Drinking in Atlantic salmon presmolts ( Salmo salar L.) and juvenile rainbow trout ( Oncorhynchus mykiss Walbaum) in response to cortisol and sea water challenge

Abstract

Circulating levels of cortisol in freshwater Atlantic salmon presmolts (15–30 g) and juvenile rainbow trout (20–30 g) were significantly increased by cortisol implants (80 mg kg −1) over a 3-week period, compared to sham implanted fish. In freshwater fish, cortisol implants were without effect on blood ionic content and drinking rate, about 0.1 ml kg −1 h −1 in salmon presmolts and 0.25 ml kg −1 h −1 in juvenile rainbow trout. Transfer of cortisol implanted salmon presmolts to sea water (33 ppt) resulted in development of a more immediate and fuller drinking response and after 7 days they imbibed at about 3.81 ± 0.01 ml kg −1 h −1 compared to 1.89 ± 0.27 ml kg −1 h −1 for sham implanted fish. A similar response was noted in cortisol implanted juvenile rainbow trout transferred to 28 ppt sea water. In both species cortisol implants resulted in improved hypoosmoregulatory capacity especially with respect to blood plasma Cl −1 concentration. The osmoregulatory role of cortisol in control of drinking in salmonids is discussed.