Stoner, A.W.

English

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835
THE ROLE OF LARVAL SUPPLY IN THE POPULATION
DYNAMICS OF QUEEN CONCH AND THE NEED FOR
METAPOPULATION ANALYSIS
ALLAN W. STONER
Caribbean Marine Research Center
805 E. 46th Pl.
Vero Beach, Florida 32963, USA
INTRODUCTION
The large gastropod Strombus gigas (queen conch) represents one of the
most important fisheries of the Caribbean region with a total annual value of
approximately $40 million US (Appeldoorn, 1994). Queen conch stocks have
declined significantly throughout the region over the last 10-20 years, and various
forms of catch and size limits have been imposed in most nations (Appeldoorn, et
al., 1987; Berg and Olsen, 1989; Appeldoorn, 1994). Despite complete closure of
the fishery in the United States in 1985, queen conch stocks have shown little sign
of recovery (Berg and Glazer, in press). This lack of recovery is poorly understood,
in part because of limited knowledge of early life history, larval abundance, and
recruitment processes.
Detailed larval descriptions for identifying larvae of the different Strombus
species (Davis, et al., 1993) and the first analyses of veliger abundance (Stoner, et
al., 1992; Posada and Appeldoorn, 1994; Stoner, et al., 1994) appeared only
recently. The recruitment problem is compounded by the fact that queen conch
larvae spend approximately 3 weeks in the water column and may drift hundreds
of kilometers from parental stocks before settling to the benthos. As a result, local
populations are probably replenished from distant sources, and stock management
for queen conch is a multinational problem (Berg and Olsen, 1989).
This report represents a preliminary analysis of the relationship between
larval supply and population distribution and year-class strength. Full analyses are
underway and will be reported in the future (e.g. Stoner, et al., in review). Because
of important correlations recently observed between larval supply and population
size in queen conch, it is critical to consider population dynamics in terms of larval
ecology and metapopulation theory. These concepts are introduced.
METHODS AND MATERIALS
Plankton collections were made in queen conch nursery sites near Lee
Stocking Island in the southern Exuma Cays, Bahamas, and in the Florida Keys
during the primary reproductive season (June through September). In the Bahamas
collections have been made at several nurseries since 1988, and in Florida since
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
836
1992. All larval collections were made by towing a simple conical plankton net
(202 micron mesh, 0.5 m diameter) behind a small boat at approximately 100
cm/sec. All collections were made in the near-surface water during the daytime.
Strombus species were sorted from the samples using a dissecting microscope,
identified according to Davis, et al. (1993), and measured for shell length. Tow
volumes were determined with a calibrated flow meter, and conch larval densities
were standardized to number/10 m3.
RESULTS
Concentrations of queen conch veligers in the Florida Keys were generally
low, with mean seasonal densities < 0.91 conch larvae/10 m3 at four nursery sites
in 1992. Concentrations in 1993 (0.007-0.67 larvae/10 m3) were similar to 1992,
except one mid-July collection in Looe Key National Marine Sanctuary yielded 161
larvae/10 m3, all very small individuals. Larvae in the Florida Keys occur in two
primary size classes, newly hatched larvae representing local reproductive stock,
and late-stage, competent larvae (near 1.0 mm shell length) that occur sporadically
along the reef tract in apparent association with Florida Current meanders.
Concentrations of conch veligers in nursery areas near Lee Stocking Island, Exuma
Cays, normally averaging 1-4 veligers/10 m3 for the reproductive season, tended
to be higher and more consistent through time than those in the Florida Keys.
Correlations between larval supply and juvenile population size occurred
on a spatial scale (across different nursery habitats), as well as on a temporal scale
(between different years). When larval concentration was examined at nine nursery
sites in the Florida Keys and near Lee Stocking Island, where numbers of juvenile
conch ranged from a few hundred to > 100,000 individuals, there was a direct
positive correlation between juvenile population size and the mean concentration
of larvae at each site. Furthermore, five years of sampling at one nursery site near
Lee Stocking Island revealed a positive, linear relationship between the mean
concentration of queen conch veligers and juvenile population size two years later.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
837
Fig
ure
1: Conceptual model of metapopulation dynamics. The model assumes a
general circulation of water carrying larvae from a Population 1 to 2 to 3. See
text for definition of the model parameters.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
838
DISCUSSION
Correlations between larval supply and juvenile population size over both
spatial and temporal scales, along with data from transplant experiments to test
juvenile growth (Stoner and Sandt, 1991; Stoner, et al., 1994; Ray and Stoner,
1994), suggest that populations of queen conch are recruitment limited, not habitat
limited. Larval limitation implies that pre-settlement phenomena, such as mortality
during planktonic stages and larval transport may be critical to population dynamics
in queen conch. The positive relationship between larval supply and population size
suggests that we need to understand transport processes and the mechanisms
affecting larval supply to nursery grounds in order to understand recruitment
process and year-class strength. The role of larval supply is frequently debated for
commercially significant species, and relatively few data are available on the
relationship between larval supply and population size for any species. For queen
conch larval supply appears to be critical.
Because conch populations appear to be heavily dependent upon larval
supply, it is particularly crucial to determine the sources of larvae and to conserve
reproductive adults in upsteam areas. Biochemical data for queen conch indicate a
high degree of gene flow among Caribbean populations (Mitton, et al. 1989;
Campton, et al., 1992), undoubtedly maintained by drift of pelagic larvae. Although
little is known about the abundance and transport of larvae of any species in the
Caribbean region, it is unlikely that the reproductive sources for most populations
in the region are local. This “open” nature of the populations requires that
population dynamics be considered from a metapopulation perspective (see Gilpin
and Hanski, 1991). In the theoretical model presented in Figure 1 there are three
subpopulations connected by larval transport. Population 1 is maintained by local
recruitment (RL) and has no recruitment by immigration from other sources (RI).
Reproductive (larval) output from Population 1 is greater than local mortality (M),
and some of that output is exported to downstream populations (E). In
metapopulation terminology, this population is a ‘Source’. Populations 2 and 3 are
downstream from Population 1 and receive larvae both from local spawners (RL)
and from upstream sources (RI). By definition, Population 3 is a ‘Sink’ because
reproductive output is less than local mortality, and most larval production is lost
from the system. Population 2 is a ‘Source’ for Population 3, but may also be a sink
depending upon the relationship between RL and M2.
Examples of ‘Sources’ and ‘Sinks’ can be seen, or at least hypothesized,
in the Caribbean region. The Windward Islands are probably ‘Source’ locations,
analogous to Population 1 in the model because of the general east-to-west
circulation of surface waters through the Caribbean Sea. In the eastern Caribbean,
populations of queen conch and other species with pelagic larvae must be
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
839
maintained by local recirculation patterns. Island-scale self-recruitment mechanisms
have been discussed in general by Farmer and Berg (1989), and more specifically
for Bermuda (Schultz and Cowan, 1994) and Barbados (Cowan and Castro, 1994)
which are probably dependent upon local retention of fish larvae. Florida Keys
populations may receive larvae from local spawning populations by retention in the
Pourtales or Tortugas Gyres (Yeung and McGowan, 1991; Lee, et al., 1992).
However, these retention mechanisms appear to be relatively inefficient, and the
Keys may actually be a ‘Sink’ with heavy dependence upon upstream sources of
larvae. This is supported by the fact that late-stage queen conch larvae are sparse
in the shelf waters of the Florida Keys but abundant in the Florida Current.
Competent larvae arrive along the reef tract in association with meanders of the
Current (Stoner, et al., in review) and indicate that recruitment from the Caribbean
may be important to population maintenance in the Keys. Important
conch-producing locations such as Belize and Pedro Bank are probably more
analogous to Population 2 in the model, with characteristics of both ‘Sources’ and
‘Sinks’.
Position within the metapopulation structure can have important
management consequences. For example, a source population will be highly
vulnerable to recruitment overfishing and emphasis needs to be placed on
maintaining an effective and sustainable reproductive stock quality. Downstream
populations are also dependent upon larvae from these source populations. A
sink-type population is more susceptible to management practices occurring in the
upstream source locations than effected by local management practice. Recovery
of depleted stocks requires an adequate source of larvae which may or may not be
local. For these reasons, a strong effort should be made to identify the sources of
larval recruitment for target populations, and stock management should be based
upon the associated metapopulation structure. In the Caribbean region, management
of the queen conch resource must be considered a multinational issue (Berg and
Olsen,1989).
LITERATURE CITED
Appeldoorn, R.S. 1994. Queen conch management and research: status, needs, and
priorities. Pages 301-319 in R.S. Appeldoorn and B Rodriguez Q., eds.
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Roques, Caracas, Venezuela.
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demersal resources of Puerto Rico and the lesser Antilles region. in R.
Mahon, ed. Report and proceedings of the expert consultation on shared
fishery resources of the lesser Antilles region. FAO Fish. Rep. 383:
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840
36-106.
Berg, C.J., Jr. and R.A. Glazer. Stock assessment of a large marine gastropod
(Strombus gigas) using randomized and stratified towed-diver censusing.
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conch (Strombus gigas) fisheries in the Caribbean. Pages 421-442 in J.F.
Caddy, ed. Marine invertebrate fisheries: their assessment and
management. Wiley, New York.
Campton, D.E., C.J. Berg, Jr., L.M. Robison, and R.A. Glazer. 1992. Genetic
patchiness among populations of queen conch, Strombus gigas, in the
Florida Keys and Bimini. Fish. Bull., U.S. 90: 250-259.
Cowan, R.K. and L.R. Castro. 1994. Relation of coral reef fish distribution to
island scale circulation around Barbados, West Indies. Bull. Mar. Sci. 54:
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Posada, J. and R.S. Appeldoorn. 1994. Preliminary observations on the distribution
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15-28.
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distribution of queen conch nursery habitats: implications for stock
enhancement. Pages 169-189 in R.S. Appeldoorn and B Rodriguez Q.,
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Los Roques, Caracas, Venezuela.
Stoner, A.W. and V.J. Sandt. 1991. Experimental analysis of habitat quality for
juvenile queen conch in seagrass meadows. Fish. Bull., U.S. 89: 693-700.
Stoner, A.W., V.J. Sandt, and I.F. Boidron-Metairon. 1992. Reproductive activity
of queen conch, Strombus gigas, and the abundance of veligers. Fish.
Bull., U.S. 90: 161-170.
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distribution of phyllosoma larvae of Panulirus, Scyllarus and Scyllarides
in the Florida Keys in May-June, 1989. Bull. Mar. Sci. 49: 699-714.


A comparison of larval development, growth, and shell morphology in three Caribbean Strombus species.

Circulation around islands, gene flow, and fisheries management.

Conservation and management of queen conch (Strombus gigas) fisheries in the Caribbean.

Differences in inshore-offshore and vertical distribution of phyllosoma larvae of Panulirus,

Experimental analysis of growth and survivorship in a marine gastropod aggregation: balancing growth with safety in numbers.

Experimental analysis of habitat quality for juvenile queen conch in seagrass meadows.

Genetic patchiness among populations of queen conch,

Influence of Florida Current, gyres and wind-driven circulation on transport of larvae and recruitment in the Florida Keys coral reefs.

Large-scale distribution of queen conch nursery habitats: implications for stock enhancement.

Metapopulation dynamics: empirical and theoretical investigations.

Population structure, larval dispersal, and gene flow in the queen conch, Strombus gigas, of the Caribbean.

Preliminary observations on the distribution of Strombus larvae in the eastern Caribbean. Pages 191-199

Queen conch management and research: status, needs, and priorities.

Recruitment of coral-reef fishes to Bermuda: local retention or long-distance transport?

Relation of coral reef fish distribution to island scale circulation around Barbados, West Indies.

Reproductive activity of queen conch, Strombus gigas, and the abundance of veligers.

Review of shared demersal resources of Puerto Rico and the lesser Antilles region.

The role of larval supply in the population dynamics of queen conch and the need for metapopulation analysis

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The role of larval supply in the population dynamics of queen conch and the need for metapopulation analysis

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