Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Sapporo, Hokkaido 062-0922, Japan
Until recently, it was thought that Japanese populations of chum salmon Oncorhynchus keta consisted entirely of hatchery fish. However, the presence of a significant proportion of wild fish has now been documented in some populations. To allow for differential management of both wild and hatchery salmon, it is important to understand the differences and similarities that exist in a number of measurable biological traits. We compared age and body size at maturity of wild and hatchery fish collected from three rivers in which the otoliths of all hatchery-origin fry were thermally marked. The difference between wild and hatchery fish varied among rivers. In one river, wild fish were older and larger than hatchery fish. In the second river, there was no difference between hatchery and wild fish, whereas in the third river, the age-related increase in body size of wild fish was smaller than that of hatchery fish.