Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Sapporo, Hokkaido 062-0922, Japan
It is widely assumed that almost all chum salmon Oncorhynchus keta in Japan originate in hatcheries, but there are no studies that actually show the contribution of naturally-spawned fish (wild fish) to total production. In this study, we estimated the contribution of wild chum to the total chum salmon catch in rivers where chum salmon hatcheries are located and hatchery-reared chum salmon are released. The catch of wild and hatchery chum salmon was estimated at weirs located on eight rivers in Hokkaido, northern Japan, by identifying the ratio of otolith thermal-marked hatchery fish and unmarked wild fish. In total, the contribution of wild fish to the total catch in these rivers was estimated to be 28.3±1.2% (15.9±0.6% for rivers where the percentage of marked fry released was 100%), but the value varied considerably among rivers and years (range: 0 to 50%). This study showed that the contribution of wild chum salmon is not negligible; rather, it is large enough to constitute fishery production.