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

Abstract

Investigations of fish community dynamics in the pelagic and littoral zone of the lowland Sulejow reservoir (Central Poland) have revealed an occurrence of strong inter- and intra-cohort cannibalism in the population of the key predator: pikeperch, Stizostedion lucioperca (L.). In early summer the number of young-of-the-year, YOY, pikeperch in stomach contents of older conspecifics positively correlated with density of juveniles in the pelagic zone. In two years (1994, 1997), when high density of YOY pikeperch (0.6 and 0.8 individual per m³, respectively) was observed, more than seven juvenile pikeperch per stomach of older conspecifics were found. In years of low reproductive success of pikeperch (0.05 YOY per m³), the average number of juvenile pikeperch in stomachs of predatory conspecifics was below 1. The strong density-dependent predation resulted in a sharp decline of YOY pikeperch toward late summer. Cannibalistic pressure was also observed among YOY pikeperch. In the reservoir, juvenile cohorts displayed a bimodal size distribution 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 survival). As an effect of this cannibalistic self-regulation, long-term stabilisation of pikeperch year-class strength has been observed.