Saving Sea Turtles with Light

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LED lights on a gillnet. [Image: ProDelphinus]

Forget plastic straws—sea turtles are under attack from another inadvertent enemy: the fishing industry. Gillnets, mesh netting designed to ensnare fish by the gills as they try to back out of the net, are a widespread small-scale fishing method with environmentally damaging consequences. While effective, gillnets are indiscriminate—catching target species as well as non-target species, or “bycatch.”

A U.K.–Peruvian research team has illuminated floating gillnets in Peru with LEDs, providing a warning beacon to non-target species, such as dolphins, seabirds and sea turtles (Biol. Conserv., doi: 10.1016/j.biocon.2019.108277). In the study, the team cut the accidental bycatch of sea turtles and dolphins by more than 70% without affecting the amount of target species caught.  

The bycatch dilemma

Many countries, such as Peru, are dependent on gillnet fisheries—the total length of gillnets set annually in Peru exceeds 100,000 km. On the conservation side, this poses a major threat to many marine taxa. One report states that bycatch of small cetaceans, such as dolphins and porpoises, in Peru most likely exceeds 10,000–20,000 animals per year, and some of these species are conservation priorities.

Historically, few solutions have been developed and implemented to reduce the bycatch rates of gillnet fisheries. Sensory cues have been tested as a bycatch reduction technology (BRT). However, past BRTs were typically designed with one specific taxon in mind—such as acoustic deterrent devices that co-opt dolphins’ echolocation to indicate that there is fishing gear in the water.

The University of Exeter team, in partnership with the Peruvian conservation organization ProDelphinus, wanted to test a BRT that would be effective for multiple threatened marine taxa. Building upon the success of previous net-illumination studies, the team investigated the efficacy of net illumination on the bycatch rates of sea turtles, sea birds and cetaceans in Peru’s coastal gillnet fisheries.

Green light = gillnet

During the three-year experiment, the team equipped gillnets with green 500-nm LED lights, placing the lights every 10 m along the float line of 864 gillnets. Each experimental gillnet was paired with an unlit control net, placed 200 m away from the illuminated net, as a point of comparison.

Using statistical modeling, the team calculated  the expected bycatch of non-target taxa as well as the catch per unit effort (CPUE) of target species. According to their data, the researchers found that the sea turtle bycatch probability was reduced by 74.4% with illuminated surface nets, and by 70.8% for cetaceans. For seabirds, the team calculated that the mean bycatch per unit effort (BPUE) was reduced by 84% in the presence of LEDs (although the authors note that seabird entanglement was rare during the study timeframe).

Interestingly, the team found that gillnet illumination didn’t seem to harm the catch rate for the target fish species. That’s a major consideration if fisheries are actually to implement the illuminated nets, according to the researchers, as the inexpensive LEDs combined with the unaffected catch rate would keep the economic burden for fishers low.  Based on these findings, the team concluded that net illumination is an effective and economically viable multi-taxa bycatch reduction technology.

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