Cannibalism as a regulatory force of pikeperch, Stizostedion lucioperca (L.), population dynamics in the lowland Sulejow reservoir (Central Poland)

Investigations of fish community dynamics in the pelagic and littoral zone of the lowland Sulejow reservoir (Central Poland) have revealed an occurren ce of strong inter- and intra-cohort cannibalism in the population of the k ey predator: pikeperch, Stizostedion lucioperca (L.). In early summer the n umber of young-of-the-year, YOY, pikeperch in stomach contents of older con specifics positively correlated with density of juveniles in the pelagic zo ne. In two years (1994, 1997), when high density of YOY pikeperch (0.6 and 0.8 individual per m(3), respectively) was observed, more than seven juveni le pikeperch per stomach of older conspecifics were found. In years of low reproductive success of pikeperch (0.05 YOY per m(3)), the average number o f juvenile pikeperch in stomachs of predatory conspecifics was below 1. The strong density-dependent predation resulted in a sharp decline of YOY pike perch toward late summer. Cannibalistic pressure was also observed among YO Y pikeperch. In the reservoir, juvenile cohorts displayed a bimodal size di stribution in early summer. Small (zooplanktivorous) individuals served as a food for bigger (piscivorous) pikeperch. During shortages of alternative food (perch, cyprinids), cannibalism by YOY from the upper modal group may enhance their recruitment, at the cost of small juveniles (low winter survi val). As an effect of this cannibalistic self-regulation, long-term stabili sation of pikeperch year-class strength has been observed.