Stock assessment of sockeye salmon Oncorhynchus nerka by using adaptive framework virtual population analysis

In Lake Toya, sockeye salmon Oncorhynchus nerka is heavily caught by recreational angling and its population has widely fluctuated from year to year. Adaptive framework virtual population analysis (ADAPT VPA) was conducted as a step towards sound management of the stock. Surveys by mail, access point, and questionnaire were carried out to collect data on fork length, wet body weight, species, and catch number of sockeye salmon from anglers. Additionally, we carried out gillnet sampling whose data included wet body weight, and fork length which was used in age length key of sockeye salmon. The catch data of sockeye salmon from Lake Toya Fisheries Cooperative Association (Lake Toya FCA) and anglers were used to calculate total catch. Catch per unit effort (CPUE) from anglers was used as an abundance index for the ADAPT VPA. The stock of sockeye salmon in the lake was estimated to be 8.31 tons in 1998, but decreased noticeably to 4.13 tons in 2002. This was followed by a considerable increase to 17.26 tons in 2009. In 2012, the estimated stock weight was 7.77 tons.