LAUSR

In chum salmon (Oncorhynchus keta) homed to the Sanriku region, Japan, most of fish is matured in bays and spawn near river mouths in coastal short rivers; therefore, their upriver migration is extremely short, but the behavioural characteristics have remained unknown. Upriver migration in the Otsuchi River, a typical coastal river, was evaluated from behavioural and physiological aspects. Homing salmon tracked in Otsuchi Bay held in the inner bay for less than 1 day to more than 10 days before river entry. The varied holding duration was negatively correlated with plasma 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) concentration, an indicator of maturation. After river entry, however, most fish were captured in weirs near the river mouths within 2 days regardless of the DHP concentration. Of thirty-four fish released in the river, on the other hand, eighty and five fish were seen next day in the main spawning sites located at about 1.5 km upstream and in the branch creek, respectively, and 85% of the fish held position there until their death. The mean survival time of released fish was 5.8 days. Plasma DHP level suggested that preparations for spawning were already completed at the timing of the release. Taken together, homing salmon completed spawning preparation in the bay; and then, they moved to their spawning sites immediately after river entry and spawned there during a short remaining life. This upriver migration contrasts with those of other populations, such as early migrants and long river migrants, whose maturation is completed during upriver migration. This article is protected by copyright. All rights reserved.