Impacts of Habitat Degradation and Hatchery Operations on Masu Salmon (Oncorhynchus masou) in Hokkaido, Northern Japan

Since 1980s, intensive hatchery program for masu salmon has been conducted on the island of Hokkaido, northern Japan, particularly in the Sea of Japan area. Despite such efforts, coastal landings of masu salmon have continued to fall. The principal reason is a decline in the wild population, which is major component of the masu salmon stock on Hokkaido, due to riverine habitat degradation. In particular, dam construction has significantly affected recruitment by preventing the upstream migration of fish for spawning. In addition, the damming of rivers has reduced the carrying capacity of areas below the dam site for masu salmon juveniles. Furthermore, changes in the sediment regime due to dam construction has affected prey abundance via the degradation of stream-bed conditions. In addition to habitat degradation, interaction with hatchery-reared fish also has an impact on wild populations. A recent study revealed that, in spring, the number of newly emerged fry in enhanced rivers, before stocking, was significantly lower than in non-enhanced rivers. This suggests that competition between hatchery and wild fish and/or declining reproductive performance of hatchery-reared fish adversely affects masu salmon stocks.