Seasonal variations in testicular germ cell stages and in plasma concentrations of sex steroids in male rainbow trout (Salmo gairdneri) maturing at 2 years old.

Male rainbow trout of an autumn- and a spring-spawning strain were sampled every 28 days over a period of 2 years, starting when they were approximately 6 months old. Plasma concentrations of testosterone (T), 11-ketotestosterone (11-K), and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) were measured by radioimmunoassay. A histometric method was used to determine the absolute quantities of several germ cell stages, including spermatogonia B, spermatocytes, spermatids, spermatozoa, and resorbing spermatozoa. The general pattern of gonadal development in the two strains was similar, e.g., steroids peaking in the order T----11-K----17,20 beta-P. However, there were a large number of differences--in the timing of first appearance of various parameters, in the timing and magnitude of various peaks, in the histological appearance of the regressing testes, and in the heterogeneity of the male population. T and 11-K levels were not associated with any particular germ cell stage in the two strains. Peak 17,20 beta-P levels coincided with the period when ovulated females were present and milt production was maximal.