The stress response predicts migration failure but not migration rate in a semelparous fish

• There exist few examples of a direct link between the stress response and fitness.
• The stress response was quantified in sockeye salmon homing to spawning grounds.
• Being semelparous, a failure to spawn indicates zero fitness.
• Fish migrating past the most difficult area had a reduced stress response.

Recent findings from iteroparous species suggest that glucocorticoid secretion following acute stress can mediate behavior and survival strategies, ultimately influencing fitness. However, these correlates of the stress response may not exist in semelparous animals given the inability to maximize fitness by delaying reproduction. We measured baseline and stress-induced cortisol concentrations in semelparous sockeye salmon (Oncorhynchus nerka) following exposure to an acute stressor at the mouth of the Fraser River in British Columbia. The homing fish were then radio-tagged and tracked throughout their in-river migration. Findings reveal that the stress response (i.e. change from baseline to stress-induced cortisol) was predictive of mortality; fish failing to leave the release site had a significantly greater stress response (mean ± SE = 1004.0 ± 75.3 ng/mL) compared to fish capable of successfully migrating beyond one of the most difficult areas of passage over 100 river kilometers upstream (mean ± SE = 780.7 ± 66.7 ng/mL). However, there were no associations between swimming behaviors, both immediately following release and to last point of detection, and the stress response. This study also introduced an unique method of tagging migrating salmon that allows for rapid capture and sampling and thus provides the first assessment of true baseline cortisol concentrations at river-entry for migrating Pacific salmon in the wild. Results show the stress response to be linked to survival in a semelparous species and therefore set the stage for further exploration into how the evolutionary theories underlying relationships between stress responsiveness and fitness may differ between semelparous and iteroparous species.