Long-Term Changes in the Importance of Aboriginal and Invasive Fish Species for Feeding of Predatory Fish of the Rybinsk Reservoir

Influence of biotic and abiotic environmental factors on feeding of the predatory fish in the Rybinsk Reservoir in 1953-2012 has been analyzed. It is revealed that the feeding spectrum of the majority of piscivores (sander Sander lucioperca, burbot Lota lota, pike Esox lucius and adult perch Perca fluviatilis) includes 16-18 fish species, while only seven in Volga sander Sander volgensis. Three aboriginal fish species (perch, ruff Gymnocephalus cernuus and roach Rutilus rutilus) and two invasive species (European smelt Osmerus eperlanus and Black-Azov tyulka Clupeonella cultriventris) are the main food items for piscivores. In some years, these species accounted for 97% of the total number of prey. Introduction and naturalization into the reservoir of smelt (during the period of climatic norm) and Black-Azov tyulka (during regional climate warming) considerably changed proportions of species serving as potential prey for predatory fish. During these periods, the share of invasive species in the ration of pelagic zone piscivores was increasing, but this increase was not proportional to the abundance of these prey species. After decline in abundance of smelt and tyulka (while, at the same time, these species were still dominant) their shares in the feeding of predators decreased, while the role of more available mass aboriginal species increased again. It is revealed that predatory fish are selective to certain prey items: sander, Volga sander, and large perch prefer to feed on perch yearlings; burbot, on yearlings of perch and on ruff; and pike, on roach.