Tuesday, 6 August 2013

UV-emitting diodes help keep endangered sea turtles out of gillnets

Although sea turtles and fish occupy the same marine habitat, their lifestyles are different enough that the two groups tend to vary in their sensory abilities. This is good news to conservationists looking to modify fishing gear so that it continues to catch fish, but no longer ensnares endangered turtles. One promising BRT, or bycatch reduction technology, is a diode that emits ultraviolet (UV) light. The rays can be detected by green, loggerhead, and leatherback sea turtles, but not by many of the most commercially valuable fish species. A recent study has shown that fishing gear fitted with these lights traps significantly fewer turtles, while continuing to capture normal amounts of target fish. Thus, the devices may help ease tensions between fishermen looking to make a living, and environmentalists hoping to protect threatened wildlife.

Green turtle, Chelonia mydas. Image courtesy of NOAA.
The diodes are the latest in a series of devices and modifications proposed for use by fishermen, and, in fact, are only the latest incarnation of net-illuminating gadgets. For example, one previous study found that green sea turtles tended to stay out of nets fitted with green light-emitting diodes (LEDs) and chemical lightsticks. The current study was conducted after researchers compared the visual ecology of sea turtles and fish and realized that the former were more sensitive to UV light than the latter. Though it seemed likely that use of UV lights could keep turtles away from nets without also warning off fish, the only way to test this theory was to deploy the devices in the field.

The researchers performed separate experiments (both in Mexico) in order to test their two hypotheses. The first experiment, conducted in a turtle-rich area known as Punta Abreojos, sought to determine whether the UV LEDs really did reduce sea turtle capture rates. Pairs of nets--one fitted with active LEDs and the other fitted with bulbs that weren't turned on--were deployed between sunset and sunrise. The researchers checked the nets every 90 minutes in order to retrieve, measure, and tag any turtles that had become ensnared. The second experiment, conducted in a commercial fishery in Bahia de los Angeles, was designed to investigate whether the LED devices affected fish capture. This part of the study used a paired-net design similar to that described above, only the nets were about four times as large and were left undisturbed all night. The next morning, all captured fish were retrieved and categorized as target species, bycatch, or "other" (e.g., kept by the fishermen as food or bait).

How a gillnet works. Image courtesy of the AFMA.

Of 332 green turtles captured during the first part of the study, only 123 (37%) were from UV-illuminated nets; the remaining 209 were captured from the nets without diodes. Although 123 is a sizeable number, the difference between lit and unlit nets was significant, indicating that use of these BRTs could substantially reduce turtle bycatch rates. Results from the second part of the study were similarly encouraging. Unlit nets only caught 46 more target fish than those fitted with UV lights (355 vs. 309); likewise, both types of net yielded catches with comparable average market values ($15.10 vs. $15.00).

These findings appear to be an excellent first step in the development and eventual distribution of a new turtle-deterring device. The UV LEDs may be improved by further behavioral studies exploring just how they work. In particular, it would be interesting to know whether turtles are avoiding the LEDs themselves, or whether the UV rays help the turtles see the fishing nets better. It will also be necessary to perform additional work that combines the two parts of this study into a single experiment; by choosing a field site that is both a commercial fishery and an area of high turtle density, scientists can investigate whether these BRTs are a practical solution to the problem of turtle bycatch.

Light-emitting diodes, or LEDs. Image courtesy of Instructables.
The authors of the study also point out that the current results may not be applicable to all fisheries, since different types of target fish have different visual abilities. Thus, the UV LEDs may work better in some sites than in others. Local conditions, such as time of day and turbidity of the water, may also affect the efficacy of the devices. While some tweaking may be necessary before the LEDs can be widely distributed, these preliminary findings suggest that the future of the apparati may be bright.

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Wang, J., Barkan, J., Fisler, S., Godinez-Reyes, C., and Swimmer, Y. 2013. Developing ultraviolet illumination of gillnets as a method to reduce sea turtle bycatch. Biology Letters 9(5), online advance publication.

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