Variations in plasma cortisol concentrations over a 24-hour period in the rainbow trout Salmo gairdneri

Abstract

Plasma cortisol was measured in rainbow trout ( Salmo gairdneri), killed at intervals over 24-hr periods. In trout killed at a fish farm in July, under a long photoperiod, plasma cortisol titres were higher at night than in the light hours, with maximum levels at 2400 hr. No circadian variations were apparent in trout killed at the farm in December, under short photoperiod conditions when cortisol levels were lower than in summer fish. Trout were also maintained in small tanks in an aquarium under similar photoperiods and temperature as in the field. Basal cortisol concentrations remained low. In July, the peak of plasma cortisol seen at 2400 hr in the field continued to be expressed, in exaggerated form, at the beginning of the light period in the morning. In June, maintenance in constant light appeared to advance the 2400 hr peak to 1800 hr and to raise cortisol levels in general, but firm interpretation of these observations awaits further work.