Growth and survival rates of the anadromous trout,Salmo trutta, from the Vardnes River, northern Norway

Synopsis

The anadromous brown trout,Salmo trutta L., from the Vardnes River feed in coastal waters during the summer, but spend the winter in freshwater. Mean increase in weight during the sea sojourn ranged from 125 g for first time migrants to 704 g for the largest fish. A loss in weight, increasing with fish size, occurred during the winter residence in freshwater. Growth was positively correlated with the duration of the sea sojourn, which was generally longer in years when higher sea temperatures prevailed. The mean growth rate of the females during the sea sojourn, was generally higher than that of the males. About 37% of first time migrants survived the sea sojourn (mean duration 70 days), compared to 56%–68% of repeat migrants. Minimum estimates of the survival rate in freshwater (duration about 290 days) lay between 66% and 74%. Annual minimum survival was 25% for first time migrants, 37% for second time migrants, and 50% for older fish. The survival rates of the males of both the first- and the second-time migrants were generally higher than those of the respective females. Prolongation of the sea sojourn, due to low water-level in the river at the normal time of ascent, resulted in a further decrease in the survival rate over and above that resulting from the increase in residence time per se.