Seawater Adaptability in Baltic Salmon (Salmo salar): A Bimodal Smoltification Pattern in Previously Mature Males

Abstract

In three years, maximum seawater adaptability in 2-yr-old Baltic salmon (Salmo salar) was found during a period of about 2 wk in late May – early June. At this time the previously mature males developed in one of two directions. One group (8–44%) developed a seawater adaptability comparable with that of immature fish whereas the other did not. Previously mature males that did not adapt to seawater had larger gonadosomatic indices than males with good performance in seawater. Individually tagged previously mature males and immature fish released into the river in spring had a total adult recapture rate of 1.6 and 11.6%, respectively. In both groups the recapture rate was highest for fish released about 2 wk before the time of optimal seawater adaptability. The effect of size on recapture rate was more pronounced among previously mature males than among immature fish, with a particularly low recapture rate of small previously mature males. It is suggested that previously mature male parr can either mature sexually again or complete smoltification in spring and that the probability for smoltification and seaward migration is affected by body size and the conditions for growth during the previous autumn–winter.