Cheap Magnets Could Keep Sharks Out of Fishing Nets

By: Jesslyn Shields

fishing nets, magnets
Trawling nets like this one sink to the sea floor, picking up whatever's down there. Countless sharks, sea turtles, seals, dolphins, rays and fish of all descriptions needlessly die before they can be thrown back overboard. Philippe Colombi/Getty Images

We kill 100 million sharks every year. I hate to come out with guns blazing on this one, but that's just the way it is.

For starters, some people eat sharks, so they kill them on purpose for that reason. However, the big shark-killing culprit is accidental bycatch. Bycatch is the collateral damage of the large-scale commercial fishing industry.

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It works like this: Say you're in charge of a commercial shrimping boat, and it's outfitted with all the gear you need to catch shrimp, including a trawling net the size of a sports field, which sinks to the seafloor and bumps along on the bottom, picking up whatever's down there. When it's hauled back in, some of what's in the net is shrimp — but it also contains sharks, sea turtles, seals, dolphins, rays, countless fish of all descriptions. Some of these needlessly die before they can be thrown back overboard, some are hauled into port and their bodies disposed of later. The same goes for baited longlines and gill nets, which entangle everything that swims into them. The result is that as many as 2 billion pounds (almost 1 metric tonne) of marine life is killed and wasted by the fishing industry each year.

Which puts the whole 100 million sharks thing into perspective.

But there might be hope, at least for the sharks. A cheap, simple fix for a big problem like this is rarely forthcoming, but according to a study published in the December 2018 issue of the journal Fisheries Research, the answer to the shark bycatch problem might be magnets.

Sharks and other elasmobranchs, the group of cartilaginous fishes that also includes rays and skates, have special sensory organs around their nostrils called ampullae of Lorenzini that look like little craters all over the shark's snout. Behind these little pores are sacs of jelly that can sense electromagnetic fields — they help sharks hunt and, scientists think, might also help them migrate using the Earth's magnetic field.

The research team hypothesized that incorporating magnets into fish traps might alert elasmobranchs to their presence, and reduce overall shark bycatch. Because bony fishes (which are often what fishing outfits are looking to catch) have low sensitivity to electromagnetic fields, they figured that even if it worked to keep sharks steering clear of the traps, it wouldn't lower catch rates of desirable fish.

To test their hypothesis, the Australian research team monitored 1,000 fish traps off the coast of Sydney, New South Wales, all of which were baited to catch Australian snapper — a segment of the industry that accidentally catches sharks about 10 percent of the time. One third of the traps were set with $22 ($30 AUD) worth of cheap magnets around the entrances, another third with metal bars at the entrance to provide a physical barrier and the remainder were left alone as controls.

"Developing ways to reduce bycatch is a priority for many fisheries. We found that traps with magnets had roughly 30 percent less likelihood of catching sharks and rays compared to traps without. In addition, those traps with magnets would catch roughly 30 percent more targeted fish, which is a rare win-win for fisheries," said coauthor Rhys Richards of the School of Environmental and Life Sciences, University of Newcastle, in a press release.

Win-wins are great, but we've got a long way to go before we make a dent in that 100 million sharks per year. The magnets seem to work well for traps, but magnets don't work on longlines — the lines are fitted with metal hooks, so the magnets tangle the gear.

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