Abstract
A spawning pheromone in the milt (semen) and testes of the Pacific herring, Clupea harengus pallasi, triggers spawning in sexually mature fish of both sexes and is thought to facilitate school spawning of this species. We found the response to the pheromone to be a stereotyped behavioral sequence consisting of a graded extension of the gonadal papilla, release of gametes, and spawn deposition behavior. The response is triggered by an olfactory stimulus, as demonstrated by the elimination of the response by occlusion of the nares. Stimulus concentrations of an approximate 1:500 dilution of fresh milt or the equivalent of 0.02 g of fully mature testes per milliliter were required to elicit a response in 50% of ripe herring that are responsive to the pheromone. Female fish appeared to be less sensitive to the pheromone in milt than males early in the spawning season, but not thereafter. The average duration of responses of male fish was longer after exposure to concentrated milt than to testes extracts, but no consistent difference in response times of the two sexes was detected. Factors other than the spawning pheromone, maturity of the fish, and stress also were found to influence the spawning response. For example, exposure to shallow (3 cm) water in a small tank induced "spontaneous" papilla extension and spawning approximately 20 min after refilling the tank; occluding the nares prevented this response. Also, the presence of floating kelp (Macrocystis) resulted in prolonged spawning in a large tank after pheromonal stimulation.