Seasonal variations in parameters of milt production and in plasma concentration of sex steroids of male rainbow trout (Salmo gairdneri).

Concentrations of testosterone (T), 11-ketotestosterone (11-KT) and 17 alpha-hydroxy-20 beta-dihydroprogesterone (17 alpha 20 beta P) were measured by a direct radioimmunoassay technique in the plasma of male rainbow trout, Salmo gairdneri, from two winter-spawning strains and one autumn-spawning strain. There was no evidence that milt production, its volume and density or the ionic composition of the seminal plasma could be related to the levels of 11-KT or T. Peak plasma T levels were reached just before production of milt could first be detected by handstripping. The concentration of 17 alpha 20 beta P remained elevated throughout the period of milt production and there was evidence that this steroid might be associated with the control of the ionic composition of seminal plasma.