Seasonal distribution of yearling masu salmon released from the Sea of Japan side of southwestern Hokkaido, Japan

We investigated the seasonal distribution and migration patterns of hatchery-reared masu salmon Oncorhynchus masou in the area surrounding Hokkaido, the northern island of Japan. From 1993 to 1999, we tagged and released 313,000 yearling masu salmon smolts into the coastal waters of the Sea of Japan adjacent to southwestern Hokkaido. From May of the release year through July of the following year, a total of 1,693 fish were recaptured in the coastal waters of northern Japan. Just after their release, masu salmon juveniles had two migration routes; one group moved northward along the western side of Hokkaido in the Sea of Japan, while the second group passed through the Tsugaru Strait and then moved eastward into the Pacific Ocean. It was estimated that both groups reached the Sea of Okhotsk at 30-40 d postrelease. The summer distribution of tagged fish was unknown because none were recaptured. Young fish appeared in the Sea of Okhotsk again in autumn and then migrated southward, retracing the above two routes. They reached the Tsugaru Strait and Pacific coastal waters ranging from the northeastern Shimokita Peninsula in Honshu to the Iburi district of Hokkaido. Tagged masu salmon juveniles were recaptured in these areas until the following March, which indicates that these sites were the most important overwintering areas. Adult fish rapidly returned to the area of release from April to June. Our results suggest that hatchery masu salmon migrated in the coastal waters of northern Japan within a relatively short period.