Purpose. To determine the current species composition of fish and analyze the structure of ichthyofauna of the Stryi River catchment on the basis of field studies, published works and survey data.Methodology. Ichthyological studies were carried out in 2017-2019 on the Stryi river, as well as its tributaries Rybnik, Zubrytsya, Skhidnytsia and Opir. Adult fish were caught using hook and line gear and hand nets, the larvae and fry were collected using sweep nets and plankton nets. Species identification and morphological analysis of fish were performed according to standard methods. Incomplete biological analysis of most fish, including all individuals of the Red book species, was carried out in situ, with fish being subsequently released in the river.Findings. The data obtained indicate that at least 27 fish species belonging to 8 families are present in the Stryi river. The typical riverine (rheophilic) species predominate in fish fauna. The limnophilic species in conditions of mountain river are rare, and the distribution of invasive species is limited for now. Ichthyofauna of the Stryi has features of endemism and relictism. 6 species are listed in the Red Book of Ukraine, 10 are in the Berne Convention. Most fishes carry out long-range migrations within the main channel as well as its tributaries. In order to preserve native riverine ichthyofauna, in particular the rare fish species, it is necessary to carefully approach any economic activity within the river basin which disrupts the natural hydrological regime or leads to water pollution.Originality. For the first time, a comprehensive study of fish fauna in the Stryi River was conducted, the distribution of rare and endangered as well as invasive fish species, was analyzed.Practical value. The data obtained are necessary for planning measures for the protection of rare and endangered fish species and for the regulation of recreational fisheries. The results of this study were used as a part of the environmental impact assessment of small hydroelectric power plants construction on the Stryi River. Information on the ichthyofauna composition, the distribution of rare and endangered fishes, particularly those, which undertake spawning migrations, is important for consideration when planning any economic activity that disrups the morphology and hydrological regime of the river bed (gravel extraction, bank stabilization, damming etc.).

The qualitative analysis of fish fauna of the Stryi River and the upper and middle sections of the Dniester River is conducted, and the taxonomy and faunistic structure are analyzed. The fish assemblage investigation was carried out during 2017–2020 in different seasons of the year. Fish sampling was performed by gill nets and sweep nets. Totally 15 locations were investigated and 465 specimens were caught. It has been established that the ichthyofauna includes 24 species in the studied section of the Upper Dniester and 22 species in the section of the Stryi River. The share of rare species is rather high: for the Dniester section, 33.3 % of the species listed in the Bern Convention and 12.5 % listed in the Red Book of Ukraine, and for the Stryi River, 40.9 % and 22.7 % respectively. The similarity of the Upper Dniester and the Stryi River ichthyofauna according to Shorigin was 53.9 %. The similarity of communities according to Jaccard was 0.59. Weinstein's index of biocenological similarity of ichthyocenoses in the studied rivers was low: 0.32. The low values of similarity indices are explained by the specificity of faunal structure in the studied rivers. Ichthyofauna of the Stryi and the Upper Dniester includes six ichthyocomplexes (boreal piedmont, boreal plain, tertiary plain freshwater, Pontocaspian freshwater, Pontocaspian marine, Chinese plain). The main similarities in the ichthyofauna of these rivers are formed due to the representatives of Pontocaspian freshwater, boreal plain and Chinese plain complexes: up to 72.7 % of total ichthyofauna. There has recently been an intensive advance of alien and invasive fish species into the piedmont of the Dniester River basin, which use transformed sections of the river with the silted bottom as refuges where they settle, increase in number and spread upstream. The share of alien species was rather high and amounted to 29.2 % in the Upper Dniester and 22.7 % in the Stryi River.

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The results of the assessment of levels of radionuclide contamination of fish during 2013 - 2019 in the reservoirs of the Chornobyl Exclusion Zone (lakes Azbuchyn, Vershyna, Hlyboke, Daleke, Yanivsky Backwater, Chornobyl NPP cooling pond) are presented. It was found that the concentration of 137Cs in representatives of ichthyofauna of water bodies during the study period continued to decrease, while the concentration of 90Sr, with some exceptions, remained unchanged or increased. The concentration of radionuclides in fish of the studied reservoirs was about 60 - 5000 times for 90Sr and in 3 - 200 times for 137Cs higher than the permissible levels for fish products accepted in Ukraine.