The article highlights the ichthyological studies carried out in the Sysola river basin (a second order tributary of the Severnaya Dvina river), where rainbow trout is reared in cages for commercial purposes in the waters of regulated tributaries. The forecast for trout naturalization seems to be poor due to the fact that trout of different ages enter the river system every year as a result of technological accidents. It has been found that, despite the numerous cases of trout occurring in the natural conditions during 40 years, in the basin of the Sysola river has not formed a self-reproducing population of rainbow trout, and cases of its by-catch are rare. The data on the climatic and ecological conditions of the region are given. It has been inferred that the main factors preventing the naturalization of trout in the water bodies of the region are unfavorable temperature and hydrological regimes during the spawning and early ontogenesis of fish. The short summer feeding period does not allow trout to prepare for successful long wintering, as evidenced by a sharp decrease in the growth rate of fish that have fallen into natural conditions, compared to fish grown in cages. Trout of cage origin obviously do not form stable groups capable of reproducing the species and do not pose a threat to the aboriginal fish population, including Atlantic salmon, whose spawning and a significant part of the life cycle take place on many rivers of the north-eastern European Russia, including the basins of the Severnaya Dvina and Pechora rivers
commentary: Tree refugia and slow forest development in response to post-LGM warming in North‐Eastern European Russia
