Using Lights On Nets To Reduce Bycatch
Gillnets are panels of mesh that hang vertically, like walls, in the water. The mesh is wide enough for fishes’ heads to poke through, but too narrow for their bodies. Consequently, fishes get stuck at the gills. One problem with gillnets — beyond the suffering already inherent in fishing — is that they entangle animals who are not the intended target of the fishing operation. Millions of animals such as sea turtles, sharks, seabirds, marine mammals, and “unwanted” fishes are killed in gillnets each year. This accidental killing of non-target animals is known as “bycatch.”
This study examined whether using green LED lights on gillnets reduced bycatch of elasmobranchii (sharks), Humboldt squids, loggerhead turtles, and finfishes off the coast of Mexico’s Baja California peninsula. Researchers partnered with local expert gillnet fishers to conduct the study, setting out 28 pairs of gillnets overnight. In each pair, one net was illuminated by green LED lights, and the other net was conventional (i.e., the unlit control). The nets in each pair were placed at the same depth and over the same substrates (e.g., rock ledges and sandbars). Taken together, the 28 pairs of nets added up to 10,000 meters of nets set for nearly 700 hours. Researchers compared bycatch rates, target fish catch rates, market value, and haulback time (i.e., time required to retrieve and disentangle nets) between the illuminated and conventional nets.
Overall, 27 different animal species were discarded as bycatch, and 14 target fish species were kept for sale. Compared to conventional nets, illuminated nets reduced the total biomass of bycatch by 63%. Illuminated nets decreased the biomass of shark bycatch by 95%, Humboldt squid bycatch by 81%, and “unwanted” finfish bycatch by 48%. The difference in bycatch biomass of loggerhead turtles was not statistically significant. But it’s important to note that nearly double the number of individual loggerhead turtles (17) were caught in conventional nets compared to illuminated nets (9).
While illuminated nets decreased bycatch, they did not significantly reduce the biomass of target fishes caught or the market value of the target catch. In addition, illuminated nets reduced haulback time by 57%, saving fishers an estimated 55.5-71 minutes per 100m of gillnet. By reducing bycatch, illuminated nets may also decrease human injuries that occur when detangling large animals such as sharks. These results about operational efficiency are important because they may help persuade fishers to illuminate their nets.
Researchers report that the lights were easy to use and required no training. However, the specific LED lights used in the study were relatively expensive (especially for developing countries). Plus, they required batteries. Another limitation of the study is that — except for loggerhead turtles — researchers did not count individual animals. Counting individual animals is important for understanding illuminated nets’ effects on different species and on different individuals within a species. For example, the visual system (i.e., eyes) that helps a fish avoid gillnets may change over the fish’s lifespan.This study establishes green LED lights as a potential technology for reducing gillnet bycatch. The lights used in the study are the first identified bycatch reduction technology for cephalopods (e.g., Humboldt squid) and elasmobranchii (i.e., sharks).
All in all, the lights are a promising, feasible strategy for harm reduction in the fishing industry. Animal advocates can collaborate with fishers, NGOs, governments, and technology developers to spread the use of green LED lights on gillnets. Animal advocates can also support more research on best practices for using the lights to reduce bycatch of different species. In a world where fishing, gillnets, and fish consumption are common, the lights may protect the welfare of at least some animals.
https://doi.org/10.1016/j.cub.2021.12.050