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October 2011    Download the Entire Issue (PDF) Available to the Public Vol. 37, No. 10   RSS Feed for Undercurrent Issues
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Shark-Repelling Magnets for Divers

they might work, but do you need them?

from the October, 2011 issue of Undercurrent   Subscribe Now

Few sport divers think they need a safeguard against sharks. Regardless, various companies have tried to make them, offering specialized sunscreens, chemical sprays, visual tricks and disguises, but with little documented success. But for those divers who want to keep the big boys at bay, magnets may send sharks fleeing. Why? The interaction of saltwater and charged metals produces a weak electrical field. When a shark nears that field, it apparently disrupts its' special sixth sense of electroreception, the detection of minute changes of electricity in seawater.

One scientist studying magnets' effectiveness is Craig O'Connell, a marine biologist working for SharkDefense ( ), a research company developing magnetic shark repellents for commercial and recreational use ( ). O'Connell, has had two research papers on his results published in the past year. In the British journal Marine and Freshwater Behaviour and Physiology, O'Connell describes how he tested bull and nurse sharks in the Florida Keys by tempting them with two piles of fish carcasses lying behind clay bricks - - one of the sites had barium-ferrite magnets, the other was not magnetized. After keying in on the bait, bull sharks on the magnetized-brick side kept swimming away in frustration (although nurse sharks that swam in from the opposite side easily ate the bait). After discovering the other pile, they fed a number of times from the non-magnetized side.

For a study in Bimini waters, O'Connell traded in bricks for beach nets. Six young lemon sharks were given the choice to swim through a magnetic or control opening in a net-like fence. In the first trial, all six avoided the magnetic region and preferred to swim through the control region fence. But after re-testing three of the sharks that were rested for 24 hours, the magnets no longer affected their swimming behavior, and they often swam through the magnetic opening. "However, these results may not mean that permanent magnets are an ineffective long-term repellent," O'Connell writes in his study, published in Ocean & Coastal Management. "[The sharks] may have habituated to the repeated magnetic exposure, although ... it was unclear whether it was due to the stress of repeated testing over a short duration, or due to sensory habituation to the magnetic fields."

"A lot of species are very different and have predatory strategies, so we can't say all sharks will respond that way," O'Connell told Undercurrent. He recently tested great whites, and next up are hammerheads and tiger sharks.

But they aren't foolproof. First, magnets only have a short range. A U.S. patent application filed in 2007 by SharkDefense describes installing Lanthanide electropositive metals into fins, a bracelet and a belt that divers can wear to ward off sharks, but also states that the gear would only repel sharks "within a few inches" of approaching them. Because the magnetic field only deters sharks up to a foot away, you'd need multiple magnets scattered across your body for enough security. What sport diver would do that?

Secondly, you may be in trouble if a shark-feeding frenzy occurs. When sharks are hungry enough, they may continue to pursue something despite an uncomfortable electrical field, says Ralph Collier, founder of the Shark Research Committee and author of Shark Attacks of the 20th Century. "If you're using a baiting situation with dead fish, sharks are aware the fish aren't alive, so their behavior is methodical and slow moving. But if it's moving at 30 miles per hour in a predatory attack, and your area of protection is in the middle of that sphere, there's an excellent possibility that the shark will pass through the magnetic field and strike before it feels the effect. Sharks' type of behavior and feeding mode has a great deal to do with the effectiveness of magnets. They only work on a limited basis."

O'Connell disagrees. He says his life was saved by a SharkDefense product, although it had no magnets attached. It was Scuba/Spear MiniCan, an aerosol spray containing the semiochemical repellent shark necromone, literally "shark corpse." You spray it into the water, shrouding you in the reek of decomposing shark. "We were running an experiment in the Bahamas, which had me surrounded by eight Caribbean reef sharks," says O'Connell. "They were feeding at the surface, so I was going to swim away, but instead I drifted directly into the center of the feed. Prior to biting, sharks tend to bump the victims, I got bumped several time. One was so hard, I blacked out. Another researcher threw two aerosol cans at me, and one hit me on the head hard, so I woke up. I sprayed, and the sharks were completely gone by the time I was about to get out of the water. The aerosol is quick acting, and will move in the current, but it did save my life." But we have yet to see this written up in a scientific journal.

He admits magnets may make a greater impact on the commercial fishing industry - - SharkDefense has found a way to magnetize fishing hooks (so sharks won't accidentally get caught by fishing boats) and beach nets installed around swimming areas.

Commercial divers - - maybe even those who feed sharks for tourists - - may not be deterred by the $500 average price for SharkShield products. SharkDefense products are much cheaper - - a magnetic ankle bracelet is $25, a can of aerosol spray is $15. Given the paltry number of sport divers who get bitten by sharks, we doubt the products will be flying off the shelf. However, if you're a worrywart, it's cheap insurance - - if the products work.

- - Vanessa Richardson

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