1Research Division, National Salmon Resources Center, Fisheries Research Agency, Sapporo, Hokkaido 062-0922, 2Chitose Field Station, National Salmon Resources Center, Fisheries Research Agency, Chitose, Hokkaido 066-0068, Japan
The migration patterns and origins of adult masu salmon Oncorhynchus masou masou that passed through or were captured at a trapping site in the Chitose River, Hokkaido, northern Japan, in 2007 and 2008 were investigated. From the results of fin clip marks and confirmation of otolith thermal marks, all adult masu salmon sampled near their spawning grounds in the upper reaches were identified as wild fish. On the other hand, both hatchery and wild fish were confirmed among the fish caught at the trapping site. From the results of snorkeling observations, it was found that once wild-origin spawners reached the point beneath a dam (the upper limit for fish migration in the Chitose River) from July to the end of August, they then moved back to the spawning area located downstream from early October to early November.