Changes in physiological indices of smolting during seaward migration of wild coho salmon, Oncorhynchus kisutch

Wild juvenile coho salmon (Oncorhynchus kisutch) were captured in traps or by seining during the course of parr-smelt transformation and migration fro m a small coastal stream in Oregon. Physiological measurements indicated th at gill (Na + K)-ATPase specific activity was significantly greater in migr ants than in non-migrants, but there was no significant difference in kidne y (Na + K)-ATPase specific activity in the two populations. Glycogen concen trations and triglyceride concentrations were significantly lower in migran t populations than in non-migrants. Guanine concentrations tended to be low er in non-migrant fish than in migrant fish after May. There were no signif icant differences in plasma thyroxine or tri-iodothyronine concentrations b etween migrant and non-migrant fish. The decrease in energy storage compoun ds supports the hypothesis that migration is caused by an ionic imbalance d uring smelting, but the lack of changes in kidney (Na + K)-ATPase specific activity during smelting of wild fish does not support this hypothesis or t he results from studies with laboratory reared coho salmon. (C) 1998 Elsevi er Science B.V. All rights reserved.