Abstract

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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