Seawater adaptability in three different river stocks of Baltic salmon ( Salmo salar L.) during smolting

Abstract

Two-summer-old Baltic salmon ( Salmo salar) from three different stocks in northern Sweden held at Heden, Norrfors and Bergeforsen hatcheries were subjected to five successive seawater challenge tests each from 2 May until 5 July 1983. A pronounced seasonal pattern of plasma sodium levels was observed in sexually immature fish at Norrfors but not at the other two hatcheries. Response surface analysis of sodium data from the Norrfors fish predicted minimum plasma sodium concentration (i.e., maximum seawater adaptation) for transfer of 14.8-cm fish to sea water on 4 June. Precocious male parr at Norrfors had significantly higher plasma sodium concentrations after seawater challenge than did sexually immature fish in all tests. At the time of the fifth seawater challenge test in early July, a number of males at Bergeforsen and Norrfors were becoming sexually mature again and they suffered heavy mortality in sea water. The inhibitory effect of low temperature on testis regression in precocious male parr and its effect on their ability to smoltify is described. It is suggested that the seasonal variation in ambient water temperature near the time of the smolt-run might affect the proportion of precocious males which migrate from northern rivers. We view the seasonal pattern of seawater adaptability in juvenile Baltic salmon as an indicator of the endogenous annual timing system involved in the control of smolting and conclude that it can assist in deciding when to release Baltic salmon smolts from hatcheries.