Fishing for Effective Conservation: Context and Biotic Variation are Keys to Understanding the Survival of Pacific Salmon after Catch-and-Release

Abstract

Acute stressors are commonly experienced by wild animals but their effects on fitness rarely are studied in the natural environment. Billions of fish are captured and released annually around the globe across all fishing sectors (e.g., recreational, commercial, subsistence). Whatever the motivation, release often occurs under the assumption of post-release survival. Yet, capture by fisheries (hereafter "fisheries-capture") is likely the most severe acute stressor experienced in the animal's lifetime, which makes the problem of physiological recovery and survival of relevance to biology and conservation. Indeed, fisheries managers require accurate estimates of mortality to better account for total mortality from fishing, while fishers desire guidance on strategies for reducing mortality and maintaining the welfare of released fish, to maximize current and future opportunities for fishing. In partnership with stakeholders, our team has extensively studied the effects of catch-and-release on Pacific salmon in both marine and freshwater environments, using biotelemetry and physiological assessments in a combined laboratory-based and field-based approach. The emergent theme is that post-release rates of mortality are consistently context-specific and can be affected by a suite of interacting biotic and abiotic factors. The fishing gear used, location of a fishery, water temperature, and handling techniques employed by fishers each can dramatically affect survival of the salmon they release. Variation among individuals, co-migrating populations, and between sexes all seem to play a role in the response of fish to capture and in their subsequent survival, potentially driven by pre-capture pathogen-load, maturation states, and inter-individual variation in responsiveness to stress. Although some of these findings are fascinating from a biological perspective, they all create unresolved challenges for managers. We summarize our findings by highlighting the patterns that have emerged most consistently, and point to areas of uncertainty that require further research.

Capture severity, infectious disease processes and sex influence post-release mortality of sockeye salmon bycatch

“…In the present study, the bulk of mortality following entanglement in freshwater did not occur until 5 days after the event, continuing at high rates for another 5–7 days, then subsiding in both treatment groups. The growing body of evidence for delayed mortality following capture in freshwater (Davis, 2002; Arlinghaus et al ., 2007; Donaldson et al ., 2010, 2012; Raby et al ., 2012) and associated sex-specific effects (Martins et al ., 2012b; Robinson et al ., 2013; Donaldson et al ., 2014; Gale et al ., 2014) points to causal factors beyond short-term impacts, such as anaerobiosis or cardiovascular collapse, and instead towards interactions between the capture-related stress and infectious disease processes (Gilhousen, 1990; Raby et al ., 2015) that can differ between sexes.…”

“…Post-release mortality of adult salmon has repeatedly been shown to be context specific and dependent on numerous factors (Raby et al ., 2015). The mechanisms of mortality following capture are therefore complex and include internal and external influences (Fenkes et al ., 2016).…”

“…Pacific salmon productivity is in a state of decline in many natal watersheds, especially at southern range extremes (Hinch et al ., 2012; Peterman and Dorner, 2012). Pre-spawning and en route mortality of adult Pacific salmon have likely contributed to these declines and have been attributed to several factors, including thermal and fisheries stressors encountered during freshwater migration (Gilhousen, 1990; Donaldson et al ., 2011; Martins et al ., 2012b; Gale et al ., 2013; Raby et al ., 2015). Disease processes are also known to influence the survival of wild salmon but have been notoriously difficult to study due to the logistical constraints inherent in monitoring wild animal populations under natural conditions, especially for highly migratory species (Altizer et al ., 2011; Miller et al ., 2014).…”

“…Variability in these physiological responses is common within and among species and stocks (Cooke and Suski, 2005), and is associated with the severity of the capture event (Gale et al ., 2011), the condition of the individual at capture (Davis, 2002; Donaldson et al ., 2012) and the animal's ability to recover (Farrell et al ., 2001; Robinson et al ., 2013). Condition at capture and subsequent recovery are also suspected to be associated with infectious disease processes (Gilhousen, 1990; Raby et al ., 2015). Stress and injury caused by a gear encounter can provide opportunities for infection (Chopin and Arimoto, 1995; Baker and Schindler, 2009; Baker et al ., 2013), elicit enhanced immune surveillance and responses by the host (Dhabhar, 2002; Neeman et al ., 2012) and promote physiological disturbances such as osmoregulatory imbalance that can impair overall host health and resilience (Gale et al ., 2011; Donaldson et al ., 2012; Cooke et al ., 2013).…”

Gill‐Net Fishing Effort Predicts Physical Injuries on Sockeye Salmon Captured near Spawning Grounds

“… and Raby et al. ). A recurring finding from these studies is that fish sustain external injuries such as skin abrasions and disruption of the mucous layer (Davis ).…”