Sexual maturation versus immaturity: different tactics with adaptive values in Baltic salmon (Salmo salar L.) male smolts

Abstract

Precocious 2-year-old male parr of the Baltic salmon (Salmo salar L.) were retained in freshwater or exposed to brackish water (4.6–5‰) in net pens after smoltification. Comparisons of growth rates and the incidence of sexual maturation revealed great differences between the two groups. In the freshwater group, the growth rate was retarded from mid-July and the fish acquired parr markings. In autumn, they all matured. The group in brackish water became rapidly growing, immature postsmolts. Only 7% matured in the autumn. The increase in mean weight of the postsmolts was 500% from the start of the experiment compared with 90% for the fish retained in freshwater.It is hypothesized that these alternative tactics expressed by salmon male smolts, i.e., sexual maturation combined with a low growth rate while in freshwater compared with sexual immaturity and a high growth rate while in sea, are mutually exclusive. Furthermore, it is suggested that these processes are controlled by annual endogenous rhythms (circannual rhythms) in interaction with the environment–neuroendocrine axis.