Gillnet illumination as an effective measure to reduce sea turtle bycatch

Abstract

The growing demand for fish around the world is an immediate threat to marine megafauna that are unintentionally captured in commercial and artisanal fishery operations. Bycatch mitigation strategies, such as turtle excluder devices, circle hooks, and net illumination, have successfully reduced this risk in some fisheries. We explored the effectiveness of gillnet illumination to reduce sea turtle captures in 2 artisanal fisheries (Mankoadze and Winneba, Ghana) under normal fishing conditions. We first quantified sea turtle bycatch in Ghana's artisanal gillnet fishery from 15 boats for 12 months. We then quantified catch of targeted species and sea turtle bycatch from 20 boats for 15 months (7427 net sets). For 10 of these boats, we placed a Centro Economy green light (1 LED) at each 10-m interval on the net. We also quantified target catch and sea turtle bycatch from 30 boats for 8 months (2250 net sets). In 15 of these boats, a Centro Deluxe green light (3 LEDs) was installed at 15-m intervals. Boats with economy lights and those with deluxe lights both exhibited an 81% decrease in sea turtle captures (W = 1, p < 0.001, n = 20; W = 215, p < 0.001, n = 30, respectively) compared with control boats without lights. Illuminated nets resulted in fewer turtle catches for leatherback (Dermochelys coriacea), olive ridley (Lepidochelys olivacea), and green sea turtles (Chelonia mydas) (p < 0.05 for all species). Target catch (mass) (W = 53, p = 0.853 n = 20; W = 76, p = 0.449, n = 23) and value (W = 50, p = 1, n = 20; W = 69, p = 0.728, = 23) were not different across treatments. Our study affirms net illumination can reduce capture rates of 3 species of sea turtles, including the imperiled leatherback. Gear modification methods can successfully reduce bycatch if they are affordable and have broad applications for multiple species in different fisheries.

Net illumination reduces fisheries bycatch, maintains catch value, and increases operational efficiency

“…Unlike previous studies, [26][27][28][29][30][31][32][33] we did not find a significant decrease in sea turtle bycatch associated with net illumination. Nonetheless, reductions in mean rates of biomass and number of loggerhead turtles in illuminated nets were high (51% and 50%, respectively; Table 1), indicating a potential trend for reduced bycatch that warrants further investigation.…”

“…25 Over the past decade, net illumination has emerged as a promising tool to reduce bycatch of sea turtles in coastal gillnet fisheries while maintaining target catch. [26][27][28][29][30][31][32][33] More recently, illuminated nets have been found to decrease bycatch of other marine megafauna without affecting target catch, including seabirds 34 and small cetaceans. 32 In theory, net illumination provides a visual cue within the appropriate sensitivity range that selectively deters or alerts non-target species to the presence of nets.…”

“…The significant reduction in total discarded biomass was driven predominantly by decreases in large (i.e., non-finfish) bycatch species (Figure 1; Table 1), which follows a pattern of previously published studies whereby net illumination decreased bycatch of marine megafauna species while maintaining catch rates of target fish in different fisheries and ocean basins worldwide. [26][27][28][29][30][31][32][33][34] Trophic downgrading through removals of consumers occupying high trophic levels can have profound effects on the structure, function, and resilience of marine ecosystems. [16][17][18] Reductions in bycatch of large consumers, such as those observed in this study, may have important ecological benefits through ameliorating the potential impacts that bycatch might have on coastal ecosystems.…”

“…In other gillnet fisheries, net illumination has been estimated to cost as little as 16 to 34 USD to prevent a sea turtle bycatch event. 28,31 We encourage conservation practitioners, fishery managers, and other stakeholders to work with industry to develop new technologies, domestically manufacture LED lights, and seek new methods to increase efficiency and availability.…”

By-catch of sea turtles in Pacific artisanal fishery: Two points of view: From observer and fishers

“…Firstly, we recommend (i) evaluating and using the knowledge of the fishers as a tool to diagnose the situation in a time efficient manner (Vaśquez-Carrillo and Pelaéz-Ossa, 2021), (ii) implementing the most turtle friendly type and size of hook and/or type of bait (larger, circular hooks and fish as bait) (see Swimmer et al, 2020), (iii) evaluating the fishers' willingness to adopt bycatch reduction technologies (e.g. LEDs; Bielli et al, 2020;Darquea et al, 2020;Allman et al, 2021), (iv) implementing economic compensation, such as ecological labels or subsidies that would provide added value or incentives for fishers carrying out sustainable practices (Leduc and Hussey, 2019). Finally, (v) the implementation of education and awareness campaigns is also essential, since 32% of fishers do not see this situation as a problem.…”