Bycatch mortality of endangered coho salmon: impacts, solutions, and aboriginal perspectives

Abstract

We used biotelemetry and human dimensions surveys to explore potential solutions to migration mortality of an endangered population of coho salmon caught as bycatch in an aboriginal beach seine fishery. From 2009 to 2011, 182 wild coho salmon caught as bycatch in the lower Fraser River (Canada) were radio‐tagged and tracked as they attempted to complete their migrations to natal spawning areas over 300 km upstream. Failure to survive to reach terminal radio receiving stations averaged 39% over three years. This mortality estimate is low compared to those obtained from telemetry studies on other salmon fisheries in the Fraser River. However, this value is markedly higher than the mortality estimate currently used to manage the fishery's impact. It is also in contrast to the perceptions of the majority of aboriginal fishers, who did not think survival of coho salmon is affected by capture and release from their fishery. Increased probability of survival was associated with lower reflex impairment, which is consistent with previous findings. Reflex impairment was positively correlated with entanglement time, suggesting that greater efforts by the fishers to release bycatch from their nets quickly would minimize post‐release mortality. Survey responses by aboriginal fishers also suggested that they are receptive to employing new bycatch handling methods if they are shown to increase post‐release survival. However, attempts to facilitate revival of a subset of captured fish using cylindrical in‐river recovery bags did not improve migration success. Fisheries managers could use the new information from this study to better quantify impacts and evaluate different harvest options. Since aboriginal fishers were receptive to using alternate handling methods, efforts to improve knowledge on minimizing reflex impairment through reductions in handling time could help increase bycatch survival. Such a direct integration of social science and applied ecology is a novel approach to understanding conservation issues that can better inform meaningful actions to promote species recovery.

A synthesis to understand responses to capture stressors among fish discarded from commercial fisheries and options for mitigating their severity

“…For fish discarded from fisheries, behavioural impairment due to stress and/or a failure to recover from the stress event can have fitness effects both directly (e.g., increased probability of predation; Raby, Packer, et al., ; Raby, Packer, Danylchuk, & Cooke, ) and indirectly (e.g., impaired foraging or swimming abilities; Gregory & Wood, ). Extensive comparative physiology research has established that the duration and magnitude of physiological disturbance following acute stress, such as a capture event, are proportional to the severity of the stressor (Chopin & Arimoto, ; Cook, Hinch, Watson, et al, 2018).…”

“…Air exposure is often unavoidable in commercial fisheries and causes acute hypoxia, a deficiency in oxygen reaching the tissues. Although air exposure is most frequent during sorting, hypoxia may also occur if ventilation is restricted (i.e., the operculum cannot move), or with localized oxygen depletion in crowded nets/traps (as observed in Raby, Packer, et al., ). Gill lamellae, the respiratory organs responsible for gas exchange, collapse during air exposure, ceasing aerobic respiration (Ferguson & Tufts, ).…”

“…Fish are progressively corralled into a smaller volume of water during containment; avoidance behaviour and exhaustive exercise increase as the net becomes more constricted, as do forced physical interactions with both the gear and other captured organisms. Dissolved oxygen levels can decrease during crowding, and in some cases result in hypoxic water conditions (Raby, Packer, et al., ). However, these low oxygen conditions can also reduce activity and potentially prevent some struggling‐related injuries (Schurmann & Steffensen, ), though this has not been tested in the context of seine fisheries.…”

“…Reducing handling times is the most practical means to improve survival through a reduction in air exposure and exhaustion. Prioritizing by‐catch during sorting (Poisson, Séret, Vernet, Goujon, & Dagorn, ; Raby, Packer, et al., ) or before brailing (with larger by‐catch species; Hutchinson, Itano, Muir, & Holland, ; Hall et al., ) to expedite their release can be effective. Having the vessel equipped with a canvas sling is a similar solution for the safe release of large by‐catch species reducing risk of injury (Poisson et al., ).…”

Perceptions and Actions of Commercial Fishers in Response to Conservation Measures in Canadian Pacific Salmon Fisheries

“…Concerns remain regarding the survival of Pacific salmon released from these fisheries and the accuracy of fish mortality estimates used in management models more generally (Raby et al. ; Patterson et al. , ; Cook et al.…”