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.

An assessment of the hydrochemical parameters and the abundance of the bacterial complex was carried out in the water of a cage farm in the Yaroslavl region in spring 2022, followed by the isolation and identification of cultivated isolates. Ten adult rainbow trout (Oncorhynchus mykiss), aged 2+, which were in a state of physiological dormancy, were studied. An assessment of the microbial contamination of the skin, internal organs and feces was given. The taxonomic diversity of the skin microbiome and immunophysiological parameters of blood were studied. A low number of microorganisms was revealed in the environment. Bacteria of the Aeromonas-Pseudomonas complex of the cage part of the reservoir are opportunistic pathogens and, under stressful conditions, can lead to an outbreak of bacterial infections. Despite the low values of the microbial contamination of the reservoir, the contamination of the digestive tract and excretory system was revealed. Opportunistic bacterial species posing a potential danger to fish have been found in the skin microflora of Oncorhynchus mykiss. The study of indicators of respiratory function and natural blood resistance of commercial trout, age 2+, under farm conditions, indicates chronic exhaustion of the organism and suppression of humoral and cellular immunity factors. Further monitoring of changes in the ecological situation and the sanitary and epizootic state of the reservoir is necessary, which will allow timely measures to be taken to correct the composition of the fish microbiome.