On the Ocean age of Masu Salmon Oncorhynchus masou in a Natural Population, Shiretoko Peninsula, Japan

In the Orumappu River, northeastern Hokkaido, 667 wild masu salmon smolts were tagged and released. During annual monitoring within the river, 10 individuals were recaptured, and an additional individual was reported from a coastal fishery. All adult masu salmon were recaptured the year after their release, and all had spent one winter at sea. When examining scales of marked fish, four individuals lacked winter annuli and one individual had 2 or 3 resting zones (closely spaced circuli) in the oceanic zone. Accordingly, we believe it is uncertain to assess age of adult masu salmon based solely on scale annuli.