PLASMA PROLACTIN AND CORTISOL CONCENTRATIONS DURING SALINITY CHALLENGES OF COHO SALMON (ONCORHYNCHUS-KISUTCH) AT SMOLT AND POST-SMOLT STAGES

Circulating prolactin (PRL) and/or cortisol were measured in relation to hydromineral balance (internal osmotic pressure or plasma Na+ and K+) in smolt or post-smolt coho salmon (Oncorhynchus kisutch) to evaluate effects of salinity challenges. Freshwater (FW) challenge of seawater (SW)-adapted fish resulted in a peak of circulating PRL concentration 3 days after transfer, irrespective of the developmental stage or the date of transfer. Plasma PRL decreased thereafter, but remained higher at 14-15 days following FW entry than in the SW controls. Circulating cortisol levels were unchanged but a slight reduction of plasma Na+ and osmolality occurred. Seawater challenge of FW-adapted fish elicited a rapid decline in circulating PRL concentrations to levels found in SW-acclimated fish; levels remained low thereafter. Plasma PRL showed a negative relationship with plasma cortisol and with internal osmotic pressures or natremia. Conversely, high circulating cortisol concentrations were associated with poor hypoosmoregulatory ability, probably reflecting a stress reaction. © 1990.