Endocrine profiles in the males of a twice-annually spawning strain of rainbow trout, Salmo gairdneri

Abstract The male reproductive cycles of a twice-annually spawning strain of rainbow trout were studied by monitoring the plasma gonadotropin (GtH) and steroid hormone levels in individual fish for more than a year using thirty-five 2.5-year-old mature males. Twenty-five males survived the whole experimental period and were divided into four groups according to the amount of milt and endocrine profiles. In the summer breeding season, milt amount was negligible in Group I and small in Group II with low plasma testosterone, 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOH-P) levels, whereas both groups showed a large amount of milt and a distinct increase in steroid levels in the winter breeding season (November–January). Group III expelled a large amount of milt in both the summer and winter breeding seasons, and plasma testosterone, 11-KT, and 17α,20β-diOH-P showed a clear peak in each breeding season. In Group IV, milt was expelled from December to July, and plasma steroid levels remained high until June before declining to the basal levels; however, these fish failed to mature in the ensuing winter breeding season. Plasma GtH levels in Groups III and IV were significantly higher than those in Groups I and II in the summer breeding season. These results clearly indicate that fish in Group III are twice-annual spawners.