Post-Capture Survival and Implications for By-Catch in a Multi-Species Coastal Gillnet Fishery

Abstract

As fisheries shift towards ecosystem-based management, the need to reduce impacts on by-catch has been increasingly recognised. In this study the catch composition, discard rate, and post-capture survival of species caught by gillnets in Tasmania, Australia, was investigated. Over half the commercial gillnet catch was discarded, with discard rates of ~20% for target and >80% for non-target species. Capture condition, including initial mortality, was assessed using simple criteria for a range of species and related to soak duration. Delayed mortality was also assessed using tank trials and related to capture condition. By combining initial and delayed mortality rates post-capture survival was estimated. Longer soak durations generally resulted in slight, but significant, declines in capture condition and lower initial survival. Nonetheless, when combined with delayed survival, four of the five most commonly caught species (Cheilodactylus spectabilis, Latridopsis forsteri, Aplodactylus arctidens, Cephaloscyllium laticeps) exhibited high post-capture survival (83–100%) for soak durations within the maximum regulated range. Post-capture survival of the one remaining commonly caught species, Notolabrus tetricus, (typically discarded) declined with increased soak duration from 84% to 62%, suggesting that this species would benefit from a further reduction in maximum soak duration. Initial and delayed survival rates for the species retained for tank trials exhibited a significant linear relationship, which was used to estimate delayed survival rates for the rarer species. This method enabled the estimation of potential post-capture survival rates for a diverse range of species and may have application in other data-limited situations where the relationships between fishing practices and by-catch survival are uncertain. Overall, our results suggest that soak duration regulation has been effective in reducing by-catch mortality for many species, noting that some species have low survival rates regardless of soak duration.

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

“…The amount of time a hanging net is deployed, soak time, can have serious implications for both survival probability, and exposure to predation (Bell & Lyle, 2016;Uhlmann & Broadhurst, 2015).…”

“…Reducing soak times may be the most effective mitigation option for most hanging net fisheries, but reduced catch-per-unit-effort (CPUE) will often result (Buchanan et al, 2002;Frick, Reina, & Walker, 2010;Uhlmann & Broadhurst, 2015) and some species may suffer high mortality regardless of soak duration (Bell & Lyle, 2016). Shorter deployments may be favoured in some fisheries given improved catch quality (Uhlmann & Broadhurst, 2015), but in others, the level of damage incurred may be so high that reducing soak times would not sufficiently improve catch quality (Savina et al, 2016).…”

Assessing relative post-release mortality for the transparent goby fishery: Environmental drivers and the utility of vitality metrics

“…The implementation of ecosystem-based fishery management and the need to restrict harvest levels have highlighted issues relating to discard mortality [5,6]. Therefore, to fully assess the effects of fishing at the individual, population and ecosystem scales, it is necessary to quantify the levels of discarding and associated rates of fishery-induced mortality [7].…”

“…Therefore, to fully assess the effects of fishing at the individual, population and ecosystem scales, it is necessary to quantify the levels of discarding and associated rates of fishery-induced mortality [7]. The fraction of the catch that may survive when released is highly variable, depending on species, gear, depth, water temperature or fish size [5,6,8]. In some cases, very high survival rates have been reported [9,10].…”

Natural and Anthropogenic Factors Influencing the Pre-Spawn Migration of Steelhead Trout

“…Differences exist across species, population, and fisheries that can influence the severity that capture has on an individual (Cooke and Suski, 2005;Raby et al, 2015). Fish captured in net fisheries (gillnet, seine net, tangle net, fyke net) may experience physical damage to organs, flesh, scales, and the mucous layer due to entanglement (Vander Haegan et al, 2004;Smith and Scharf, 2011;Colotelo et al, 2013;Raby et al, 2015;Bell and Lyle, 2016). Physical damage may also occur for fish captured by angling due to damage at the hooking location (Hühn and Arlinghaus, 2011) and…”