Cortisol and its effects on plasma thyroid hormone and electrolyte concentrations in fresh water and during seawater acclimation in yearling coho salmon, Oncorhynchus kisutch.

Plasma cortisol, thyroxine (T4), and triiodothyronine (T3) concentrations increased during seawater (SW) acclimation in yearling coho salmon, Oncorhynchus kisutch. Maximal concentrations of cortisol (220 ng/ml) occurred within 1.5 hr after the ambient water was changed from fresh water (FW) to SW; after 21 days in SW, cortisol levels were still slightly elevated (23 ng/ml) compared to those in FW fish (8 ng/ml). Plasma T4 concentrations peaked (14 ng/ml) at 12 and 72 hr after exposure to SW, and they were higher than those in FW fish (4 ng/ml) at all samples times. Maximal concentrations of T3 (8 ng/ml) occurred within 12 hr after exposure to SW, followed by a return to FW control levels (4 ng/ml) within 24 hr. Chronic treatment with cortisol significantly lowered plasma T3 concentrations in FW and during SW exposure, but it had no significant effect on T4 concentrations. Cortisol treatment lowered gill Na-K-ATPase activity in FW fish, but it did not affect plasma osmolarity, Na, K, Ca, or Mg in fish in FW or during SW acclimation.