Stimulation of gonadotropin secretion after castration in rainbow trout

The effects of bilateral castration on plasma gonadotropin (t-GtH), as measured by radioimmunoassay, were investigated in male rainbow trout at several stages in the reproductive cycle: at the start of spermatogenesis at the end of May, after its completion in September, and before and after the beginning of spermiation in October and December. Castration resulted in increased level of circulating immunoreactive t-GtH but the response varied with the season. The rise after castration was four times the initial value in May and September, and increased twofold in October and sevenfold in December. The duration of the response following castration was 4 months in May, 2 months in September, and less than 1 month in October and December