The role of gonadal steroid hormones in the sexual behaviour of male rainbow trout (Oncorhynchus mykiss)

The effect of exogenously administered steroid hormones on the spawning behaviour of hatchery-raised and wild-stock male rainbow trout (Oncorhynchus mykiss ) was investigated to determine if male sexual behaviour could be reinstated in castrated individuals. The study with the hatchery-raised fish investigated the effects of silastic implants of testosterone (T), 11-ketoandrostenedione (KA), and a combination of both on the spawning behaviour of castrated rainbow trout in the presence of actively nesting females. Blood samples taken from the males confirmed that the implants raised the plasma levels of the corresponding hormones. Intact males that received an empty implant showed the highest levels of spawning behaviour, followed by those implanted with K A . Males implanted with T and T+KA showed the same levels of behaviour as the castrated controls. Further treatment of the T, K A , and T+KA males with an injection of 17a,20|3-dihydroxy-4-pregen-3-one (P) indicated that P may facilitate the effect of K A on male spawning behaviour. The KA+P group of males exhibited spawning behaviour (attending) as intense as the sham-operated males. A pilot study using injections of salmon pituitary extract did not reinstate spawning behaviours. Treatment of intact males with a prostaglandin inhibitor, indomethacin (IM), had no effect on spawning behaviour. When castrated, wild-caught males were paired with nesting females 3,7, and 10 days after gonadectomy, no differences in spawning behaviours were found when compared with intact males. Treatment of intact males with cyproterone acetate (CYA), an androgen receptor blocker, reduced quiver response 10 days after administration, supporting the hypthesis that androgens play a causal role between androgens and male sexual behaviour. Castrated males were implanted with silastic capsules of K A or solid elastomer pellets containing P. There were no differences in the responses of the gonadectomized controls and the P males. K A males showed a higher level of quivering than the gonadectomized controls, and attending levels similar to those found for the intact controls. The highest level of quivering was found in the K A males following treatment of the groups with pituitary extract. The results of the experiments involving both hatchery-raised and wild-stock male rainbow trout indicate that K T plays a causal role in the maintenance of spawning behaviourScience, Faculty ofZoology, Department ofGraduat