GENERALISTS AS SEQUENTIAL SPECIALISTS - DIETS AND PREY SWITCHING IN JUVENILE SILVER PERCH

Diets of silver perch, Bidyanus bidyanus, in organically fertilised aq uaculture ponds were dominated by chironomid larvae, Daphnia and calan oid copepods. Insects and crustaceans contributed approximately 80% an d 20% by weight to the diet respectively. Classification of the stomac h contents of individual fish revealed 8 diet groups, 4 of which were dominated by planktonic crustaceans and 4 by insects. Each diet group was strongly dominated by a different prey type. Fish from the same sa mple tended to belong to the same diet group and there was a non-rando m distribution of diet groups across ponds. Perch diets were influence d by the method of pond fertilisation (livestock effluent or pellet fe ed). Shifts in the representation of groups indicated that fish switch ed from one diet group to another over a 2-4 week period. The selectio n of planktonic prey by perch was related to prey densities in the pon ds. Fish preferred Daphnia when these prey were common, but switched t o calanoids and insects when Daphnia were scarce. A perfect rank corre lation between the mean body size of planktonic prey and their contrib ution to the diet suggested that prey choice involved comparative deci sions based on prey size. These findings indicate that, while classifi ed as dietary generalists, silver perch exhibit consumption patterns w hich at the individual level are highly specialised at any given time. These patterns can be predicted, given information on prey densities in the environment.