Illumination of Gillnets Reduces Sea Turtle Bycatch

Coastal gillnet fishing is a common practice throughout the world and is often problematic because of its non-selective effects on fish populations and other biota including sea turtles. Gillnet fisheries have been documented with significant sea turtle bycatch rates. For example, the coastal gillnet fishery based in Lopez Mateo, Baja California, Mexico interacts with up to 800 loggerhead turtles (Caretta caretta) per year, while gillnet fisheries off the coast of Northern Peru catch over 300 green sea turtles (Chelonia mydas) per year. Fisheries bycatch impedes the recovery of threatened and endangered sea turtle populations. More recently, it has become a motivating factor to improve the balance between species protection and commercial fishing interests.

Experiments with chemical lightsticks show a 40% decrease in turtle catch rates (P<0.05). Control nets caught 209 turtles, while 
        illuminated nets caught 123.
Experiments with chemical lightsticks show a 40% decrease in turtle catch rates (P<0.05). Control nets caught 209 turtles, while illuminated nets caught 123.

Spectral sensitivity (Y-axis) vs. wavelength (X-axis).
        Top graph: Loggerhead turtles (top line) and leatherback turtles (bottom line) have relatively high sensitivity to UV light 
        (wavelengths <400 nm, shown by pink bar.)  Bottom graph: Summer flounder, on the other hand, are insensitive to UV light.
Spectral sensitivity (Y-axis) vs. wavelength (X-axis). Top graph: Loggerhead turtles (top line) and leatherback turtles (bottom line) have relatively high sensitivity to UV light (wavelengths <400 nm, shown by pink bar.) Bottom graph: Summer flounder, on the other hand, are insensitive to UV light.

Behavioral and physiological studies indicate that visual cues play important roles in sea turtle foraging and orientation. This has led researchers at PIFSC, the University of Hawaii Joint Institute for Marine and Atmospheric Research and their collaborators to examine visual-based strategies to reduce sea turtle interactions with gillnets. In other fishery settings, the use of visual cues to alert non-target species to the presence of fishing gear has been studied, but in coastal gillnet fisheries, using visual alerts to deter sea turtles from entanglement is a novel approach.

Experiments with illuminated gillnets show a reduction of green sea turtle interaction rates by 40% to 60%. Researchers hypothesize that illuminating nets creates enough of a visual cue to alert sea turtles to the presence of a barrier, allowing them to avoid it.

For a catch reduction technology to be successfully implemented, it must have a limited impact on the market value of the fishery. In studying a commercial bottom-set gillnet fishery in Baja California, researchers found that net illumination does not significantly affect total catch rates of target species or the total catch value of the fishery.

Most recently, researchers have tested the use of ultra-violet (UV) lightsticks to illuminate gillnets with a view to reducing sea turtle bycatch. Many pelagic and coastal fish species have eye structures that prevent UV vision and physiological experiments show that the visual systems of some fish species are insensitive to UV light (wavelengths < 400nm.) On the other hand, behavioral and physiological studies indicate that the visual systems of sea turtles are sensitive to UV light. Taken together, these results suggest that UV light may be used to selectively communicate with sea turtles and those fish species that are sensitive to UV light.

The Baja California study suggests that UV illumination of bottom-set gillnets reduced sea turtle interactions in the fishery by 40% without reducing total target catch rates. In fact, for the primary target species (and the species with highest value per kilogram), California halibut (Paralichthys californicus), catch rates in the UV illuminated nets increased by 45%. In contrast, shark bycatch decreased by 41% overall and by 55% for scalloped hammerheads (Sphyrna lewini). These results suggest that lightsticks of different wavelengths can be used to illuminate gillnets and change their selectivity in specific ways. Thus they might be used to fulfill needs in specific fisheries, reduce bycatch of particular species, and address specific conservation needs.

Current net illumination experiments have focused on testing the use of this strategy to reduce sea turtle bycatch in coastal and pelagic gillnet fisheries based in Peru, Baja California Sur, Chile, Brazil, and Indonesia. In many of these fisheries the gear interactions involve several species including green turtles, leatherbacks, hawksbills, and loggerheads.