Seawater adaptability of New Zealand's sockeye ( Oncorhynchus nerka) and chinook salmon ( O. tshawytscha): physiological correlates of smoltification and seawater survival

Gill Na +-K +-ATPase activities, haematocrit, and plasma osmolarity, cortisol, chloride and sodium concentrations were monitored over a 1-year period in freshwater stocks of salmon and in 24-h-seawater-challenged salmon. Salmon (0+ to 1 year old) failed to adapt to seawater from December to March (austral summer) when seawater temperatures were higher than 16 °C. In freshwater-sampled salmon, smoltification coincided with high gill Na +-K +-ATPase activities and plasma cortisol levels (35 ng/ml) and low plasma chloride levels ( < 120 mmol/l). Seawater survival was highly correlated with the 24-h-seawater-challenge samples of plasma cortisol and ionic levels which provide the best indicators for successful seawater adaptation.