Physiological profiles of sockeye salmon in the northeastern Pacific Ocean and the effects of exogenous GnRH and testosterone on rates of homeward migration

We captured 196 adult Fraser River sockeye salmon (Oncorhynchus nerka) in the Gulf of Alaska and collected blood and tissue samples to describe their energetics and physiology at an early stage of homeward migration. Somatic energy concentrations differed significantly among population (run-timing) groups, with the earliest entering group (the Early Summer- runs) possessing less energy than Summer-run and Late Summer-run sockeye. Conversely, Early Summer-run fish had invested significantly more in testicular development relative to the other run-timing groups (76.1 ± 9.8g vs. 47.0 ± 8.0 g and 39.0 ± 5.4g). Egg production followed a similar trend but was only marginally significant. Plasma testosterone was also nearly twice as high in Early Summer sockeye relative to Late Summer- run sockeye (89.01 ± 13.12ng mL-1 vs. 38.69 ± 5.61 ngmL-1). To test the pleiotropic effect of reproductive hormones on migratory behaviour, we implanted these same 196 sockeye with gonadotropin-releasing hormone and/or testosterone and examined travel times via acoustic telemetry. Relative to controls, there was no significant relationship between hormonal treatment and travel times, which suggests that exogenous treatment had little effect though sample size was small (N = 13). Nonetheless, pre-treatment levels of testosterone correlated significantly with travel times (r = -0.813), irrespective of treatment.