On the importance of considering sublethal stress and injury effects in discard and escapee survival and fitness

Shark release, NOAA

Sublethal stress and injury are important factors to consider in discard and escapee survival and fitness (Wilson et al. 2014). Fishing gears are often designed to enhance escape of animals that might otherwise become bycatch. Captured animals are discarded from fisheries for economic, regulatory, conservation ethic, and political reasons. Observation of animal impairment (Davis 2010) and immediate mortality for discards on board fishing vessels is straightforward. For animals that are alive when discarded or that escape from fishing gear, the effects of stress and injury can alter behavior, growth, and reproduction, or result in delayed mortality. Delayed mortality of escapees (Suuronen 2005) and discards (Revill 2012) has been documented for commercially important species. However, few studies have measured potential fitness effects of sublethal stress and injury on surviving discards or escapees.

Fig. 1. Conceptual diagram outlining the immediate and long-term effects of escape or release from commercial fishing gear and how it relates to each level of biological organization. Question marks (?) denote areas for which no primary literature exists, and present future avenues of research (Wilson et al. 2014).

Sublethal effects of capture, escaping, and discarding can occur at individual, community, and population levels of organization. For individuals, immediate sublethal effects are physiological responses, injury, and reflex impairment. Delayed sublethal effects are behavioral impairment, altered energy allocation, wound healing, immune function and disease, reproductive success, and offspring quality and performance. Few studies have been conducted for responses at community and population levels, and clearly these are important to consider.

Wilson et al. 2014 summarize:

“The obvious gap that emerges is the lack of research linking at-release measurements with latent sublethal fitness outcomes such as foraging, energetics, growth, reproduction and offspring quality. The dearth of knowledge in this area is likely based on two realities: (1) a justifiable focus on simply quantifying and reducing bycatch mortality, and (2) the difficulty of long-term monitoring of fitness outcomes in wild animals. Of the reviewed studies, several indicated that physiological disturbance, injury or behavioural impairments may have had long-term implications for growth and reproductive fitness. Further study of sublethal effects could clarify previously unaccounted-for population level consequences of fisheries and better conservation practices to mitigate the impacts of fisheries.”

Also of importance to discard survival and fitness is consideration of predation that can occur after escape or discarding of captured animals (Raby et al. in press). Controlling factors for predator-induced mortality include fishery type, stress and injury, barotrauma, predator behavior and abundance, fish size, and temperature. Summary of the Raby et al. review suggests research directions:
  “The important first step is for fishers, managers and researchers to identify systems where predation is likely to be a substantial contributor to unobserved fishing mortality. Most study of capture-and-release mortality involves quantifying the effects of factors such as temperature, capture depth or fight time. Predator type and abundance could be considered new ‘phantom’ factors that are dynamic and would be challenging to incorporate into research. A conservative approach would be to assume a constant level of predation threat for a given fishery and focus on examining the capacity of released fish to evade predators and the accompanying rates of predator-induced mortality. PRP is a unique contributor to mortality because it is probably most often characterized by a short period (minutes or hours) of risk, which could simply be overcome by using pre-release techniques that reduce the impairment of fish being released (Farrell et al. 2001; Broadhurst et al. 2009).”

Future studies that can be used to assess the presence of delayed sublethal fitness effects in fisheries escapees and discards include allostasis, biotelemetry, reproductive success of individuals, measurement of genetic material contributions to next generations, and tank or net pen holding studies to determine behavior, growth rates, and reproduction. In all these types of studies, ongoing collection of fisheries observer data on reflex impairment and injury using a vitality scoring system (RAMP) would be needed to link vitality scores of at-release discards or escapees back to fitness outcomes for individuals.