The Effects of Fisheries Capture on the Physiology and Post-Release Fate of Adult Pacific Salmon

All animals encounter acute stressors during their lifetimes, and while the immediate response to those stressors is well understood and presumed to be adaptive, relatively few studies have linked those responses with subsequent fitness outcomes. This problem finds particular relevance in the fisheries realm, where there is interest in developing a) an understanding of what leads to mortality for caught-and-released animals, b) methods for reducing post-release mortality, and c) predictors of delayed mortality. Pacific salmon are a tractable model for studying post-release mortality because the migration success of individuals after release can be easily and effectively tracked, and migration failure means zero lifetime fitness. In this thesis I report on research in which I used physiological assessments and tracked Pacific salmon fitness outcomes in the wild to examine the response to and recovery from capture, and whether individual differences in responses could be linked to migration or spawning failure. A key finding that arose throughout was that reflex impairment is an effective indicator of the whole-animal response to capture stressors, is correlated with dermal injury, reflects underlying physiological processes, and can predict delayed mortality. I demonstrate that mortality rates currently used in management models are likely inaccurate, but use several lines of evidence to show that mortality could be reduced using different capture and handling techniques. Specifically, more proactive efforts to reduce handling time reduced physiological disturbance and reflex impairment. Revival using industry-standard revival totes and novel in-river recovery bags did not reduce delayed mortality, although the latter and forced-flow revival boxes appeared effective at expediting short-term revival. I found some evidence that sensitivity to capture stressors may change dynamically throughout the spawning migration, with fish becoming particularly resilient once reaching spawning areas. Well-controlled experiments are required if the knowledge gaps arising from this thesis are to be addressed: namely, how does resilience to capture stressors change over the course of the spawning migration, and when does facilitated revival benefit fish survival? Collectively, the work presented in this thesis provides a useful addition to our understanding of the effects of fisheries capture on the physiology and survival of fish.