On land, electric lights are notoriously bad for sea turtles. Their glow can lure newly hatched babies inland, outshining the stars that would otherwise lead them to sea.
For turtles that do escape their birth beach, however, scientists have found that certain kinds of electric lights can be surprisingly beneficial — at least when it comes to protecting the ancient reptiles from another man-made danger: gillnets.
In a new study, researchers added green light-emitting diodes (LEDs) to gillnets at a small Peruvian fishery, reducing the number of sea turtles deaths by 64 percent — and without affecting the nets' intended haul of guitarfish, a type of ray. Sea turtles rely heavily on visual cues when foraging, and the green lights presumably helped them (but not the guitarfish) see the looming nets before it was too late.
"This is very exciting because it is an example of something that can work in a small-scale fishery, which for a number of reasons can be very difficult to work with," says lead author Jeffrey Mangel, a research fellow with the Darwin Initiative and research coordinator for Peruvian NGO ProDelphinus, in a statement. "These lights are also one of very few options available for reducing turtle bycatch in nets."
Every year, thousands of sea turtles worldwide are fatally entangled in gillnets, a type of fishing gear designed to catch fish by their gills. Typically made of nylon, gillnets create a wall of mesh that hangs in the water column. Anglers who use them rarely mean to kill sea turtles, but sentiment alone doesn't stop it from happening.
"Turtles encountering a gillnet can quickly become entangled around their head or flippers as they try to escape," explains the U.S. National Oceanic and Atmospheric Administration, which helped fund the study. "Entangled turtles will drown if held under the water but have a higher chance of survival if they can reach the surface to breathe. The nylon can tighten around a turtle's soft body parts and cause deep cuts potentially leading to infections, limited movement or complete loss of the limb."
Conducted in northern Peru's Sechura Bay, the new study represents the first time lighting technology has been scientifically tested in a working fishery, according to its authors. Each of the green LEDs cost $2 (£1.40), and the researchers calculate the cost of saving one turtle is about $34 (£24) — an already reasonable price that would be further reduced if the method is rolled out at a larger scale, they note. "[T]he costs to outfit the entire gillnet fishery in Sechura Bay can be as low as $9,200," they write.
The study featured 114 pairs of nets, roughly 500 meters (1,640 feet) in length. One net in each pair was unlit, while the other was illuminated with green LEDs placed every 10 meters (33 feet) along the gillnet floatline. The unlit mesh caught 125 green sea turtles (Chelonia mydas), while 62 were caught in the illuminated nets.
That's still too many, but 50 percent less bycatch is nonetheless a step in the right direction. LEDs offer a low-cost way to protect endangered sea turtles, the study's authors say, while also supporting Peru's economically important fishing industry.
Green sea turtles are one of several species that forage in Peruvian coastal waters, joined by olive ridleys, hawksbills, loggerheads and leatherbacks. The country's gillnet fleet sets at least 100,000 kilometers (62,000 miles) of net per year, an inadvertent death trap for thousands of turtles who don't see it coming until they're tangled.
"The turtle populations in the eastern Pacific are among the world's most vulnerable," Mangel says, "and we are hoping that by reducing bycatch, particularly in gillnets, will help with the management and eventual recovery of these populations."
The new findings back up other recent research, like a 2013 study that found ultraviolet-lit nets caught 40 percent fewer sea turtles than unlit nets did. Jesse Senko, a postdoctoral researcher at Arizona State University, has also found that LEDs curbed loggerhead bycatch by about 50 percent at night in the North Pacific. Not only did the catch of target species not diminish, Senko writes for the Safina Center, but LED nets caught more halibut and less non-turtle bycatch.
"Fishers were particularly excited about the prospect of not having to remove all that bycatch, turtles included — a time-consuming process that also uses more fuel because the nets become heavier and more difficult to haul!" Senko writes.
LEDs are one of a few clever ways to save sea turtles and other animals from fishing nets. Turtle excluder devices, or TEDs, are used in trawl nets to give turtles an escape hatch, for example. The U.S. also requires pingers on gillnets in certain fisheries, designed to prevent bycatch of dolphins and porpoises. Weak links are sometimes added to help nets break under the force of a swimming whale. The United Nations also banned large-scale, high-seas driftnets in 1991, and many countries have set limits on where, when and what type of fishing nets are legal.
Yet despite all that, fishing nets still pose a widespread risk for sea turtles. And combined with other threats, from light pollution to plastic pollution, that risk warrants a sense of urgency, especially since sea turtles are so slow to reproduce. The authors of the new study are now working with larger fisheries in Peru and with different colored LEDs to see if the results can be replicated — not just with green sea turtles, which are endangered, but also more critically endangered species.
"It is exciting to be part of research that is highlighting innovative methods that may assist the move toward sustainability in these fisheries," says co-author Brendan Godley, a conservation biologist at the University of Exeter. "Understanding [cost] will help emphasize the need for institutional support ... to make possible widespread implementation of net illumination as a sea turtle bycatch reduction strategy."