The study aims to determine the radionuclide contamination formation patterns in ichthyofauna representatives of some Forest Steppe of Ukraine water bodies and to establish reference values for the specific activity of 90Sr and 137Cs in the fish body as of 2021. To determine the radionuclide pollution levels in the industrial ichthyofauna representatives in the water areas of the Kaniv Reservoir, the Kosivskyi, the Bila Tserkva Middle, and the Bohuslav Reservoirs on the Ros River and in the unregulated sections of the Ros River (above the city of Bila Tserkva) and Hnyliy Tikych (the village of Stavyshche) as of 2021, the following fish species were selected: red snapper - Rutilus rutilus L., flatfish - Blissa bjoerkna L., common bream - Abramis brama L., silver crucian carp - Сarassius gibelio (Bloch), bream - Alburnus alburnus alburnus L.; pike - Esox lucius L. (peaceful species); pike perch – Stizostedion lucioperca L.; European-Asian river perch – Perca fluviatilis fluviatilis L. (predatory species). The specific activity of 90Sr and 137Cs was determined in the entire fish body by generally accepted radiochemical and gamma spectrometric methods. In 2021, the registered average specific activity of 90Sr in fish of various species of the Kaniv Reservoir ranged from 0.6 to 1.3; 137Cs - from 2.4 to 13.2 Bq/kg. The average specific activity of 90Sr in fish of different species of the Kosiv Reservoir ranged from 0.1 to 0.5, that of 137Cs – from 0.2 to 2.1 Bq/kg, Bila Tserkva Middle ranged, respectively, from 0.2 to 0.8 and from 0.8 to 4.0, Bohuslav – from 0.1 to 0.5 and from 0.5 (bream) to 3.1 Bq/kg. The average content of 90Sr and 137Cs in the body of fish fished in unregulated sections of the Ros and Hnyliy Tikych rivers was 0.1–0.5 and 0.5–2.0 Bq/kg, respectively. The specific activity of 90Sr in peaceful and predatory fish of most of the studied reservoirs probably did not differ, and we can note only the tendency to the formation of higher levels of accumulation of the radionuclide by peaceful species. However, the values of the specific activity of 137Cs of predatory species exceeded the corresponding values that were registered in peaceful fish. The levels of fish radionuclide contamination in all the studied reservoirs, except for the Kaniv Reservoir, did not differ reliably. The increased content of radionuclides in the Kaniv Reservoir fish compared to other reservoirs of the Forest Steppe can be explained by the constant influx of radionuclides from the ecosystem of the Kyiv Reservoir through the Kyiv HPP dam. During the research period, 58–74% of the total activity of peaceful fish in forest-steppe reservoirs was formed by 137Cs, that of predatory species made 82–93%. The research results made it possible to determine reference and screening values in the specific activity of radionuclides in the fish of the forest-steppe reservoirs as of 2021. For peaceful fish, the value of the reference 90Sr activity in the studied reservoirs of the Forest Steppe is 1.0, the screening activity - 3.0, predatory fish - 0.5 and 2.0 Bq/kg, respectively. For 2021, the reference value of the specific activity of 137Cs in the body of peaceful fish of the Kaniv Reservoir makes 3.2, the screening value - 10.0, the Kosiv, Bila Tserkva Middle, Bohuslav reservoirs,the Ros and Hnyliy Tikych rivers are 2.0 and 6.0 Bq/, respectively kg; that of predatory fish of the Kaniv Reservoir - 12 and 36, respectively, Kosiv, Bila Tserkva Middle, Bohuslav Reservoirs, the Ros and Hnyliy Tikych rivers - 3.0 and 10.0 Bq/kg, respectively.

Key words: water bodies of the Forest Steppe, fish, radionuclides.