Gonadotropin-Releasing Hormone Analog and Sex Steroids Shorten Homing Duration of Sockeye Salmon in Lake Shikotsu

Abstract

Gonadotropin-releasing hormone analog (GnRHa), testosterone (T) or 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) was implanted in lacustrine sockeye salmon (Oncorhynchus nerka) to examine their effects on homing behavior prior to spawning. Maturing adult fish in Lake Shikotsu were captured by a set net adjacent to their natal hatchery in September and October, 1997. They were tagged, implanted with hormones and released at the center of lake. More than 70% of released fish returned to the hatchery. In September, GnRHa-implanted fish returned significantly earlier than the controls regardless of sexes. DHP also shortened homing duration in the females but not in the males. T tended to reduce homing duration in the males, however it did not have any significant effect in the females. In October, fish in all groups quickly returned within a few days after the release. Hence, the shortening effect of GnRHa on homing duration was not seen. The October fish may be already well primed by endogenous hormones. The present study showed that, in September, GnRHa consistently shortened homing duration, whereas sex steroid actions varied depending on sex in lacustrine sockeye salmon.