An illuminating idea to reduce bycatch in the Peruvian small-scale gillnet fishery
Introduction
Gillnet fisheries are found in the coastal waters of all continents (Gilman et al., 2010; Waugh et al., 2011) and for many countries, gillnet fisheries comprise the largest component of their small-scale fishing fleets (Alfaro-Shigueto et al., 2010; Žydelis et al., 2013). Incidental catch, or ‘bycatch’, in gillnets is a major threat to many marine taxa and contributes to the population decline of numerous threatened marine species (Alfaro-Shigueto et al., 2011; Read et al., 2006). Gillnet fisheries are regarded as some of the largest sources of mortality for sea turtles (Lewison et al., 2014; Peckham et al., 2007), cetaceans (Dawson and Slooten, 2005; Lowry et al., 2018; Read et al., 2006; Reeves et al., 2013) and seabirds (Crawford et al., 2017; Žydelis et al., 2013). However, solutions to the problem of bycatch in net fisheries, have been difficult to identify and implement (Martin and Crawford, 2015; Žydelis et al., 2013).
In Peru, the total length of gillnets set is estimated to exceed 100,000 km per year (Alfaro-Shigueto et al., 2010). This fleet also has frequent interactions with threatened taxa such as marine mammals, seabirds and sea turtles (Alfaro-Shigueto et al., 2010; Majluf et al., 2002; Mangel et al., 2010). Mangel et al. (2010), reported that bycatch of small cetaceans in Peru was likely in excess of 10,000–20,000 animals per year. Two of the most common small cetacean bycatch species caught - the dusky dolphin (Lagenorhynchus obscurus) and the Burmeister's porpoise (Phocoena spinipinnis) - are considered conservation priorities by the IUCN Cetacean Specialist Group (Reeves et al., 2003). The Burmeister’s porpoise is also listed as Near Threatened by the IUCN Red List of Threatened Species (IUCN, 2018). Coastal gillnets in Peru are also thought to be a population sink for multiple protected sea turtle species (Alfaro-Shigueto 2011), including leatherback (Dermochelys coriacea), hawksbill (Eretmochelys imbricata), loggerhead (Caretta caretta), green (Chelonia mydas) and olive ridley (Lepidochelys olivacea). While seabird bycatch rates have not been as thoroughly documented in Peru’s small-scale fisheries (SSF), the Peruvian coast hosts more than 90 species of pelagic birds (Spear and Ainley, 2008), including species of conservation concern like the waved albatross (Phoebastria irrorata), pink-footed shearwater (Ardenna creatopus), white-chinned petrel (Procellaria aequinoctalis) and Humboldt penguin (Spheniscus humboldti), all of which are documented to interact with Peruvian SSF (Awkerman et al., 2006; Jahncke et al., 2001; Majluf et al., 2002).
Strategies to reduce this bycatch have included examining methods to utilize the sensory capabilities of these animals to alter their behavior around fishing gear (Jordan et al., 2013; Southwood et al., 2008). For example, as cetaceans primarily employ echolocation for many aspects of their ecology (Wartzok and Ketten, 1999), acoustic deterrent devices have been tested as a method to reduce cetacean bycatch or depredation (Schakner and Blumstein, 2013). A recent study by Mangel et al. (2013) showed that acoustic alarms, or ‘pingers’, had the potential to reduce small cetacean bycatch in the gillnet fisheries based in Salaverry, Peru.
The use of visual cues has also been suggested as a potential method to reduce bycatch in fisheries (Southwood et al., 2008; Wang et al., 2007). Visual cues play important roles in the behavioral ecology of many marine vertebrates. Sea turtles rely primarily upon visual information to help guide their foraging behaviour (Constantino and Salmon, 2003; Southwood et al., 2008; Swimmer et al., 2005) and orientation (Wang et al., 2007; Witherington and Bjorndal, 1991). Many species of seabirds use a combination of visual and olfactory cues to find their food (Martin and Crawford, 2015; Silverman et al., 2004; White et al., 2007). In addition, marine mammals not only rely on acoustic cues, but also on vision for important biological functions such as feeding, orientation and individual recognition (Griebel and Peichl, 2003; Wartzok and Ketten, 1999). A potential (and until now elusive) benefit here is that, if effective, a bycatch reduction technology (BRT) based upon visual cues may work across taxa. BRTs have typically been designed to address interactions with one particular taxon (e.g. acoustic pingers for dolphins, circle hooks for turtles (Read, 2007), hookpods for seabirds (Sullivan et al., 2018). A multi-taxa BRT could derive multiple benefits such as effectiveness across a range of fisheries, reduced cost and eased implementation in fisheries with bycatch of multiple taxa.
Net illumination (a type of visual cue) has recently been tested as a bycatch reduction technology on sea turtles (Ortiz et al., 2016; Wang et al., 2013, 2010, 2007) and seabirds (Mangel et al., 2018) and showed significant reductions in bycatch interactions for both taxa when fishing nets were illuminated. Ortiz et al. (2016) reported that green LEDs placed on the floatlines of a demersal set gillnet fishery in Peru reduced the incidental catch rate of green sea turtles (C. mydas) by 63.9 % without any significant reduction in target catch per unit effort (CPUE) or catch value. Mangel et al. (2018), in a companion study of this same fishery reported an 85.1 % decline in the catch rate of guanay cormorants (Phalacrocorax bougainvillii) in the illuminated nets compared with the non-illuminated control nets.
Given the high levels of bycatch reported in coastal gillnet fisheries and their massive annual fishing effort both in Peru and globally, bycatch mitigation solutions are urgently needed. Building upon the success of previous net illumination trials, this study aimed to test the efficacy of LEDs as a multi-taxa BRT in Peru’s small-scale coastal gillnet fisheries. More specifically, the present analysis investigated the effect of gillnet illumination on (i) the probability of catching sea turtles, seabirds and cetaceans and (ii) catch per unit effort of target species.
Section snippets
The fishery
This study was conducted under true fishing conditions aboard six small-scale gillnet fishing vessels departing from the ports of San José (6° 46′S, 79° 58′W), Salaverry (8° 12′S, 78° 58′W), and Ancon (12° 02′S, 77° 01′W; Fig. 1). Small-scale vessels have a maximal storage capacity of 32.6 m3, maximum length of 15 m and rely on manual work during fishing operations (Reglamento de la ley general de pesca, 2001).
Gear characteristics varied somewhat between sets and ports. Surface driftnets were
Descriptive summary
During the experiment 131 sea turtles were captured incidentally of which 86.2 % were green turtles. Loggerhead and olive ridley turtles were captured in smaller numbers. Of the 53 small cetaceans captured, 47.2 % were long beaked common dolphins, 26.4 % were dusky dolphins and 24.5 % were Burmeister’s porpoises. Of the 46 seabirds captured during the experiment 71.7 % were white-chinned petrels and 17.4 % Humboldt penguins, with pink-footed shearwaters also captured in smaller numbers (Table
Discussion
In this study, fishing nets illuminated by LEDs achieved reductions in sea turtle bycatch probability without negatively affecting target species catch rates. The expected sea turtle bycatch probability per set was reduced by 70.0 % and 74.4 % in bottom set nets and surface driftnets, respectively. This corroborates the findings of Ortiz et al. (2016) which reported that net illumination reduced sea turtle BPUE by 63.9 % in bottom set nets from the Constante, Peru landing site. The use of net
Declaration of Competing Interest
We confirm that this manuscript is original research and that all the listed co-authors have agreed to their inclusion and have approved the submitted version of the manuscript. The manuscript is in its original form and has not been submitted elsewhere. All funding sources have been acknowledged and the authors have no direct financial benefit from publication. All research not carried out by the authors has been acknowledged in full. All necessary permits were obtained to conduct the research.
Acknowledgements
We thank the participating fishers and their families in San Jose, Salaverry and Ancon for their support throughout the project. We also thank the entire ProDelphinus team that participated in data collection. This work was supported by the DEFRA Darwin Initiative, University of Exeter, NOAA Pacific Islands Fisheries Science Center, NOAA Pacific Islands Regional Office, National Fish and Wildlife Foundation, Sea Life Trust, World Wildlife Fund, and Birdlife International - Albatross Task Force.
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