Adding cheap magnets to the traps reduced shark and ray bycatch by a third and increased fish hauls by just as much, according to a new study
Sharks are having a rough go of it. It’s estimated that humans kill 100 million sharks of all species each year, many hunted for shark-fin soup while others are killed as bycatch when fishermen accidentally hook or trap them. While there’s a lot of work to be done to save sharks, Ben Millington at the Australian Broadcasting Network reports that researchers have come up with a simple and cheap strategy for preventing some shark bycatch: outfitting fish traps with magnets.
Sharks have special sense organs called ampullae of Lorenzini, small jelly-filled pores located on their snouts and around their head. This sixth sense can pick up the electrical field of small fish, which helps sharks locate prey in dark or murky water. The ampullae may also allow sharks to detect the Earth’s magnetic field, which they might use to navigate the oceans. Other groups of cartilaginous fish in the subclass elasmobranchii, including rays and skates, also have the electricity-sensing organ.
Researchers at the University of Newcastle in Australia developed new tech that uses sharks’ electrical sensitivity to deter them from fishing traps. According to a press release, team studied 1,000 fish traps deployed near Sydney, Australia, over the course of eight months. The traps were targeted at Australasian snapper, but at least 10 percent of their catch is typically elasmobranchs, including blind sharks, Wobbegong sharks and electric rays. The team outfitted a third of the traps with magnets around their entrances, another third with metal bars and the rest were unchanged and left alone a control.
The magnets had a big impact. “Across the summer we deployed close to a thousand traps and we found pretty significant effects from those magnets, which basically reduced the total catch of the sharks by about 30 percent, which is great for that fishery,” marine biologist Vincent Raoult, co-author of the study in the journal Fisheries Research tells Millington. Because the magnets successfully deterred sharks, there was more room for fish in the traps. In total, magnet traps had about 30 percent more fish in them than usual.
“This is an awesome example of how to mitigate the impact of fisheries on sharks and at the same time have benefits to those fishermen, because they’re catching more fish,” Millington adds.
Raoult tells Joshua Rapp Learn at National Geographic that the strong magnets confuse the animals’ senses when they get close to the fish trap, the equivalent of getting hit in the face with a strong stink. “It’s just an unpleasant thing for the animal as far as we can tell,” he says.
Another advantage of using magnets is that they are cheap—about $2 a piece, meaning a trap can be outfitted for about $30 depending how many openings it has. That can have a big impact for fishermen.
For a minimal investment they can increase the productivity of their traps and perhaps even reduce the number of traps they fish with, which also reduces costs. “It’s definitely better for the business and the sharks, for them not to get in the traps, and you can catch more fish. It’s win-win for everyone,” commercial fisherman Mark Cranstone tells ABC’s Millington.
The research team wants to communicate their finding to the fishing community around the world since this technique can be implemented quickly and on a large scale. But Learn reports the magnet trick really only work on fish traps, currently. Using magnets with other types of fishing gear is a much stickier situation, literally. Tests using magnets in longline fishing, which uses metal hooks, showed that they attracted the hooks and tangled the gear.
Other studies showed that some shark species overcame their aversion to the magnetic field and went after baited lines. Some species, like blue sharks and shortfin makos, even seemed to be attracted by the magnets.
But researchers have had some success in using magnet technology to replace shark nets and barriers, which can tangle up fish and other marine wildlife. The SharkSafe Barrier is made of magnet-filled tubes, which look like a kelp bed, which sharks dislike