Fish-zooplankton interactions during spring in a deep reservoir

Abundance, biomass and production of zooplankton was monitored during April -June in the Rimov Reservoir, Czech Republic. Gut content analyse of three dominant fish species, perch (Perca), roach (Rutilus) and bream (Abramis) w as performed during the same period. The dominant year classes of adult per ch were subjected to more detailed analyses of food ration. Consumption of planktonic prey was low before and during the peak of perch spawning (late April). An exponential increase of perch feeding activity coincided with th e end of their spawning and with the formation of the warm epilimnion. Larg e-sized daphnids (adults>1.0 mm) were positively selected irrespective of t heir relative abundance. This was particularly important during the period of low density of daphnids in early spring when the population of perch was able to eliminate a significant proportion of the Daphnia production. Late r, in the end of May, some perch (70% of females, 18% of males) switched to feeding on littoral organisms such as insect larvae, Polyphemus and cyprin id fish eggs, while the rest consumed pelagic zooplankton. Roach and bream consumed a wider spectrum of planktonic organisms including Bosmina and sma ll Daphnia. Inability to prey on small plankton and the absence of larger p rey items probably led to the collapse of the perch population in the subse quent two years.