Urine of reproductively mature female rainbow trout, Oncorhynchus mykiss (Walbaum), contains a priming pheromone which enhances plasma levels of sex steroids and gonadotrophin II in males

Urine of reproductively mature female rainbow trout was shown to contain a priming pheromone which raised the levels of 17 a ,20β‐dihydroxy‐4‐pregnen‐3‐one (17,20β‐P), testosterone and gonadotrophin II in the blood plasma of reproductively mature male rainbow trout. Milt volumes, however, were unaffected. Because it had been established in a previous study that the sulphated form of 17,20β‐P is abundant in the urine of spawning rainbow trout, synthetic 17 a ,20β‐dihydroxy‐4‐pregnen‐3‐one 20‐sulphate was also tested for pheromonal activity. It was found to have only a smalt and inconsistent priming effect on steroid levels and did not alter the orientation or spawning activity of males. It was also shown that, in the majority of experiments, there was a significant drop, over the course of 24 h, in the levels of 17,20β‐P and testosterone in the control groups of males.