Effects of sex steroids on the smoltification of masu salmon, Oncorhynchus masou.

Testosterone (T; 25-micrograms/g diet), estradiol-17 beta (E2; 2.5-, 25-micrograms/g diet), and 5 alpha-dihydrotestosterone (DHT; 2.5-, 25-micrograms/g diet) were administered to yearling masu salmon from February 3 through May 8. T and E2 inhibited the natural smoltification in spring and stimulated gonadotropin (GtH) accumulation in the pituitary, but DHT had no effect. T (25-micrograms/g diet) was administered to underyearling masu salmon from September 30 through December 14. At the end of the T treatment, fish body color darkened and pituitary GtH content increased in the T-treated group. On May 8 in the next spring, however, plasma T levels and pituitary GtH contents already decreased and smoltification occurred in both control and T-treated groups. On October 29, underyearling masu salmon reared under the short daylength (8 L:16 D) for 4 months were switched to a long daylength (16 L:8 D) and simultaneously administered with T (25-micrograms/g diet) or 11-ketotestosterone (11-KT; 25-micrograms/g diet). On December 17, the proportion of smolts was 52.8% in the control, whereas T and 11-KT administration inhibited smoltification under the artificially increased daylength. Pituitary GtH content significantly increased both in the T-treated and 11-KT-treated groups, though it was much lower in the latter. These results indicate that the springtime smoltification which is induced by increasing daylength is inhibited by biologically active sex steroids such as E2, T, and 11-KT.