Layering with depth in a heterospecific fish aggregation

Abstract

The schooling behavior and interspecific interactions within a nearshore planktivorous fish aggregation in Bermuda was observed during the summer (July–Sept) and winter (Dec–Jan) of 1986–87. Four species of morphologically and ecologically similar fish were, studied: the silverside Allanetta harringtonensis, the round herring Jenkensia lamprotaenia, the anchovy Anchoa choerostoma, and the pilchard Harengula humeralis. Heterospecific aggregations of these fish layered with depth into discrete horizontal bands 30 to 60 cm in width. Layers were categorized by unique sets of species as well as by age offish (measured as fish length). The median depth of the layer was also a function of whether the fish were feeding or resting. The median depth and the width of each layer were consistent across three sites and midmorning versus midafternoon observation times. Reduction in predation per individual is suggested as a possible force acting to maintain the structure of this fish aggregation.