SHARK-REPELLING MAGNETS MAY be the perfect antidote to unwanted shark attention while fishing.
According to new research, magnets could ward off sharks and rays, preventing them from being accidentally caught by baited fish traps.
“We were surprised at how successful this was,” says Vincent Raoult, a marine ecologist at the University of Newcastle in Australia and one of the authors of a study published recently in Fisheries Research.
Sharks have sensory pores on the front of their heads that allow them to detect the electrical currents generated by the muscle contractions of their prey.
“They can basically sense where their food is, even if they can’t see it or smell it,” Raoult says.
But strong, unnatural magnetic fields can confuse these senses—Raoult describes it as the equivalent of opening a door and suddenly being hit with a strong stink. “It’s just an unpleasant thing for the animal as far as we can tell.”
The team tested whether magnets would deter sharks from bait traps, which are large boxes made of chicken wire that are filled with bait and used to attract fish. They put four three-inch magnetic bars made of ferrite around each funnel. While they aren’t necessarily much larger than your average fridge magnet, they are thicker and much stronger.
In the Hawkesbury River estuary north of Sydney, Australia, commercial fishing vessels using bait traps are primarily fishing for Australasian snapper and yellowfin bream, but the traps also draw intruders such as blind shark, Wobbegong sharks, Port Jackson sharks, and sometimes electric rays. In this area, Raoult says about 10 percent of fishermen’s entire catch by weight is made up of unwanted sharks and rays.
While the sharks usually survive the ordeal, it can cause them stress and tire them out, making them easy prey for larger sharks and fish upon their release. The sharks also affect fishing’s bottom line, as they take up space meant for snapper.
Good News for Sharks
After monitoring nearly 1,100 traps for eight months, the researchers found that on average, traps with magnets had 30 percent less shark bycatch than other traps. Without sharks taking up space, the traps also pulled in 30 percent more snapper.
“Coming up with a way to reduce bycatch is cool on its own,” Raoult says, but it also has to make economic sense. The fact that a few magnets costing five to ten dollars per trap can reduce shark bycatch and increase snapper haul makes shark conservation financially viable, he adds.
On top of this, some fishers said that since they can catch their target load more efficiently, they could also put out fewer traps. Fewer traps means that less gear gets lost at sea, where it can cause problems for marine life that becomes stuck in it.
Raoult is optimistic about using magnets for fish traps in other parts of the world. While the sharks caught in his study aren’t vulnerable species, Raoult says that magnet-armed fish traps could help protect endangered or threatened sharks in other areas.
Not a Cure-All
While the magnets seem to work well in repelling sharks from fish traps, broader applications of this technology are mixed. Since the magnets used are the type known as permanent magnets, they retain their magnetic properties without needing an outside current or magnetic field.
But they emit a field for only about eight inches, Raoult says. This means that magnets may be effective at trap openings, where sharks have to stick their heads past the magnets to get the bait, but using them on hooks maybe trickier. Likewise, swimmers would be ill-advised to try to ward off sharks by carrying magnets.
One study found initial success using magnets to deter Galapagos sharks from hooks, but competition between sharks eventually pushed some individuals to get over the repellent effect and go for the bait. According to other research, the magnets even had the opposite effect on blue sharks and shortfin makos, two of the shark species most affected by longline fishing.
Magnets that can fit on dozens of longline hooks may also be too small to repel larger sharks, and would be more work to put on and maintain, says biological oceanographer Sebastian Biton Porsmoguer, a coauthor of the shortfin mako study.
But Craig O’Connell, a shark biologist with the O’Seas Conservation Foundation and National Geographic grantee, has had success keeping great whites and bull sharks away from larger areas by setting up a barrier made from magnets inside a long PVC pipe — even with bait on the other side. He hopes such a method can eventually replace the nets used to keep sharks away from beaches, which can entangle marine life.
O’Connell says that finding new fishing techniques is crucial to improving the health of ocean ecosystems.
“At the end of the day, we need our fishing practices to be a bit more responsible. If we can reduce our shark bycatch, that’s a step in the right direction,” he says.