The Rybinsk Reservoir is one of the largest artificial reservoirs in Russia. The Sheksna reach of the Reservoir is worst anthropogenically affected due to the waste waters of the Cherepovets industrial plants. Heavy metals en-tering the Reservoir can accumulate in the body of fish including the predators ‒ the last link in the trophic chains of the Reservoir. Purpose of the study is to register the concentration of heavy metals in the muscles of the pikeperch Sander lucioperca in the Rybinsk Reservoir and to define its compliance with the Russian and international standards. The fish were caught in the feeding period with a trawl in the Sheksna and Volzhsky reaches, measured, and fatness coefficients were calculated according to Fulton and Clark. The content of Cd, Cr, Pb, Cu, Mn, Fe, Al, Co, Ni, V was determined in the fish muscle tissues. Fatness of pikeperch from the Rybinsk Reservoir, regardless of the level of the local anthropogenic pollution, corresponded to the mean values typical to this species. In the pikeperch muscles the content of nickel and vanadium was below the detection limit. On average, the tissues contained Cd 0.004, Cr 0.095, Pb 0.028, Cu 0.734, Mn 0.205, Fe 4.986, Al 12.538, Co 0.002 in mg/kg wet weight. Concentration of Fe, Al, Mn in the muscle tissues of pikeperch from the Sheksna reach significantly exceeded the values of Fe, Al, Mn in the muscle tissue of pikeperch in the Volga reach. Consumption of pikeperch meat from the Rybinsk Reservoir is not potentially hazardous to human health, since concentrations of heavy metals in the muscle tissues of the studied species do not exceed the permissible levels of elements, according to the Russian and international standards.

Abstract. The rapid growth of the aquaculture industry using modern intensive farming methods has led to a number of problems related to water quality and the spread of infectious diseases in fish. The intestinal physiological microbiota of fish and the surrounding aquatic environment forming an ecosystem, the balance of which is a key factor in maintaining the health of aquaculture. However, optimizing the microbiota is still a challenging task due to a lack of studies about the dominant microorganisms in aquacultures and the influence of environmental factors on it. The aim of this work was to study the structure and taxonomic composition of the cultivatable bacterial gut community of the rainbow trout Oncorhynchus mykiss under the conditions of the “Yaroslavskaya Trout” cage farm. Methods. To identify the isolated strains and characterize the composition of common bacterial communities were used phenotypic and biochemical characteristics, as well as amplification and sequencing of 16S rRNA gene fragments and subsequent phylogenetic analysis. Scientific novelty. Data on the taxonomic diversity of the cultivated rainbow trout gut microbiome have been obtained. A comparative analysis of fish intestinal microbiocenoses and the surrounding aquatic environment was carried out. Results and practical significance. Gammaproteobacteria was the most abundant bacteria in the structure of the studied microbial communities. Bacteria of the genera Pseudomonas, Stenotrophomonas, and Aeromonas were found in the microbiome of both the rainbow trout intestines and the surrounding aquatic environment, which are opportunistic pathogens and can lead to an outbreak of bacterial infections under stressful conditions. However, studies have shown that the surrounding aquatic microbiome does not always reflect the microbiome of aquatic organisms. A comparative analysis of the freshwater microbiomes and the gut microbiome of rainbow trout revealed the ability of the host organism to concentrate beneficial probiotic microorganisms even under conditions of pathogenic presure. The resulting collection of microorganisms can be further used for screening potential probiotic cultures.