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 sample 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.