The significance of olfaction and social cues in milt availability, sexual hormone status, and spawning behavior of male rainbow trout (Oncorhynchus mykiss).

Male rainbow trout orientate to a "releaser" pheromone emitted by an ovulated female. This chemical signal is not necessary for the completion of spawning behavior: anosmic males spawned as readily as intact males when paired with ovulated females. The amount of "strippable" milt and plasma concentrations of testosterone (T) and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) were greater in males paired for 3 hr with ovulated females than in isolated males, or males placed with nonvitellogenic adult females or in all male groups. Milt and plasma T, 17,20 beta-P, and 11-ketotestosterone (11-KT) were reduced in anosmic males paired with ovulated females, indicating the existence of a priming pheromone. Milt, T, 11-KT, and 17,20 beta-P increased in intact males within 4 hr after injection of salmon gonadotrophin-releasing hormone analogue, lyophilized salmon pituitary extract, or homogenized fresh rainbow trout pituitary material. The results indicate that the priming pheromone acts through the hypothalamus-pituitary-gonad axis.