Gonadotropin-releasing hormone analog and sex steroids shorten homing duration of sockeye salmon in Lake Shikotsu

Gonadotropin-releasing hormone analog (GnRHa), testosterone (T) or 17 alpha , 20 beta-dihydroxy-4pregnen-3-one (DHP) was implanted in lacustrine sockey e salmon (Oncorhynchus nerka) to examine their effects on homing behavior p rior to spawning. Maturing adult fish in Lake Shikotsu were captured by a s et net adjacent to their natal hatchery in September and October, 1997. The y were tagged, implanted with hormones and released at the center of lake. More than 70% of released fish returned to the hatchery. In September, GnRH a-implanted fish returned significantly earlier than the controls regardles s of sexes. DHP also shortened homing duration in the females but not in th e males. T tended to reduce homing duration in the males, however it did no t have any significant effect in the females. In October, fish in all group s quickly returned within a few days after the release. Hence, the shorteni ng 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 st eroid actions varied depending on sex in lacustrine sockeye salmon.