and Hylkema and Vogelaar (2012). Salinity and

dissolved oxygen levels were subject to tidal movements or fresh water supply through rain or inflow

from the surrounding land, but no specific information on this subject was available. Regarding depth,

no correction for tidal movements has been made, because the effect on conch distribution was

expected to be very small as the water level fluctuations were maximal 0.3 m.

The method of sediment characterization gives a quantitative insight in the distribution of sediment

types in Lac Bay, and is therefore more informative and more reliable than earlier data obtained with a

visual method (Engel, 2008; Hylkema and Vogelaar, 2012). Using this quantitative method, the coarser

fractions might be overestimated, and the finer fractions might be underestimated. Sediment particles

may not be completely detached during sieving because of sticky clumps in the samples; a longer

sieving time could reduce this uncertainty. Thereby, finer particles slipped back in the water easily

during sampling, together with the excess water that was poured off.

Not all algal taxa observed in this study were also reported by Hylkema and Vogelaar (2012), such as

Halimeda decipiens, Turbinaria turbinata, Champia sp., Enteromorpha sp. and Ramicrusta sp. Some of

these taxa occur only on the sandy plateau, an area that was not included in the study of Hylkema and

Vogelaar.

The sampling technique for identification of vegetation was based on the example of Hylkema and

Vogelaar (2012), but they did not extrapolate the sampling area to a larger area. In the present study,

4 m2 was used as a characterization of the vegetation composition of an area of approximately 30 by

30 metres, through which important plant species could have been missed. Distributing at least 4 m2

over the quadrant instead of one corner could already improve the sampling technique, because in

this way the patchiness of the vegetation cover would have been taken into account. Now a few notes

were available about plant species observed elsewhere in the quadrant, but these could not be used in

the multivariate analysis although the species could have been important. For quadrant 9 for example,

the quadrant with the highest juvenile density, most individuals were found next to plant species

(S. filiforme and H. incrassata) that were not observed in the species sampling area.

Insight in the role of each environmental variable and the relationship with juvenile densities was

obtained using multivariate analysis. Most of the quadrants with a high number of juveniles ≤150 mm

per m2 showed a positive trend with depth and the finer sediment fractions. These quadrants were

situated in the central part of the lagoon and were associated with several plant taxa. A few of these

species were observed only once or twice, and then only in one of the quadrants with high juvenile

densities; Avrainvillea nigricans, Champia sp., Dasya sp. and Polysiphonia sp. are therefore considered

less important. Plant species that showed a high percentage cover and therefore were more important

are H. stipulacea, S. filiforme, T. testudinum, the red cyanobacterial growth on the sand and also the

28

group of turf-like algal species. Turf algae were also observed in areas with high densities of juvenile

conch in Carlisle Bay, Barbados, by Phillips et al. (2010).

In Lac Bay, T. testudinum was present in all quadrants with a higher density of juveniles ≤150 mm, but

a high density of juveniles ≤150 mm does not automatically imply the presence of T. testudinum.

Higher densities of juveniles ≤150 mm were found in those areas in Lac Bay where at least a

combination of abundant species was found (table 4.1).

Table 4.1 Percentage cover of the combination of plant species that are associated with the distribution of

juveniles ≤150 mm in Lac Bay.

Quadrant percentage cover of most abundant plant species

H. stipulacea S. filiforme T. testudinum red cyanobact. turf species

1 93.5 - 5.3 - -

3 *1 88.3 10.3 - 84.0

6 - - 33.5 - 63.5

7 - - 19.5 - 0.8

9 - *2 39.0 34.5 -

12 - - 62.0 26.0 -

18 79.0 - 45.0 1.3 -

19 0.8 95.0 33.0 26.0 -

20 - 5.8 20.0 - -

*1 Also high percentage cover of H. stipulacea, but not reported because it was not present in

vegetation sample area. *2 In quadrant 16 juveniles ≤150 mm were mainly found in that part of quadrant

with high percentage cover of S. filiforme. This species was not reported because it was not present

in vegetation sample area.

Quadrants 1 and 20 are, according to the multivariate analysis, similar to most of the quadrants in the

central bay. They have high numbers of juveniles and have a combination of abundant plant species.

However, contrary to the other quadrants in that cluster, they were characterized by a high

percentage of larger sediment particles. In quadrant 1 an algal species from the rarely observed

Ceramium genus was present, and in quadrant 20 a relatively high number of unknown algal species

were observed.

Some quadrants, e.g. 2 and 5, are conform the characteristics of the cluster, but show low densities of

juveniles. Other, not in the analysis included factors, must be underlying the scarcity of juveniles.

Predation is plausible for quadrant 5, were almost 50 percent of the observed shells were empty, and

many of these were found next to an octopus shelter. Quadrant 4 is the deepest site of the survey, and

only here Caulerpa cupressoides was observed. Quadrant 24 differs because of a high percentage of

gravel and the single observation of Valonia ventricosa.

On the sandy plateau at the oceanside of Lac Bay, the quadrants are characterized by coarser

sediment and shallow waters. According to the multivariate analysis, the quadrants all have a

characteristic combination of plant taxa. Quadrant 30 and 31 are not remarkably different regarding

the vegetation composition, but the sediment at quadrant 30 is relatively finer, as at quadrant 31 it

contains an even higher percentage of large sediment particles than the other quadrants at the

plateau. Also quadrant 33 differs by a higher percentage of large sediment particles and the presence

of Cladophora species.

Lott (2001) and Engel (2008) studied the spatial distribution of the conch population in Lac Bay,

including juveniles of all size and also adults; it appears that high densities of juveniles ≤150 mm in our

study are observed in similar areas of the lagoon as where high conch densities were found in the

29

previous studies (appendix II fig. C). This means that juveniles ≤150 mm do not occupy specifically

different areas than larger conspecifics. Aggregation might occur in order to decrease the predation

risk, or in areas with sufficient availability of food. According to Stoner et al. (1998) simple habitat

maps are not sufficient in determining important habitats and predicting conch distribution. It is true

that, based on the environmental variables investigated in our study, no clear delineation can be made

of the spatial distribution of juveniles in Lac Bay, and it is recommended to include other variables

such as hydrography. But still, trends can be recognized in sediment type and combination of plant

species, and juveniles are mainly observed in the central region of the lagoon. Within this region

conchs move between different locations; as Engel (personal comm.) once said that if one day you find

a lot of conchs, a few days later you might find none at the exact same location.

Day and night surveys showed no significant difference between the numbers of observed juveniles

≤150 mm. However, as we can see in literature a significant difference is possible. Danylchuk et al.

(2003) observed a significant higher number of juveniles between 55 and 90 mm, during the night

compared to day surveys. In the present study the analysis was based on only four samples. Moreover,

day and night samples were not done within the same day, so juveniles could have migrated already

over longer distances. And, as in other studies such as Sandt and Stoner (1993), it was not easy to find

infaunal juveniles, especially because in the present study a method which disturbed the environment

as little as possible was used (e.g. no dredging or towing like de Jesus-Navarete and Valencia-Beltran,

2003). Juveniles observed in this study were not found smaller than 74 mm, but smaller conchs should

be present. Infaunal juveniles must have emerged from the sediment and preyed upon by octopuses,

as very small empty shells were observed in Lac Bay close to octopus shelters (Engel, personal comm.).

Smaller, infaunal juveniles could be found during night surveys, if a good and effective night sampling

method will be developed. Considering the method used in this study, one improvement would be if

sampling would take place in the morning and after sunset on the same day, using e.g. a visual

censuses technique. With this technique, a survey on the same location could be repeated in a short

time period, because the area would be less disturbed and the juveniles will not be stresses as much as

by taking them into the boat for measurements (English et al. 1994 as cited in Nagelkerken et al.

2000). Something else to consider before surveying on infaunal juveniles is the presence of calcareous

plateau underlying the lagoon. The sediment layer covering this plateau is at some places not very

thick (Lott, 2001; personal observ.), and therefore could play a role in the habitat choice. The sediment

layer must be thick enough for juveniles to be buried up to an adequate depth.

Consistent with findings of de Jesus-Navarrete and Valencia-Beltran (2003), it was expected that

infaunal juveniles would be found in areas with coarse sediment as this would be easier to be buried

in. But Danylchuk et al. (2003) suggested that coarser sediment might be more difficult for burying, as

they observed a higher number of infaunal juveniles in areas with fine sediment. This is interesting

because for Lac Bay it means that infaunal juveniles could be found in the same region as epifaunal

juveniles. Besides, this would imply that juveniles do not always move to a different area when

changing the infaunal for epifaunal state.

4.2 Population structure

The length-frequency distribution of the population in 2012 is displayed by a bell-shaped curve, and

shows a significant increase of the conch population in Lac Bay compared to 1999 and 2007. The curve

has also expanded at the extremes; especially smaller and a few larger conchs were observed in this

study. As sexual maturity is reached at a length of about 240 mm, it can be seen that the population

mainly consisted of juveniles. Remarkable is the sudden decrease in number of individuals from 100 to

30

130 mm. Compared to size ranges of year classes for Lac Bay suggested by Engel (2008), and assuming

that reproduction of the population in Lac Bay is almost year-round, these lower number can

represent juveniles from one season. Juveniles from this season might be exposed to adverse

conditions for reproduction or growth, such as a higher predation pressure or limited food resources.

Less recruitment, a lower growth rate or higher mortality might have caused a lower number of

juveniles for this season. Smaller but similar irregularities in population structure can also be seen in

the curves of 1999 and 2007.

The density of conchs in Lac Bay has increased about 5 times compared to the results of the study in

2007. However, this number must be handled very carefully, as this could induce false ideas about the

reaching of a large, healthy conch population. First, it is most likely that the density of conchs in 2012

is overestimated. Not the whole area of Lac is sampled in the present study; the focus was on areas

where more juveniles were expected. Besides, in 2012 only sixty percent of the area of the studies in

1999 and 2007 was surveyed. In the earlier studies also parts of the bay border, adjacent to the

mangrove area were included, an area where relatively less conchs were observed. A recalculation of

the densities only based on the number of observed conchs in the exact same study areas would give a

better comparison of the densities over the years. Moreover, for Lac Bay a mean conch density of

0.0226/m2 was calculated, but literature indicates significantly higher densities for the Caribbean

region. Values of 0.1-0.2/m2 are reported as typical densities for conch aggregations (Stoner and Ray,

1993) and even higher values of 0.55 up to 2.0/m2 are observed (Phillips et al., 2010; Stoner, 2003).

Juveniles ≤150 mm were observed in Lac Bay in an average density of 0.0058/m2. Danylchuk et al.

(2003) estimated mean densities of 0.07-0.2/m2 for juveniles of the same size range, in a nursery area

near South Caicos, British West Indies. Although the total number of conch in Lac Bay was not as high

as elsewhere in the Caribbean region, the increasing population, also as a result of the improved

management, proofs that Lac Bay is an increasingly important nursery area for the Queen conch.

4.3 Predation

Third aim of this study was to get an impression of the main predators on the Queen conch in Lac Bay,

as the availability of shelter to reduce predation risk is an important factor in habitat choice (Ray and

Stoner, 1995). The ratio between natural predation and poaching differed between juveniles ≤150 mm

and larger conspecifics. Juveniles ≤150 mm were only exposed to natural predation, as larger conchs

were mainly subject to poaching. Engel (in prep.) even report that 90 percent of mortality is due to

poaching. Given the high percentage of shells that has been poached, it remains a serious threat to the

Queen conch population, in spite of all anti-poaching campaigns. Important is that the results from this

study are based on personal observations on the selected sites, but during the time of research

poaching activities on very small juveniles in shallow areas of Lac Bay (Awa Blanku and near Sorobon)

were reported by other people (appendix II fig. D, a and b). This happened at the same time of the

Regatta event, an annual international sailing festival which attracts many tourists. Secondly, poached

juveniles were encountered quite often in Lac Bay, and in 2012 these were collected at Cai (appendix II

fig. D, c and d). A random sample of this collection recently poached juveniles showed a shell sizes

between 110 and 180 mm (with a mean size of 146 mm). It shows that also small juveniles do not

escape human poaching activities.

The number of crushed shells was an estimation based on shell fragments that were found, but this

estimation is uncertain for it cannot be said whether these fragments originate from one or more

shells.

31

5. Conclusion and recommendations

Aim of this study was to get an understanding of the spatial distribution of juvenile Queen conch

smaller than or equal to 150 mm (first year class) in Lac Bay, Bonaire, and thus to enable an

identification of specific important areas for the species in this lagoon. This study showed that a

pattern could be recognized in the distribution of epifaunal juveniles over Lac Bay. Highest juvenile

densities occur in the seagrass beds in the central part of the lagoon, where the bay is deepest and the

sediment has a fine structure. Aggregations of juveniles move between different areas within the

central bay. The distribution of juveniles ≤150 mm appeared to be similar as the distribution of larger

conchs, as investigated in earlier studies in this bay. Also in Lac Bay, high juvenile densities are

associated with the presence of the seagrass Thalassia testudinum, but always in combination with

species as Halophila stipulacea and Syringodium filiforme, turf-like algae and a yet unknown red

cyanobacteria seems to play a role as well.

The increase of the population of Queen conch in Lac Bay has continued over the last decade, both in

total number and in size range as more and smaller juveniles are present. Poaching activities are the

main cause of death for the total population of Queen conch in Lac Bay. Although juveniles ≤150 mm

are mainly subject to natural predation, they do not escape poaching. A continuation of the

restoration project, which includes conservation and awareness raising, is therefore important for a

vital conch population in Lac Bay. In this context, further research focussing on the aggregation

dynamics of juveniles in Lac Bay would also be useful. Understanding of aggregation patterns and

spatial movements of juveniles is important for a more adequate protection, as aggregations are

attractive for poachers.

As in many other studies, observations of infaunal juveniles remained difficult. The development of an

efficient and adequate method for night surveys is crucial for gaining insight in the occurrence and

distribution of infaunal conch. Important are the frequency and timing of the day and night surveys.

Particle size of the sediment might be a major factor in the distribution of these juveniles, but it is still

uncertain what type is preferred. If it appears to be true that infaunal juveniles prefer smaller

particles, it is possible that infaunal conchs live in the same areas as epifaunal conchs, and that habitat

migration occurs only on a very small scale. Furthermore, the thickness of the sediment layer on the

calcareous plate can give an idea about the location of infaunal juveniles.

32

33

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Strombus gigas from alacranes reef versus Chinchorro bank of Mexico. Proceedings Gulf and

Caribbean Fisheries Institute, Vol. 54 (Ed, Creswell, R. L.), pp. 202-225.

Berg, C. J. (1976) Growth of the Queen conch Strombus gigas, with a discussion of the practicality of its

mariculture. Marine Biology, 34, 191-199.

Blott, S. J. and Pye, K. (2012) Particle size scales and classification of sediment types based on particle

size distributions: Review and recommended procedures. Sedimentology, 59, 2071-2096.

Danylchuk, A., Rudd, M. A., Giles, I. and Baldwin, K. (2003) Size-dependent habitat use of juvenile

queen conch (Strombus gigas) in East Harbour Lobster and Conch Reserve, Turks and Caicos

islands, BWI. Proceedings of the Annual Gulf and Caribbean Fisheries Institute, Vol. 54 (Ed,

Creswell, R. L.) Gulf Caribbean Fisheries Inst Gcfi, Ft Pierce, pp. 241-249.

Davis, M. (2005) Species profile: Queen conch, Strombus gigas. Southern Regional Aquaculture Center,

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de Jesus-Navarrete, A. and Valencia-Beltran, V. (2003) Abundance of Strombus gigas zero-year class

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Engel, M. S. (2008) Results of survey Lac Bay, Bonaire for Queen conch (Strombus gigas) and seagrass

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Iversen, E. S., Jory, D. E. and Bannerot, S. P. (1986) Predation on Queen conchs, Strombus gigas, in the

Bahamas. Bulletin of Marine Science, 39, 61-75.

Littler, D. S. and Littler, M. M. (2000) Caribbean reef plants, Offshore Graphics Incorporated,

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guide from Florida to Brazil, Airlife Publications, Shrewsbury, UK.

Lott, C. E. (2000) Research and monitoring results for the size class distribution and abundance of the

Queen conch, Strombus gigas, and seagrass characterization in Lac Bay, Bonaire. Environics

N.V. report for STINAPA. STINAPA Marine National Park, Bonaire, pp. 57.

34

Lott, C. E. (2001) Lac Bay then and now: a historical interpretation of environmental change during the

1900s - A site characterization of Lac Bay for resource managers and naturalists. Environics

N.V. report for STINAPA. STINAPA Marine National Park, Bonaire, pp. 185.

Martinmora, E., James, F. C. and Stoner, A. W. (1995) Developmental plasticity in the shell of the

Queen conch Strombus gigas. Ecology, 76, 981-994.

McCarthy, K. (2007) A review of Queen conch (Strombus gigas) life-history. In Complete Stock

Assessment Report 1, National Marine Fisheries Service, Southeast Fisheries Science Center,

Miami, pp. 8.

Moorsel, G. v. and Meijer, A. J. M. (1993) Base-line ecological study van het Lac op Bonaire. Bureau

Waardenburg bv, Culemborg, the Netherlands, pp. 168.

Nagelkerken, I., Dorenbosch, M., Verberk, W., de la Moriniere, E. C. and van der Velde, G. (2000)

Importance of shallow-water biotopes of a Caribbean bay for juvenile coral reef fishes:

patterns in biotope association, community structure and spatial distribution. Marine Ecology

Progress Series, 202, 175-192.

Phillips, M. A., Bissada-Gooding, C. E. and Oxenford, H. A. (2010) Preliminary investigation of the

movements, density, and growth of juvenile Queen conch in a nursery area in Barbados.

Proceedings of the Gulf and Caribbean Fisheries Institute, Vol. 63 San Juan, Puerto Rico, pp.

427-434.

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Marine Science, 14, 246-295.

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gastropods: the role of prey size, prey density, and habitat complexity. Journal of

Experimental Marine Biology and Ecology, 240, 303-321.

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gastropod, Strombus gigas. Marine Ecology Progres Series, 123, 83-94.

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Strombus gigas - patterns and potential mechanisms. Fishery Bulletin, 91, 516-525.

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the large-scale distribution of juvenile Queen conch in seagrass meadows. Bulletin of Marine

Science, 58, 217-233.

Stoner, A. W., Ray-Culp, M. and O'Connell, S. M. (1998) Settlement and recruitment of Queen conch,

Strombus gigas, in seagrass meadows: Associations with habitat and micropredators. Fishery

Bulletin, 96, 885-899.

35

Stoner, A. W. and Ray, M. (1993) Aggregation dynamics in juvenile Queen conch (Strombus gigas) -

population-structure, mortality, growth, and migration. Marine Biology, 116, 571-582.

Stoner, A. W. and Ray, M. (1996) Shell remains provide clues to historical distribution and abundance

patterns in a large seagrass-associated gastropod (Strombus gigas). Marine Ecology Progress

Series, 135, 101-108.

Stoner, A. W., Ray, M., Glazer, R. A. and McCarthy, K. J. (1996b) Metamorphic responses to natural

substrata in a gastropod larva: Decisions related to postlarval growth and habitat preference.

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relative to seagrass standing crop. Fishery Bulletin, 88, 573-585.

Ter Braak, C. J. F. (1986) Canonical Correspondence-Analysis - A new eigenvector technique for

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Web sources:

[1] Bower-Dennis, B. (n.d.), Queen conch life cycle, from

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[2] Hardy, E. (n.d.), Lobatus (Eustrombus) gigas, http://www.gastropods.com/7/Shell_217.shtml

(visited August 23, 2012)

36

37

Appendix I - Percentage cover for each plant taxa per quadrant

qu

adran

t

Aca

nth

op

ho

ra

spicifera

Aceta

bu

laria

crenu

lata

An

otrich

ium

tenu

e

Avra

invillea

nig

rican

s

Ca

ulerp

a

cup

ressoid

es

Ca

ulerp

a

mexica

na

Ca

ulerp

a

sertula

roid

es

Cera

miu

m sp

.

Cera

miu

m

crucia

tum

Cera

miu

m

niten

s

Ch

am

pia

sp.

Cla

do

ph

ora

sp.

Da

sya sp

.

Dyctio

ta sp

.

Entero

mo

rph

a

sp.

Feltma

nn

ia

ind

ica

Ha

limed

a

decip

iens

A sp

ic

A cren

A ten

u

A n

igr

C cu

pr

C m

exi

C sert

Cera sp

C cru

c

C n

ite

Ch

am sp

Clad

sp

Dasy sp

Dyct sp

Ente sp

F ind

i

H d

eci

1 47.8 - - - - - - 16.8 - - - - - 4.3 - - -

2 - - - 0.8 - - - - - - - - 4.3 5.3 - - -

3 2.0 - - - - - - - - - 1.8 - - 0.5 - - -

4 0.5 0.3 - - 0.5 - - - - - - - - - - - -

5 51.0 - - - - - - - - - - - - 19.5 - - -

6 42.5 - - - - - - - - - - - - 4.3 - - -

7 0.8 - - 11.3 - - - - - - - - - 58.0 - - -

8 - - - - - - - - 31.3 - - - - 33.5 - - 0.5

9 2.3 - - - - - - - - - 20.3 - 0.8 - - - -

10 - - - - - - - - - - - - - 67.0 - - 0.3

11 0.3 - - - - - - - - - - - - 41.0 - - -

12 1.3 - - - - - - - - - - - - 33.0 - - -

13 - - - - - - - - - - - 0.8 - - - - -

14 - 0.3 - - - - - - - - 0.3 - - 2.0 - - -

15 - - - - - - - - - - - - - 34.3 - - -

16 - - 41.0 - - - - - - - - - - - 39.5 - -

17 - - - - - - - - - - - - - - - - -

18 - - - - - - - - - - - - - 0.5 - - -

19 - - - - - - - - - - - - - 3.5 - - -

20 0.8 - - - - - - - - - - - - 0.3 - - -

21 - - - - - - - - - - - - - - - - -

22 - - - - - - - - - - - - - - - - 55.3

23 - - - - - - - - - - - - - - - - -

24 - - - - - 0.5 - - - - - - - 42.0 - - 26.5

25 17.8 - - - - - - - - - - - - 1.3 - - -

26 - - - - - - 26.5 - - - - - - 57.3 - - 8.8

27 - - - - - 0.5 - - - - - - - 0.3 - - 3.0

28 - - - - - - - - - - - - - 10.8 - 3.0 13.5

29 3.8 - - - - 2.0 - 0.3 - - - - - 2.3 - - 0.3

30 0.3 - - - - - - - 1.0 - - - - 0.3 - - -

31 - - - - - - - - - - - - - 2.5 - - 3.3

32 1.3 - - - - - - - 5.5 1.8 - - - - - - -

33 - - - - - - - -

- - 12.0 - - - - -

38

Appendix I - Percentage cover for each plant taxa per quadrant, continued

qu

adran

t

Ha

limed

a

incra

ssata

Ha

lop

hila

stipu

lacea

Lau

rencia

sp.

Micro

dictyo

n

bo

ergesen

ii

Pa

din

a sp

.

Pen

icillus

cap

itatu

s

Po

lysiph

on

ia

sp.

Ra

micru

sta sp

.

Syring

od

ium

filiform

e

Tao

nia

ab

bo

ttian

a

Tha

lassia

testud

inu

m

Turb

ina

ria

turb

ina

ta

Ud

otea

flab

ellum

Va

lon

ia

ventrico

sa

cyano

bacteria

(red) o

n san

d

turf-like

algae

species

un

kno

wn

algae

species

H in

cr

H stip

Laur sp

M b

oer

Pad

i sp

P cap

i

Po

ly sp

Ram

i

S fili

T abb

o

T test

T turb

U flab

V ven

t

red san

d

turf

un

kno

wn

1 - 93.5 - - - - - - - - 5.3 - - - - - -

2 0.3 0.8 - - - - - - 100.0 - 45.0 - - - 90.8 - -

3 - - - - - - - - 88.3 - 10.3 - - - - 84.0 -

4 - - - - - - - - - - - - - - - 4.0 -

5 2.3 - - - - 0.3 - - - - 52.3 - - - - 55.5 -

6 4.5 - - - - - - - - - 33.5 - - - - 63.5 -

7 1.0 - - - - - - - - - 19.5 - - - - 0.8 -

8 21.5 - - - - - - - - - 44.5 - - - - - -

9 - - - - - - - - - - 39.0 - - - 34.5 - -

10 46.3 - - - - - - - - 6.0 55.3 - - - - - -

11 2.8 - - - - - - - - - 35.8 - - - - - -

12 5.3 - 1.3 - - - - - - 2.0 62.0 0.3 - - 26.0 - -

13 - 96.5 - - - - - - - - - - - - - 90.3 -

14 0.3 - - - - - - - - - 4.3 - - - - 21.3 -

15 - 86.5 - - - - - - - - 43.5 - - - 0.8 - -

16 - 82.5 - - - - - - - - 4.0 - - - - 2.8 -

17 - 99.3 - 4.5 - - - - - - 9.3 - - - - 99.3 17.8

18 - 79.0 - - - - 3.5 - - - 45.0 - - - 3.5 - -

19 - 0.8 - - - 0.8 - - 95.0 - 33.0 - - - 1.3 - -

20 0.5 - - - - - - - 5.8 - 20.0 - - - - - 20.0

21 36.3 2.8 - - - - - - 96.3 - 85.0 - - - - 31.3 -

22 - - - - - - - - 0.0 - 99.3 - - - - 99.3 -

23 - 84.3 - - - - - - - - 84.3 - - - - 93.3 -

24 11.5 - - - - - - - - - 6.5 - - 0.8 5.3 - -

25 0.5 - - - - - - - 17.5 - 8.5 - - - - - 4.3

26 2.5 - 9.8 - - 5.3 - 1.0 - - - - - - - - 47.3

27 - - - - - - - 2.8 - - - - 0.5 - 1.5 41.0 -

28 - - - - 0.8 - - - - - - - - - - - -

29 0.3 - - - - - - - 0.3 - 3.3 - - - - 4.0 -

30 - - - - - - - - - - - - - - - 1.8 -

31 - - - - - 0.5 - - - - - - - - - - 21.5

32 - - - - - - - - - - 26.8 - - - - 5.8 -

33 - - - - - - - - - - 55.3 - - - - - -

39

0

1

2

3

4

5

6

7

8

9

0.0 1.0 2.0 3.0 4.0 5.0 6.0

dis

solv

ed

oxy

gen

(m

g/L)

depth (m)

Figure A Densities of juveniles ≤150 mm as observed in each quadrant.

Figure B Graph displaying observed levels of dissolved oxygen levels at different depths in Lac Bay.

Appendix II - Additional figures

0.000

0.005

0.010

0.015

0.020

0.025

0.030

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

juve

nile

de

nsi

ty (

juve

nile

s/m

2 )

quadrant

40

Figure C Left overview shows Queen conch distribution patterns in Lac Bay in 1999 (Lott, 2000), at the right Queen conch distribution patterns of both 1999 and 20007 are shown (Engel, 2008), all displayed in number of living conchs per grid.

Figure D Very small juveniles that were subject to poaching in the time of this research (a, b) and a collection of poached juveniles at Lac Cai (c, d). (pictures a,b: D. Sint Jago, Oct. 2012 / c,d: I. Willemse, Oct. 2012)

41