On the Issue of Efficiency of Large-Scale Hatchery Rearing of Chum Salmon (Oncorhynchus keta, Walbaum, 1792) on Iturup Island, Kuril Islands

The number of juvenile chum salmon released from the salmon fish hatcheries on Iturup Island has increased by more than 10 times over the last 20 years. An hypothesis has been made that such a substantial increase in numbers of released hatchery-reared fry on Iturup caused a decrease in the average body weight of fish and a more rapid decline in the number of returning spawners relative to the number of juveniles released than in Sakhalin. The patterns of growth in the first year of life of chum salmon that feed in the bays of Iturup Island were studied using diagrams of fish scale circuli. It has been found that the growth retardation in fish observed in cases of release of large numbers of hatchery-reared juveniles occurs only in the early marine stage of life history, while the growth pattern in open sea waters remained unchanged. Consequently, the statement that salmon older than fry deplete the carrying capacity of the North Pacific and the adjacent Far Eastern seas appears to lack grounds.