The present study examines the composition of the diet and size-related as well as seasonal changes in the prey selection of pikeperch in large eutrophic L. Võrtsjärv. The total stomach contents of 325 pikeperch with a standard length of 5–76 cm were analysed in 1994–2005 with the goal to assess how the predation pressure of pikeperch is distributed over various prey species. The frequency of occurrence (FO), the number and restored weight of prey fish as well as the index of relative importance (IRI) and Ivlev’s selectivity index (E) were used for the description of pikeperch diet. The results of our study showed that the first prey fish for young pikeperch were mainly congeners and smelt Osmerus eperlanus (L.). With increasing body size the diet of pikeperch was enriched with ruffe Gymnocephalus cernuus (L.), perch Perca fluviatilis L., and roach Rutilus rutilus (L.). According to IRI, ruffe (IRI = 56%) was the most important prey for pikeperch in L. Võrtsjärv, while roach and perch were of second-rank importance. Bream Abramis brama (L.) was a rejected prey fish (E = – 0.9) for pikeperch, while smelt (E = 0.8) was actively selected. In comparison with data from the 1950s (Erm, V. 1961. Eesti riim- ja magevete kohade bioloogilistest ja morfoloogilistest erinevustest. In Hüdrobioloogilised uurimused, Vol. 2, pp. 289–342, Tartu), the main long-term changes in the diet of pikeperch are related to shifts in the fish community structure. Vendace Coregonus albula (L.) was the main prey (FO = 54%) for pikeperch in the 1950s, but it is absent from the recent diet. At present smelt replaces vendace in the diet of young pikeperch. Cannibalism was not observed half a century ago due to the small number of pikeperch in the lake at that time.
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