Juvenile Rogue River spring Chinook salmon (Oncorhynchus tshawytscha) reared at two temperatures and photoperiods for a 2-yr period were examined for cyclic changes in gill (Na + K)–ATPase specific activity. Three peaks in activity were observed: a peak in October of the first year, a second higher peak the following May, and a peak in October of the second year. Exposure to photoperiods artificially advanced 3 mo (July–October) suppressed the October peak during the first year, but did not greatly alter the subsequent two peaks in activity. The temperature at which the fish were reared did not directly affect the changes in enzyme activity. However, growth rates varied in response to water temperature. Altered growth rates resulted in differences in the cyclic changes in gill (Na + K)-activated ATPase activity in the two temperature groups. A threshold of 8 cm was suggested as the minimal size at which the fish could respond to the appropriate photoperiod by altering the gill (Na + K)–ATPase activity. Key words: (Na + K)–ATPase, chinook salmon, temperature, photoperiod, Oncorhynchus tshawytscha.

The hypothesis that juvenile anadromous salmonids which are migrating to sea have higher levels of gill (sodium and potassium) adenosine triphosphatase ([Na+K]‐ATPase) activity than nonmigrants was examined for wild and hatchery spring and fall chinook salmon (Oncorhynchus tshawytscha) from the Rogue River, Oregon. Although migrant juvenile chinook salmon captured at river km 173 had elevated gill (Na+K)‐ATPase activity, migration from the more upstream portions of the Rogue River occurred without significant changes in gill (Na+K)‐ATPase activity. High water flows stimulated migration without increasing gill (Na+K)‐ATPase activity. Tributary‐to‐main‐stem migration of juvenile fall chinook salmon did not result in elevated gill (Na+K)‐ATPase activities. Juvenile spring chinook salmon released from Cole Rivers Hatchery (km 254) migrated rapidly to km 18 on the Rogue River without changes in gill (Na+K)‐ATPase activity. We conclude that, although the relationship between seaward migration and elevated gill (Na+K)‐ATPase activity is demonstrable in chinook salmon in many cases, such elevated ATPase activity is not a prerequisite for seaward movement of juvenile chinook salmon.

Juvenile spring chinook salmon (Oncorhynchus tshawytscha) (Rogue River stock) were reared in constant 8 °C well water under artificial lighting with photoperiods adjusted to Medford, Oregon. Juveniles were divided into six groups according to the amount of food provided. Growth (change in fork length per day) was linear from July through December for all six groups. Three distinct patterns of gill (Na + K)–ATPase activity were observed in these groups. In the two groups fed the largest rations, a peak in activity in October was observed with a smaller peak in July. Fish fed the smallest rations showed no increase in gill (Na + K)–ATPase activity throughout the year. At intermediate rations, there was a small peak in July but no peak in October. A growth rate of less than 0.027 cm/d appeared to suppress the October peak in gill (Na + K)–ATPase activity. Fish with higher growth rates showed peaks in activity in either October or NovemberKey words: chinook salmon, (Na + K)–ATPase, growth, photoperiod, Oncorhynchus tshawytscha

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