High-density aggregation in queen conch Strombus gigas: formation, patterns, and ecological significance

Abstract

Queen conch Strombus gigas L. , an important fisheries species, is often found in large aggregations of very high density (20 to 300 conch m-2), which move over the bottom in seagrass meadows of the Bahamas. Over the 6 yr period from 1987 to 1992, 25 high-density aggregations were observed in the southern Exuma Cays, Bahamas, 12 of them in a single, well-studied nursery ground. Except for 1 aggregation, all were oriented across tidal currents, but movements in both ebb and flood directions were observed. Aggregations occurred in all months, but predominantly between October and April. This 'mass migration' was hypothesized to serve the purpose of carrying conch from juvenrle to adult habitats. Given the lack of progression toward deep-water adult habitats, and the fact that aggregations consisted of different age groups from the l + year class to early adults, it is unlikely that this phenomenon represents ontogenetic movement. Aggregations sometimes formed in the center of homogeneous seagrass beds and are probably not feeding fronts. Most commonly, high-density aggregations formed when juvenile populations were large and when density was increased experimentally. suggesting that formation IS density-dependent. A tethenng experiment showed that conch in an aggregation experience lower predatory mortality than those in the surrounding nursery area. The most important ecological significance of high-density aggregation appears to be probability refuge from predation, or dilutlon effect.