Plasma thyroid hormones during smoltification of yearling and underyearling coho salmon and yearling chinook salmon and steelhead trout

Abstract

Both yearling and underyearling (zero-age) coho salmon in fresh water showed spring- and/or summer increases in plasma thyroid hormone concentrations. The peak in plasma thyroxine (T 4) was larger and occurred earlier during the spring in yearling fish compared to zero-age fish. Both groups had similarly elevated plasma triiodothyronine (T 3) during the summer months. Groups of yearling and zero-age fish transferred serially to seawater net-pens showed differential rates of survival in seawater. In the zero-age fish, the greatest survival seemed coincident with the progression of the elevation of plasma T 3 in fish in fresh water. Success in seawater of yearling fish correlated with the progression of the plasma peak of T 4 in fish in fresh water. Peaks of thyroid hormones were also observed in chinook salmon and steelhead trout in fresh water. Although the survival of the chinook salmon in seawater net-pens was comparable to that of coho salmon, steelhead trout showed unexpectedly poor survival in seawater. These data suggest that analysis of changes in plasma thyroid hormones provides useful information regarding the optimal time to transfer coho salmon from fresh water to seawater. Whether this information can be applied in a similar manner to other species of salmonids requires further investigation.