Diet shift of pikeperch (Sander lucioperca) after the collapse of smelt (Osmerus eperlanus morpha spirinchus) population in a large shallow lake

Pikeperch ( Sander lucioperca ) are known to prefer acerate-bodied prey such as lake (dwarf) smelt ( Osmerus eperlanus morpha spirinchus ) when abundant, as observed in Estonia's lakes Võrtsjärv and Peipsi. This study aimed to investigate whether, in the absence of preferred prey, pikeperch would switch to alternative prey based primarily on availability rather than morphology, and whether increasing predator size corresponds to increased consumption of larger prey items. From 2015–2024, gut content analysis of 3807 mature pikeperch revealed that after smelt declined, pikeperch did not shift to similarly shaped prey (e.g., ruffe; Gymnocephalus cernuus ) but increasingly consumed deep-bodied bream ( Abramis brama ), the lake’s most abundant species. Prey length to predator length ratio (PPR) did not drive prey choice, as high-PPR bream was consumed equally with lower-PPR species. Ivlev’s Electivity Index confirmed strong smelt preference when available, but after its decline, prey abundance dictated selection. Larger pikeperch (>50 cm) mainly consumed larger bream (>10 cm standard length), likely due to limited alternatives. Pikeperch, currently being the main piscivorous fish in Võrtsjärv, plays a crucial role in controlling cyprinid biomass and therefore maintaining ecological balance in the lake.