Interaction of Physical and Biological Factors in the Large-Scale Distribution of Juvenile Queen Conch in Seagrass Meadows

Large-scale distribution of a large, commercially significant gastropod, Strombus gigas (queen conch), was investigated in a II ,OOO-haregion of the Great Bahama Bank near Lee Stocking Island, Exuma Cays. Maps of depth and seagrass biomass, generated with Landsat thematic mapper data, and a 4-year survey of juvenile conch distribution showed that most of the juveniles were in aggregations located in 1.5-4.0-m water depth. Although general locations of juvenile conch aggregations remained the same between 1989 and 1992, their total surface area occupied only about 1.5% of the 8,300 ha of seagrass habitat available. Locations of only the most persistent long-term aggregations could be predicted on the basis of preferred seagrass biomass (30-80 g dry weight m-2); however, important conch nurseries were always located in tidal channels which brought clear, oligotrophic water from the Exuma Sound. Harmonic analysis of water temperature data from sites with and without juvenile aggregations showed that conch nurseries were subject to flushing with oceanic water on every tide, whereas non-conch sites reflected only diurnal heating and cooling of bank water. Relationships between circulation and juvenile conch distribution on the Great Bahama Bank may be related directly to larval recruitment, or indirectly to aspects of nutrient cycling and food production; evidence for both mechanisms exists. Although exact locations of conch aggregations shift from year to year, these shifts appear to occur within larger nursery habitats, the boundaries of which are set by a precise combination of physical and biological factors. Because most meadows are probably unsuitable for this severely ovcrfished species, critical nursery habitats should be identified and protected.