Cortisol effects on plasma electrolytes and thyroid hormones during smoltification in coho salmon Oncorhynchus...

Plasma concentrations of Na+, K+, triiodothyronine (T3), and thyroxine (T4) and muscle water content were measured in yearling coho salmon. Oncorhynchus kisutch, after injection of cortisol in April, May, and June in fresh water (FW) and during acclimation to seawater (SW). Cortisol (17-21 micrograms/g), injected intraperitoneally in a melted cocoa butter suspension, suppressed the rise of plasma Na+ during SW acclimation in April but not in May or June. Muscle water content increased during SW acclimation in cortisol-treated fish in April and June. These observations suggest a hypoosmoregulatory function for cortisol during SW acclimation. Cortisol treatment also induced elevations of plasma K+ in FW (April only) and SW (April and May only). Cortisol treatment increased plasma T3 during SW acclimation in June and T4 in FW in May. The results suggest that cortisol may modify osmoregulatory processes and thyroid hormone activity during smoltification and acclimation to SW in yearling coho salmon.