Seawater‐acclimation and the thyroidal response to thyrotropin in juvenile coho salmon (Oncorhynchus kisutch)

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Juvenile coho salmon (Oncorhynchus kisutch) were acclimated to seawater beginning at one of two stages of development. The first group was seawater‐acclimated before the 1‐ to 2‐month period of elevated plasma thyroxine (T4) concentration associated with the parr to smolt transformation (smoltification). The second group was seawater‐acclimated midway through the period of elevated T4 levels. Two measures of thyroid activity were used to compare seawater‐acclimated salmon with their controls remaining in fresh water. The baseline concentrations of thyroid hormones were assayed throughout a 9‐ or 10‐week period, and at the end of 9 or 10 weeks the responsiveness to a single injection of 0.2 IU bovine thyrotropin was measured. Transfer to seawater before the smoltification‐associated T4 elevation prevented the rise in thyroid hormone concentrations and reduced the response to bovine thyrotropin. Transfer of salmon during the period of elevated T4 levels did not change thyroid hormone concentrations; however, the response to thyrotropin was greater in seawater‐acclimated fish than in fish still in fresh water. We conclude that the longterm response of the thyroid to seawater acclimation is partly a function of the stage of development at the time of transfer. Copyright © 1987 Wiley‐Liss, Inc., A Wiley Company

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Journal of Experimental Zoology