Effects of thyroxine, growth hormone and cortisol on salinity preference of juvenile coho salmon(Oncorhynchus kisutch)

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Abstract
Freshwater yearling smolts of coho salmon (Oncorhynchus kisutch) weighing 25–70 g showed no preference for 33 ppt seawater from March to July; instead, seawater aversion was detected in June and July after the migratory period. In April during the migratory period, the fish showed restless behavior (swimming actively between compartments), which resulted in a high standard error of the mean. Effects of hormone administration on preference for 14 ppt seawater were examined using underyearling coho weighing 0.8–8.8 g. Fish treated with thyroxine (T4) or with ovine growth hormone (oGH) showed a significantly greater preference for 14 ppt salinity than the controls. The combination of T4 + oGH further increased this preference. Cortisol administration resulted in a less but still significant increase in preference. The finding that thyroid hormone, growth hormone and Cortisol all stimulate salinity preference of young coho, suggests the importance of these hormones in preparing smolts for downstream migration and entry into the ocean.