Seasonal variations in plasma and pituitary levels of gonadotrophin in males and females of two strains of rainbow trout (Salmo gairdneri).

Male and female rainbow trout of an autumn- and a winter-spawning strain were sampled every 28 days over a period of 2 years, starting when they were approximately 6 months old. Plasma and pituitary levels of gonadotrophin (GTH) were measured by a radioimmunoassay that preferentially detected the gonadotrophin referred to as either GTH II or "maturational and ovulatory" GTH. The general pattern of gonadal development and the profile of GTH in plasma and pituitary were similar in the two strains, except that the timing was different. Gonadal development in both sexes began about 9 months before spawning, concomitant with an increase in pituitary GTH content. In both sexes, pituitary GTH levels rose dramatically by about 50,000-fold, peaking at the beginning of spawning. They remained high many months thereafter, before falling slowly. Plasma GTH levels during the reproductive cycle were different in the two sexes. In both sexes plasma GTH levels remained very low and unchanged until shortly before full sexual maturity was reached. GTH levels then rose only slightly in males, but markedly in females. The slightly elevated GTH levels persisted many months in males, throughout the period when they could be stripped of milt, but in females they were elevated only a short time around ovulation. These results suggest that although a GTH II-like gonadotrophin is present in the pituitary gland throughout the period of gonadal growth, it does not appear to be released in any substantial amounts into the blood until near spawning.