The present paper provides the results of the study on feeding of juvenile (underyearlings and age 2+) sterlet (Acipenser ruthenus) caught in the Danube River in 2002-2003. The full spectrum of food items found in the fish stomachs was identified. The seasonal patterns of food composition are revealed. It is shown that juvenile sterlet in the Danube feeds mainly on larvae of chironomids and trichopterans, amphipods, and leeches. The organisms typical for lithophilic and lithorheophilic biocenoses play considerable role in the feeding of sterlet. Mean daily gains of body length and weight in the sterlet underyearlings are calculated.Note: OF⎯occurrence frequency, %; RN⎯relative number in the digestive tract content, %; number of fish with empty stomachs is in parentheses; number of studied fish is outside of parentheses.
The paper presents the results on the study of specific, spatial-temporal, age and trophic structures of groups of fish larvae feeding in the littoral zone of the Rybinsk Reservoir. The species composition and distribution of fish larvae were determined in biotopes of the protected vegetated coastal area in years of different water content. It has been pointed out that the dynamics of zooplankton in the spring period corresponds to the age-related changes in consumption of different groups of hydrobionts by fish larvae. There have been investigated the environmental factors ensuring the dissociation in time and space of potential competitors of the same trophic level accompanied by the divergence of food spectra and the division of food resources. Using as the example one of the structural and functional elements of the littoral biocenosis (multi-species groups of fish larvae), the role of the trophic factor which provides the organization and functioning of this biological system and gives it a balanced and integral character has been demonstrated. At the same time, the balance of food relationships was provided by such mechanisms as differentiation of individuals of different species of fish larvae according to the age principle, change of food objects during ontogenesis, and spatial separation.
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
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