Abstract
Under raceway conditions, an advanced photoperiod schedule caused migratory movements and elevation in gill Na+–K+ adenosine triphosphatase activity (Na+–K+ ATPase) to occur about 1 mo earlier than normal in yearling summer steelhead (Salmo gairdneri) from Dworshak National Fish Hatchery (Idaho). Exposure of migrants to 13 °C for 20 d resulted in serious impairment of continued migratory behavior and a reduction of gill Na+–K+ ATPase activity. Migrants outnumbered nonmigrants at fork lengths of 16 cm and longer. It is proposed that the potentially detrimental effects of warming river temperatures during the normal migratory season and delayed migration caused by dams and impoundments might be partially overcome by inducing early smolt transformation and migration with the use of advanced photoperiods.Key words: ATPase, steelhead, migration, temperature, photoperiod, smolts

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