A comparison of the female reproductive cycles of autumn-spawning and winter-spawning strains of rainbow trout (Salmo gairdneri Richardson).

Annual changes in the plasma concentration of testosterone, 17 beta-estradiol, 17 alpha-hydroxy-20 beta-dihydroprogesterone, vitellogenin, and gonadotrophin were monitored by radioimmunoassay in females of autumn- and winter-spawning strains of rainbow trout. The annual profile of steroids and of vitellogenin, and the relationships between these parameters, were similar in the two strains, except that most changes occurred 2 months later in the winter-spawning strain than they did in the autumn-spawning strain. The times during which most fish ovulated in each strain were approximately 3 months apart, while gonadotrophin peaks, which were only detected about the time of ovulation, were 4 months apart. This phase shifting between the strains is discussed in terms of the environmental control of the reproductive cycle.