Mechanisms of Size Selectivity by Planktivorous Fish: A Test of Hypotheses

The role of decision in size—selective predation by planktivorous fish is not clear. What causes size selectivity when many prey can be seem simultaneously (high prey density) is also not well understood. Suspended clay particles were used in experiments to control the number of Daphnia pulex that could be seen simultaneously by bluegill sunfish (Lepomis macrochirus) in order to test between alternative hypotheses. In clear pools, °122 Daphnia of two distinct size classes could be seen simultaneously by the fish. In turbid pools, the probability that two or more Daphnia could be seen simultaneously was <.01, although initial density was the same as in the clear pools. Size selectivity exhibited by the bluegills was independent of turbidity level and the number of prey seen simultaneously. A differential encounter rate hypothesis and the apparent—size selection hypothesis, both based on a perceptual bias for the larger size class, are rejected. The outcome of the experiments indicates that size selectivity resulted from decisions by the fish to ignore the smaller size class of Daphnia. The role of decision in prey selection and foraging strategies is discussed.