spatial distribution of juvenile queen conch (lobatus gigas · 2018. 8. 31. · spatial...
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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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34
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35
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(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