SEXUAL DIFFERENCES IN HOMING PROFILES AND SHORTENING OF HOMING DURATION BY GONADOTROPIN-RELEASING-HORMONE ANALOG IMPLANTATION IN LACUSTRINESOCKEYE-SALMON (ONCORHYNCHUS-NERKA) IN LAKE-SHIKOTSU

Adult sockeye salmon (Oncorhynchus nerka) in Lake Shikotsu were captur ed in September, October and November adjacent to their natal hatchery prior to spawning. They were sampled for hormones, tagged and release d in the center of lake. Fish were again sampled at recapture to chara cterize changes in steroid hormone levels in individual migrants as we ll as homing percentage and duration in each month. All males returned faster than females early in the breeding season, although a half of the tagged males did not return to the natal site late in the season ( November). A high percentage of females always returned, and homing du ration shortened late in the season. In males, the shortening of homin g duration coincided with an increase in serum testosterone (T) and 11 -ketotestosterone levels. In females, the shortening of homing duratio n corresponded to an elevation of serum T and 17 alpha,20 beta-dihydro xy-4-pregnen-3-one (DHP) levels, and a drop in serum estradiol-17 beta levels. Sustained administration of gonadotropin-releasing hormone an alog (GnRHa, via implants) in September greatly shortened homing durat ion, especially in females. GnRHa treatment caused a dramatic increase in serum DHP levels in both sexes on average. Individual GnRHa-treate d males which rapidly returned, however, showed higher serum T levels and lower serum DHP levels than slower returning males. The present st udy indicates sexual differences in homing profiles as well as shorten ing of homing duration following GnRHa implantation in lacustrine sock eye salmon in Lake Shikotsu which may be reflective of changes in seru m steroid hormone levels.