SPECIFIC FEATURES OF FEEDING EUROPEAN VENDACE COREGONUS ALBULA (L.) IN RYBINSK RESERVOIR

Abstract

The article describes Coregonus albula (L.) inhabiting the open pelagial of the Main reach and the river reaches of the Rybinsk Reservoir together with other fish species that form the feeding colonies of planktophagous fish. The feeding spectra of vendace for the summer-autumn period are determined. Mass species of cladocerans are present in the diet of fish feeding in the productive biotopes of the main reach of the reservoir, in the zones of stable high concentrations of feed invertebrates in the summer, during the maximum development of zooplankton. In this period of the year there dominate Bosmina zooplankton (the relative significance ratio in nutrition (IR) varies from 67.2 to 98.5%), Daphnia species (IR 98.1%) and Bythotrephes species (22.1%). The diet of vendace is more diverse in the estuaries of large tributaries of the river reaches. There they feed on the larvae and adults of amphibiotic insects Chironomidae, Simuliidae and Syrphidae, whose larvae live on the bottom, while adult insects keep a ground-air lifestyle. An invasive amphipod species Gmelinoides fasciatus presents in the diet of vendace in the near-dam zone of the reservoir in autumn. The appearance of benthic organisms in the nutrition of vendace in the autumn period coupled with a significant decrease of plankton invertebrates in the diet may be explained, on one hand, by a seasonal decrease in the biomass of zooplankton in the reservoir’s pelagial and search for new forage places, and, on the other hand, a food competition from the another plankotophage, the Ponto-Сaspian kilka. Despite the fact that vendace is morphologically adapted to feeding on small invertebrates in the water column, it demonstrates a certain degree of plasticity in the selection of food organisms in the Rybinsk reservoir, and its feeding spectrum changes depending on the season and productivity of the reservoir