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

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Plasma cortisol, thyroxine (T 4), and triiodothyronine (T 3) 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 T 4 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 T 3 (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 T 3 concentrations in FW and during SW exposure, but it had no significant effect on T 4 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.