The oceans are facing catastrophe from overfishing, and the problem is as much the things caught by accident – by-catch – as it is the fish we are trying to catch. This is particularly problematic for large, long-lived species like sharks and sea turtles, whose populations only rebound slowly. The answer, or at least a big part of it, could be attaching lights that deter non-target species, while having no effect on the fish we actually want to catch.
Although finding a way to keep unwanted animals from fishing nets makes sense, it comes with an obvious problem. When the creatures we don't want include species as diverse as mammals, reptiles, sharks, and invertebrate cephalopods, it's hard to imagine any one thing that would work on all of them without also reducing catches of target fish.
Nevertheless, scientists including John Wang of the National Oceanic and Atmospheric Administration (NOAA) report in Current Biology that's exactly what they have found in the form of green LEDs dotted along nets' floatlines. Better still, there are benefits to the fishing crew, so the idea may not need to be imposed by regulation.
Small-scale fisheries provide food and employment for large proportions of the planet. However, the gillnets they use are lethal to marine life too large to slip through their nets, including many creatures with little or no value to the crew. The accidental removal of apex predators from these ecosystems can have devastating effects on other species.
The team attached green LEDs every 10 meters (33 feet) along the edge of 28 nets set to catch grouper and halibut off Baja California, leaving other nets as controls. The illuminated nets caught 63 percent less by-catch in total, including 81 percent fewer squid and an astonishing 95 percent fewer sharks and rays by weight. Ironically, almost the least successful aspect was in saving turtles, whose excellent site in the green part of the spectrum inspired the idea initially. Nevertheless, a 51 percent reduction in deaths for the most threatened vertebrate group on the planet is nothing to be sneezed at, and the turtles narrowly benefited more than unwanted fin fish at 48 percent.
Meanwhile, there was no difference in the amount of commercially valuable fish the nets caught. Indeed more halibut, a particular target, was caught in the lighted nets, although this may have been chance.
Attaching and powering the lights obviously costs something – $140 per net to be precise – but the researchers are looking into cheaper solar-powered options, and placing them further apart.
Moreover, a lot of time is often wasted untangling by-catch from nets, and this fell 57 percent in the lighted nets. Fishing boat operators may greenlight the idea if they consider the time saved worth the cost.
"It is important for fishers to know that there are tangible benefits for them. This is critical for the adoption of such technologies by the fishing industry,” Wang said in a statement.
Previous studies have produced encouraging results for the lights for saving turtles. These have led to some adoption of the practice, although on nothing like the required scale. A few papers have even looked at other possible beneficiaries such as dolphins. What is new in this work is the examination of the total by-catch across all species, as well as the measurement of time saved in untangling.