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A cross-taxa assessment of pelagic longline by-catch mitigation measures: Conflicts and mutual benefits to elasmobranchs

Authors:
  • Pelagic Ecosystems Research Group

Abstract

Elasmobranch mortality in pelagic longline fisheries poses a risk to some populations, alters the distribution of abundance between sympatric competitors, changing ecosystem structure, processes and stability. Individual and synergistic effects on elasmobranch catch and survival from pelagic longline gear factors, including methods prescribed to mitigate bycatch of other vulnerable taxa, were determined. Overall relative risk of higher circle vs. J-shaped hook shark catch rates conditioned on potentially informative moderators, from 30 studies, was estimated using an inverse-precision weighted mixed-effects meta-regression modeling approach. Sharks had a 1.20 times (95% CI: 1.03-1.39) significantly higher pooled relative risk of capture on circle hooks, with two significant moderators. The pooled relative risk estimate of ray circle hook catch from 15 studies was not significant (RR=1.22, 95% CI: 0.89-1.66) with no significant moderators. From a literature review, wire leaders had higher shark catch and haulback mortality than monofilament. Interacting effects of hook, bait and leader affect shark catch rates: hook shape and width and bait type determine hooking position and ability to sever monofilament leaders. Circle hooks increased elasmobranch catch but reduced haulback mortality and deep hooking relative to J-shaped hooks of the same or narrower width. Using fish vs. squid for bait increased shark catch and deep hooking. Pelagic stingray (Pteroplatytrygon violacea) catch and mortality were lower on wider hooks. Using circle instead of J-shaped hooks and fish instead of squid for bait, while benefitting sea turtles, odontocetes and possibly seabirds, exacerbates elasmobranch catch and injury, therefore warranting fishery-specific assessments to determine relative risks.
... Some strategies are widely effective in mitigating by-catch of a variety of speciessuch as restricting FADs or switching from wire to mono leadersalthough target catch rates may be affected (Gilman, 2011). Other strategies are more variable depending on the context and species, and in some cases may reduce one type of by-catch but increase catch rates of another species (Gilman, Chaloupka, Swimmer, & Piovano, 2016). ...
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By-catch is the most significant direct threat marine megafauna face at the global scale. However, the magnitude and spatial patterns of megafauna by-catch are still poorly understood, especially in regions with very limited monitoring and expanding fisheries. The Indian Ocean is a globally important region for megafauna biodiversity and for tuna fisheries, but has limited by-catch data. Anecdotal and scattered information indicates high by-catch could be a major threat. Here, we adapt a Productivity Susceptibility Analysis tool designed for data-poor contexts to present the first spatially explicit estimates of by-catch risk of sea turtles, elasmobranchs, and cetaceans in the three major tuna fishing gears (purse seines, longlines, and drift gill nets). Our assessment highlights a potential opportunity for multi-taxa conservation benefits by concentrating management efforts in particular coastal regions. Most coastal waters in the northern Indian Ocean, including countries that have had a minimal engagement with regional management bodies, stand out as high risk for fisheries interactions. In addition to species known to occur in tuna gears, we find high vulnerability to multiple gear types for many poorly known elasmobranchs that do not fall under any existing conservation and management measures. Our results indicate that current by-catch mitigation measures, which focus on safe-release practices, are unlikely to adequately reduce the substantial cumulative fishing impacts on vulnerable species. Preventative solutions that reduce interactions with non-target species (such as closed areas or seasons, or modifications to gear and fishing tactics) are crucial for alleviating risks to megafauna from fisheries.
... This development is contrary to the more globally accepted approach of adopting circular hooks to mitigate the capture of bycatch (Watson et al., 2005;Ward and Hindmarsh, 2007;Pacheco et al., 2011). Circular hooks also increase the survival rates of bycatch species with little or no effect on target species capture (Gilman et al., 2016;Reinhardt et al., 2018). The circular hook has significantly lower shark catch rates compared to the J-shaped hook (Mejuto et al., 2008;Sales et al., 2010;Andraka et al., 2013). ...
Article
The decline in traditionally valuable fish stocks, coupled with an increased demand for shark fins and meat has caused many fishermen to target sharks. However, there is limited information on the fishing practices, gears, and catch distribution for the shark fisheries. This study used a semi-structured interview to characterize the shark fisheries and techniques used to catch sharks as well as the catch statistics and trade in fifteen coastal communities along the Eastern, Central, and Western coasts of Ghana. Out of the 470 fishers interviewed, 46% were specialized shark fishers while the remaining 54% landed sharks as bycatch. Specialized shark fishers captured between 7–10 individual sharks per fishing trip while by-catch shark fishers captured between 1–5 sharks. The shark species harvested include the blue shark, thresher shark, common and sand tiger, bull shark, short and longfin mako shark, hammerhead, and milk shark. Fishers along the eastern coast mostly capture sharks as bycatch, while those along the central and western coast of Ghana target sharks with specialized fishing gears. The catch data obtained from the fishers indicated higher shark landings and was linked to the development of longlines and drift gill nets with hooks attached to the footrope that target sharks, and are usually deployed along the western coastlines of Ghana. Nearly all the shark species captured in the study communities were listed by the IUCN as either “Endangered”, “Critically Endangered” or “Vulnerable”. Among fishers who target sharks, 74% stated that shark populations were declining and were spending more effort to capture the fish. Specialized shark fishers indicated a poor perception and attitude towards shark conservation along the entire coastline of the country and are not in support of shark conservation. It is recommended that the capture and trade in these species should be closely monitored to prevent the extinction of vulnerable populations.
... Several studies have been carried out in which there is evidence of the benefits of using metaanalysis, such as Gilman, E. et al (2016), Musyl, M. and Gilman, E. (2019) and Gilman, E. et al (2020). However, given the multiple advantages of meta-analysis, it is worth highlighting the difficulty in gathering the data necessary to carry them out. ...
... Interest in using mitigation measures to reduce this bycatch (also referred to as incidental catch) is increasing. Some mul-tidisciplinary (Squires et al., 2021) and necessary (Guerra, 2019) actions have been highlighted as good practices, as suggested by Gilman (2011) including: circular instead of Jshaped hooks to avoid capturing sea turtles and marine mammals (Reinhardt et al., 2018), deep sets (> 100 m) for avoiding sea turtles and sharks (Gilman et al., 2006), "tori lines" (Mancini et al., 2009;Jiménez et al., 2014), and line weighting for deterring seabirds (Santos et al., 2019), among other methods as well (Gilman et al., 2016). These actions often encounter resistance from the fishing industry and fishers due to alleged operational difficulties and high costs (Solís et al., 2021). ...
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Sustainable fisheries' managers increasingly seek to implement measures that reduce the mortality of threatened species while maintaining or increasing catch of target species. Our study proposed a simple management option for optimizing fishing over daily and seasonal scales to maximize catch success while protecting non-target species in pelagic longline fisheries. We used Bayesian beta mixed regression models to describe the effects of setting times and seasonality on catches in a pelagic longline fishery in the southwest South Atlantic Ocean (SWAO). Targeted species (swordfish, blue shark, and albacore tuna) are typically captured in fully nocturnal sets (started between 16 and 00 h), whereas shortfin mako shark and loggerhead turtles are typically captured during partially nocturnal sets (started between 00 and 04 h); probably a response to hook depth and circadian behaviours. The results suggest that it is feasible to use only fully nocturnal sets to target preferred species while reducing incidental catch of non-target species. The catch of target species was higher in austral winter, explained by the northward displacement of the subtropical convergence in the SWAO during this season. These results provide a baseline for bycatch mitigation strategies in pelagic longline fisheries at regional and global scales.
... Hook shape can significantly affect the catch risk of most pelagic shark species. There is higher shark catch risk on circle as compared to J-shaped hooks, presenting a conflict with marine turtles (Gilman et al., 2016;Reinhardt et al., 2017). ...
Article
For many years, tremendous effort has been dedicated to developing new industrial tuna fisheries, while their adverse impacts on threatened marine species have received relatively little attention. In tuna fisheries, bycatch is the major anthropogenic threat to marine megafauna in general, particularly sharks. Research on the development of gear technology for bycatch reduction and potential mitigation measures helped tuna Regional Fisheries Management Organizations adopt bycatch reduction management measures. After reviewing past research on the development of mitigation measures for pelagic longline and tropical purse seine fisheries based on pelagic species' behaviours, we describe promising new approaches integrating recent technological breakthroughs. New innovations include autonomous underwater vehicles carrying cameras along with miniaturized sensors, aerial drones, computer simulation of fishing gear geometry, environmental DNA assays, computer visualizations and deep learning. The successful application of such tools and methods promises to improve our understanding of factors that influence capture, escape and stress of caught species. Moreover, results emerging from recent ethological research explaining the power of social connection and learning in the “fish world” such as social learning from congeners, habituation to deterrents, and how past fishery interactions affect responses to fishing gear should be taken into account when developing technical mitigation measures.
... Essentially, the massive removal of sea life as a whole has been depleting the food resources of the sharks (Ferretti et al., 2010). Unsustainable fisheries are making sharks more sensitive to changes at all levels of the marine community (Gilman et al., 2016;Rosa, Rummer & Munday, 2017). The indirect pressures of the fisheries, such as bycatch, also have an impact on the shark populations (Gallagher et al., 2014). ...
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• Elasmobranchs are apex predators that play a crucial role in marine ecosystems by regulating the dynamics of food webs, as well as connecting different trophic levels across habitats. • The large-scale removal of elasmobranchs impacts the energy transfer in trophic interactions. The pressure of unsustainable fisheries is considerable, as most elasmobranchs have reproductive strategies that render them unable to recover their demographic status after depletion. • In Brazil, elasmobranchs are broadly commercialized under the generalist common name of ‘cação’ (namely, shark meat). This allows threatened species to be commercialized and makes the tracking of different species difficult. • DNA barcoding of the Cytochrome c oxidase subunit 1 (COI) gene was applied to identify the different species sold as ‘cação’ along the coastline of Brazil. Fifty-seven samples from 33 cities in 15 coastal states of Brazil were purchased and analysed. • Bioinformatic analyses revealed the presence of 17 species that were sold as ‘cação’. Among them, Prionace glauca (blue shark) was the most abundant. Other species, listed as Endangered under the IUCN Red List of Threatened Species, were also uncovered as being in the shark meat trade, such as Sphyrna lewini (scalloped hammerhead), Isurus paucus (longfin mako shark), and Squatina guggenheim (angular angel shark). • These findings have reinforced the necessity to correctly label the commercialized species. Public actions towards species-specific management plans must be applied, as well as monitoring the supervised allied educational programmes.
... Our findings contrast Afonso et al. (2012) and Ward et al. (2008), which found that cable leaders caught more sharks when compared to nylon leaders. Similarly, a literature review of longline gear selectivity found shark catches were higher on cable leaders across several longline surveys (Gilman et al., 2016). It is important to note that two large species C. leucas (n = 4) and S. mokarran (n = 1) were exclusively caught on monofilament, however with the small sample size it was not possible to determine statistical significance. ...
Article
Our study assessed the influence of leader type (monofilament vs stainless steel cable) and gear strength (i.e., hook size and leader thickness) on species composition, catch rates, and size distribution of sharks captured in a fisheries independent longline survey near Bimini, The Bahamas. A total of 28 surveys were conducted, resulting in 50,400 hook hours. Catches consisted of 167 sharks from eight species. Overall, catches were higher on monofilament leaders for light duty gear, but there was no difference for heavy duty gear. When bite-offs were combined with catches to form ‘total shark contacts’ the rates were not significantly different between the two leader types. Tiger sharks were the only species to show differences in mean pre-caudal lengths between light and heavy duty gear, likely due to the wide range of size classes encountered for this species. These results further highlight the importance of understanding selectivity of terminal gear (gear strength and leader material), especially for comparing catches across surveys conducted by multiple agencies or organizations.
... Essentially, the massive removal of sea life as a whole has been depleting the food resources of the sharks (Ferretti et al., 2010). Unsustainable fisheries are making sharks (notorious integrative energy connectors) more sensitive to changes at any level of the marine community (Gilman et al., 2016;Rosa, Rummer, & Munday, 2017). The indirect pressures of the fisheries also impact the shark populations when the catches are incidental (bycatch) (Gallagher et al., 2014). ...
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