Androgens impair seawater adaptability in smolting Baltic salmon (Salmo salar)

Sexually immature (females and males) and mature male two-summer-old Baltic salmon (Salmo salar) were implanted with silastic capsules containing testosterone or 11-ketoandrostenedione in spring before the smoltification period. The seawater adaptability of the hormone-treated fish and intact control fish or control fish implanted with empty capsules was tested periodically from May to July using a common seawater challenge test with 25‰ saltwater. Fish were selected from the Norrfors hatchery stock (Ume River, (63°50′N, 20°05′E)) and ranged in fork length from 13 to 17 cm. At all sampling times the immature control fish displayed a better seawater adaptability than the previously mature males. Both groups exhibited a seasonal smoltification pattern with maximal adaptability in mid-June, when the mean plasma sodium levels after challenge were 161 mmol in the immature and 193 mmol in the mature fish. At all times both androgens impaired the seawater adaptability of the immature fish, which displayed high sodium levels comparable to those in untreated previously mature males. Even higher sodium levels were reached in previously mature males treated with androgen capsules. However, in all categories of fish the lowest levels of plasma sodium were found at the same time in mid-June. This indicates that although there is a marked, negative effect of sexual maturation and androgens on seawater adaptability, the temporal organization of smoltification is not affected.