Ocean life is in trouble. Many fish species have already been harvested to the brink of extinction, and more and more species are declining at alarming rates. Fish are not the only animals to take into consideration when deciding whether to indulge in a seafood dinner, however. Commercial fishing nets are extremely hazardous for sea turtles. Unsuspecting turtles periodically become tangled in the nets and are hauled up as bycatch, with fatal consequences.
Finding ways to decrease bycatch of sea turtles without sacrificing fish harvest rates is an ongoing challenge for scientists and fishermen alike. Fisheries would like to be able to claim that their products are “turtle friendly,” yet they don’t want to lose revenue by curtailing harvests in prime areas or investing in expensive new equipment. While they understand that saving turtles is important in the big picture, many fishermen and commercial fisheries are understandably hesitant to sacrifice their incomes for the cause.
Numerous potential methods to decrease turtle bycatch have been proposed, but none have yet been systemically implemented on a large scale. One strategy is to modify to equipment, such as altering net float lines and tie-downs. Other solutions are more organizational, such as proposed networked advisory programs. One of the most intriguing strategies, however, is not to change the design of the nets themselves or where/when they’re placed, but to alter how marine animals perceive them in the first place.
How turtles and fish see the world
This technique requires a bit of natural history knowledge: what do we know about how turtles and fish see the world? It turns out that some reptiles, including green and loggerhead sea turtles—both of which commonly caught as bycatch—can see ultraviolet (UV) light. This ability is smattered across the vertebrate world: many insects and birds, and even some reptiles, fish, and a few mammals can detect light at UV wavelengths. As it turns out, though, several fish species that are valuable to fisheries cannot detect light at UV wavelengths. This revelation gave scientists the idea for a creative strategy to decrease turtle bycatch at commercial fisheries: illuminate the nets with UV light. In theory, turtles will see the nets and avoid them, while the hapless fish remain oblivious and swim into the nets. If the technique is successful, the fisheries stay happy, and the turtles stay free.
It’s an exciting idea, but would it work? A group of researchers from San Diego’s Ocean Discovery Institute and Hawaii’s Pacific Islands Fisheries Science Center wanted to find out, and their findings are reported in recent issue of the journal Biology Letters. The group conducted experiments to answer two important questions:
1) Will illuminating nets with UV light reduce the number of turtles caught in fishing nets?
2) Will the new illumination reduce the catch rate and/or market value of fish harvests?
The first experiment was conducted at a site called Punta Abreojos, off of the coast of Baja California. This trial was designed to determine whether illuminating nets with UV LEDs would reduce the number of turtles that become ensnared in the nets. The researchers deployed pairs of nets: one with illuminated UV LEDs placed every 5 meters along the float lines, and a control treatment with inactive LEDs placed along the float lines. The nets were opened around sunset and collected just before sunrise. The researchers chose Punta Abreojos because sea turtles are known to exist there in relatively high densities, and they needed to ensure an adequate sample size for statistical analyses.
How did the turtles fare? It turned out that although the lights weren’t a total panacea—some turtles still managed to get themselves entangled in the illuminated nets—shining UV light on the nets did significantly decrease the turtle catch rate. Overall, there was a 39.7% reduction in turtle catch rate when the nets were illuminated with UV LEDs.
So far, so good. But there was another key element involved: fishermen can’t be convinced to illuminate their nets unless they are confident that it wouldn’t harm their profit margin. For an illumination technique to be accepted, it can’t interfere with either fish harvest rates or the market value of the fish that are caught. This was the reason for the second experiment. The researchers repeated the same experimental methods in a fishery off another part of Baja California, Bahía de los Angeles. This time, the researchers tracked the nets’ success with catching fish. They calculated both the catch rate for target species and the monetary value-per-unit effort, to ensure that higher-priced fish weren’t avoiding the nets, which could decrease market value even if overall catch rates stayed consistent.
Once again, the strategy proved its mettle. There was no statistical difference in the catch rates for target species between the UV-illuminated and the dark nets—the fish can’t see UV, so the LEDs didn’t make a difference. Likewise, the market value of the harvests made with the illumination didn’t differ significantly from those made with dark nets. The experiments provided strong evidence that fishermen don’t have to worry about hurting their profits by illuminating their nets with UV light.
Although this study yielded promising evidence in favor of the UV illumination strategy, there are a few angles left to explore. First, the turtle and fish experiments were conducted in different localities. The turtles weren’t sampled within the fishery, because the researchers wanted to target an area where they’d get an adequate sample size of turtles to prove the validity of the method. It would be useful to follow up with an inclusive study, documenting the effects of UV illumination on decreasing turtle bycatch within active fisheries. Also, these experiments involved nets set only during night hours. Many fisheries deploy nets around the clock, so more experiments that include daytime hours will be useful as well.
Finding management strategies that can help restore species of concern is only one component of effective conservation efforts. The techniques can only be successful in the long-term if they do not harm the livelihoods of people that depend on the same habitats and/or resources as the animals. Without the support of locals, conservation is often a losing battle. At this point, things are looking hopeful for the UV illumination method on both counts. It may prove to be a boon to sea turtle populations as well as to fisheries and fishermen, if they can tap into the growing demand for turtle-friendly seafood. Only time will tell.
Note: You can consult Blue Ocean’s Guide to Ocean Friendly Seafood [pdf link] to see whether your favorite seafood species are associated with turtle bycatch. You may even be inspired to try new species that are just as tasty as their less ocean-friendly counterparts. The market can drive harvesting practices; vote with your menu chocies!
Anne-Marie Hodge, EGN’s Species Science Advisor, is currently a PhD student at the University of Wyoming in Ecology, and holds a Bachelor of Science in Zoology from Auburn University and a Master of Science in Biology from the University of North Carolina-Wilmington. While at Auburn, she established and served as president of Alabama’s first chapter of the Society for Conservation Biology. Anne-Marie’s research interests include community ecology, predator-prey dynamics, and conservation, with a focus on tropical carnivores. She has research experience in Mexico, Belize, Ecuador, and Kenya.