Osmoregulatory Ability of Wild Coho Salmon (Oncorhynchus kisutch) and Dolly Varden Char (Salvelinus malma) Smolts

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Abstract
Osmoregulatory ability of wild coho salmon (Oncorhynchus kisutch) and Dolly Varden char (Salvelinus malma) smolts migrating from a small stream in southeastern Alaska [USA] was assessed by plasma Na+ levels after a 24-h seawater challenge. Osmoregulatory ability of coho salmon was unaffected by time of out-migration, water temperature, and fish size. Osmoregulatory ability of Dolly Varden char was apparently affected by time of out-migration or water temperature but not by fish size. Char migrating in the first half of the migration period, when water temperature was usually < 8.0.degree. C, had lower plasma Na+ levels than did char migrating in the second half when temperatures were .gtoreq. 8.0.degree. C. A plasma Na+ threshold of 170 mmol .cntdot. L-1, used by others to separate smolts from silvery parr, indicated that 70% of the coho salmon and 80% of the Dolly Varden char we sampled were physiologically prepared to enter seawater. The remaining fish may have sufffered some level of osmoregulatory stress.