Endocrine changes associated with spawning behavior and social stimuli in a wild population of rainbow trout (Salmo gairdneri). I. Males.

Rainbow trout (Salmo gairdneri) were collected from a natural spawning population at Pennask Lake, B.C. Blood samples were taken from male trout at different stages of spawning and assayed by radioimmunoassay for gonadotropin (GtH), testosterone (T). 11-ketotestosterone (11-KT), and 17 alpha-hydroxy-20 beta-dihydroprogesterone (17,20-P). Testosterone and 11-KT were detected in the plasma of males in spawning condition: 11-KT was at a higher concentration than T and declined to undetectable levels in postspawning fish. There were striking increases in plasma GtH and 17,20-P in males actively courting nest building females. The increase in 17,20-P was evidently not dependent upon an elevation of the pituitary hormone: 17,20-P remained low in males placed with sexually inactive, unovulated females even though GtH levels rose above levels measured in males isolated from females. The significance of the social control of hormone levels is not clear. There is some evidence that 17,20-P and/or GtH may cause an increase in quantity of the milt, and thus the endocrine response to a sexually active female may serve to synchronize maximum male fertility with oviposition readiness in the female.