Phylogeny explains capture mortality of sharks and rays in pelagic longline fisheries: a global meta-analytic synthesis

Abstract

Apex and mesopredators such as elasmobranchs are important for maintaining ocean health and are the focus of conservation efforts to mitigate exposure to fishing and other anthropogenic hazards. Quantifying fishing mortality components such as at-vessel mortality (AVM) is necessary for effective bycatch management. We assembled a database for 61 elasmobranch species and conducted a global meta-synthesis to estimate pelagic longline AVM rates. Evolutionary history was a significant predictor of AVM, accounting for up to 13% of variance in Bayesian phylogenetic meta-regression models for Lamniformes and Carcharhiniformes clades. Phylogenetically related species may have a high degree of shared traits that explain AVM. Model-estimated posterior mean AVM rates ranged from 5% (95% HDI 0.1%–16%) for pelagic stingrays and 76% (95% HDI 49%–90%) for salmon sharks. Measures that reduce catch, and hence AVM levels, such as input controls, bycatch quotas and gear technology to increase selectivity are appropriate for species with higher AVM rates. In addition to reducing catchability, handling-and-release practices and interventions such as retention bans in shark sanctuaries and bans on shark finning and trade hold promise for species with lower AVM rates. Robust, and where applicable, phylogenetically-adjusted elasmobranch AVM rates are essential for evidence-informed bycatch policy.

Evidence from interpretable machine learning to inform spatial management of Palau's tuna fisheries

“…The time‐of‐day of fishing operations and fishing depth can affect the vertical overlap (encounterability) and catch risk of some pelagic marine predators whose vertical distributions can vary temporally due to diel vertical migration cycles that mirror the movements of their prey, time of day of foraging and temporal variability in diving behavior (Gilman, Chaloupka, et al, 2019; Musyl et al, 2011; Rodrigues et al, 2022). These two variables also affect species‐specific at‐vessel mortality rates (whether catch are alive or dead when retrieved during the gear haulback before being handled by crew) (Ellis et al, 2017; Gilman, Chaloupka, et al, 2022; Orbesen et al, 2017). The number of pelagic longline hooks that are attached between two floats is an approximate index for relative but not absolute fishing depth.…”

Global governance guard rails for sharks: Progress towards implementing the United Nations international plan of action

“…The IPOA should be expanded to provide this information. The gaps in bycatch mitigation approaches included mitigating (i) the production and ghost fishing by abandoned, lost and discarded fishing gear (Gilman et al, 2021; Macfadyen et al, 2009); (ii) pre‐catch, at‐vessel and post‐release mortality risk (Ellis et al, 2017; Gallagher et al, 2014; Gilman, Chaloupka, et al, 2022; Musyl & Gilman, 2019); (iii) several methods to increase fishing gear selectivity such as reducing the attractiveness of the gear by using certain bait species and artificial bait and not using light attractors (Gilman et al, 2020; Poisson et al, 2016); and (iv) bycatch offsets (Booth et al, 2020; Gilman et al, 2023). These NPOA gaps are potentially substantial opportunities to improve global chondrichthyan conservation and management measures.…”

“…For example, the IPOA states that NPOAs should aim to minimize shark discards (FAO, 1999a). While discard bans may incentivize fishers to implement more selective fishing gear designs and methods to reduce catch rates of unwanted species and sizes of catch subject to the ban, this might substantially exacerbate fishing mortality of chondrichthyan species with low at‐vessel and post‐release mortality rates (Ellis et al, 2017; Gilman, Chaloupka, et al, 2022). Therefore, this recommendation in the IPOA should be revised to improve clarity on under what conditions discard bans are likely to be effective in supporting conservation objectives.…”

“…Therefore, this recommendation in the IPOA should be revised to improve clarity on under what conditions discard bans are likely to be effective in supporting conservation objectives. For chondrichthyan species with low at‐vessel as well as post‐release mortality rates, in addition to reducing catchability, prescribed handling‐and‐release practices and policy interventions that, under certain enabling environments, reduce retention, including retention bans in shark sanctuaries, and shark finning and trade bans, can effectively reduce fishing mortality (Friedman et al, 2018; Gilman, Chaloupka, et al, 2022). The most promising approach to reduce the fishing mortality of species with either high retention rates or that have high at‐vessel or post‐release mortality rates is to employ methods that reduce catch risk.…”

Adjusting time-of-day and depth of fishing provides an economically viable solution to seabird bycatch in an albacore tuna longline fishery

“…The objectives of analyses were to assess the effect of relative depth and the time-of-day of fishing on seabird and target species catch rates. These operational factors are also informative predictors of marine turtle and species-specific elasmobranch catch and at-vessel mortality rates 8 , 15 , 18 , 20 . Findings support evidence-informed interventions to reduce the mortality of threatened seabird species in albacore tuna longline fisheries.…”