Looking Beyond the Mortality of Bycatch: Sublethal Effects of Incidental Capture on Marine Animals

Mortality rates among “bycatch” marine animals, that are caught by commercial fishers with other targets and released back into the ocean, have been heavily studied. However, there has been little research into the effect the injury and stress of capture has upon the behavior, growth, and reproduction of bycatch survivors. The authors reviewed 133 studies with the goal of examining the impact of capture stress on lifetime reproductive success, and to identify gaps to be addressed by future research. Exhaustion from trying to escape capture, injury during captivity, and increased vulnerability while recovering from physiological stress after release can impede foraging ability, reduce mating behavior, interrupt lactation, and result in reduced and damaged offspring. The authors call for increased research into the after effects of catch and release fishing to improve understanding of appropriate management and conservation.

There is a widely recognized need to understand and reduce the incidental effects of marine fishing on non-target animals. Previous research on marine bycatch has largely focused on simply quantifying mortality. However, much less is known about the organism-level sublethal effects, including the potential for behavioural alterations, physiological and energetic costs, and associated reductions in feeding, growth, or reproduction (i.e., fitness) which can occur undetected following escape or release from fishing gear. We reviewed the literature and found 133 marine bycatch papers that included sublethal endpoints such as physiological disturbance, behavioural impairment, injury, reflex impairment, and effects on reproduction, feeding, and growth for animals that survived a fisheries interaction. Of the 133 identified articles, 22 documented sublethal effects of capture using metrics directly related to fitness, life history, or population-level processes. Sublethal effects were classified as either short-term (e.g., acute stress response), which could lead to long-term or delayed sublethal outcomes (e.g., growth, reproduction), which are directly fitness-relevant and could have had population-level effects. We recommend further investigation into the effects of injury on fitness, and the effects of capture stress on reproduction. It is completely unknown whether sublethal effects can have significant consequences at the population- or ecosystem-level. To date, the potential for discards to suffer from sublethal fitness effects has been almost entirely ignored, and added knowledge on the topic could benefit both conservation and management.