ИСТИНА

Biology of Ozernaya River sockeye salmon (the most population in Asia) have been study since 1932. The spawning, ecology of juveniles in the Kurilskoye Lake (before downstream migration to the Ozernaya River), marine life and spawning migration of adult fish was studied as well during this period. However, biology of juvenile sockeye during downstream migration and early marine period has been poorly studied. Although, these stages of the life cycle are most critical for Pacific salmon, as known, because the high mortality of fish can occur. This significantly complicates for forecasting of future returns of adult fish that is necessary for effective stock management and rational fishing. The first integrated study of juvenile ozernovskaya sockeye salmon in Kurilskoye Lake, Ozernaya River (riverbed and estuary) and coastal waters of the Okhotsk Sea was conducted in June-August 2017. The obtained data confirmed that downstream migration of juvenile sockeye from Kurilskoye Lake starts in late May or early June and ends in the second half of July. Downstream migration of juveniles in the Ozernaya River run on less than one day (through a high speed of water flow). The average size of migrants age 1+, 2+ and 3+ in the riverhead (according to interannual data) are 8.1, 9.7 and 10.8 cm, respectively. We have determined that such large juvenile sockeye do not stay in the estuary and migrate to the Okhotsk Sea immediately. Hydrological conditions in river mouth area and adjacent coastal waters allow of juvenile sockeye quickly smoltificate and to adapted for the life in marine environment. As a result, a juvenile sockeye already widespread in coastal sea waters (to the north of the Ozernaya River mouth) to the end of July and early August. According to the results of trawl surveys, abundance of juvenile sockeye in the coastal waters of the Okhotsk Sea is comparable to the total abundance of migrants from the Kurilskoye Lake (correlation coefficient R = 0.86). Thus, in the life cycle of ozernovskaya sockeye salmon, downstream migration and the early marine period of life are not critical. This feature of biology is a one of the factors that makes it possible with almost 100% accuracy to prognosticate the number of returns of adult fishes in the Ozernaya River (the average accuracy of prognosis of ozernovskaya sockeye salmon returns was 103.6% for the last 10 years). The field studies were supported by the Russian Foundation for Basic Research (project nos. 17-05-01224).