Mechanisms to Explain Bycatch Mortality in Coho Salmon: Interactions Between Injury, Reflex Impairment, and Physiology

Discard research frequently involves the use of reflex impairment, injury, or blood physiology as means of quantifying vitality and predicting mortality but exceptionally few studies have used all three metrics. We conducted an experimental purse seine fishery for Pacific salmon in the Juan de Fuca Strait, with a focus on understanding the relationships between different sublethal indicators and whether mortality could be predicted in coho salmon bycatch. We monitored mortality using a 24h net pen experiment and acoustic telemetry, two approaches commonly used to assess bycatch mortality that have rarely been directly compared. Mortality was predicted by injury and reflex impairment, but only in the net pen. Higher reflex impairment was clearly mirrored by perturbations to plasma ions and lactate, supporting the notion that reflex impairment can be used as a proxy for departure from physiological homeostasis. Reflex impairment was also correlated with injury scores, while injury scores significantly correlated with plasma ion concentrations. Collectively, these results help refine our understanding of the different sublethal metrics used to assess bycatch and the mechanisms that can lead to mortality.