There has been growing concern over the sustainability of marine megafauna exposed to bycatch fishing mortality. This is especially the case for species with low intrinsic population growth rates, late maturity and other life histories that make them particularly vulnerable to elevated mortality from anthropogenic pressures (Hutchings et al., 2012; Jorgensen et al., 2022; Musick 1999). Selective fishery removals of pelagic marine apex and mesopredators can alter population and ecosystem size structure, have cascading effects down food webs in some pelagic ecosystems and cause fisheries-induced evolution (Kitchell et al., 2002; Polovina & Woodworth- Jefcoats, 2013; Ward & Myers, 2005). There has also been increasing attention to risks from bycatch to food, nutrition and livelihood security (Jaiteh et al., 2017; Seidu et al., 2022).
Tuna purse seine fisheries are a substantial anthropogenic mortality source for silky (Carcharhinus falciformis) and other species of sharks, including oceanic whitetip sharks (C. longimanus), hammerheads (Sphyrnidae) and whale sharks (Rhincodon typus). They also capture manta and devil rays (Mobula spp.), marine turtles, whales, and mainly in the eastern Pacific Ocean, sets may be made on tuna schools associated with dolphins (Dagorn et al., 2013; Filmalter et al., 2021; Hall & Roman, 2013; Kaplan et al., 2014; Lezama-Ochoa et al., 2019; Poisson et al., 2014). For some gear types and some taxa of at-risk bycatch, several methods are now available that avoid and substantially reduce catch and fishing mortality of bycatch that are also economically viable, practical, safe and support a broad range of approaches for effective compliance monitoring (Gilman, 2011; Hall et al., 2017; Poisson et al., 2016). However, there has been mixed progress in their uptake (Gilman et al., 2014; Juan-Jorda et al., 2018). This includes input and output controls, international trade bans, restrictions on drifting fish aggregating device (FAD) designs to avoid shark and turtle entanglement, restrictions on purse seine set type, handling and release practices and area-based management tools (ABMTs) (Gilman et al., 2022; Hall et al., 2017; Poisson et al., 2016).
This study identified the spatial exposure of at-risk and target tuna species to purse seine
fishery hazards in the western Pacific Ocean. The study analyzed observer data from PNG and Philippine flagged tuna purse seine vessels to estimate the effect of the spatial and temporal distribution of fishing effort on catch rates of at-risk and target species, with effort conditioned to account for other potentially informative predictors of catch risk based on fitting spatially- explicit generalized additive multilevel regression models within a Bayesian inference framework. Findings identify potential multispecies conflicts from alternative spatial management strategies so that any unavoidable tradeoffs are planned and acceptable (Gilman et al., 2019b). The study objective was to determine if there are temporally and spatially predictable hotspots and coldspots for catch rates of at-risk species and of target tunas to determine if these can be feasibly separated. Findings provide evidence to inform the design of a bycatch management strategy that incorporates spatial and temporal management to avoid catch rate hotspots of at-risk species without compromising the catch of principal market species.
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