Do Shark Repellents Really Work?

Long before the premier of Jaws and the very first Shark Week, humans have been searching for ways to ward off one of the sea’s most fearsome (and hyped) hunters. Sharks might not deserve their gnarly reputation, but every shark attack seems to renew the fever search for effective repellent, reports Brian Owens for Hakai magazine. 

Prompted by shark attacks in the Pacific during World War II, U.S. Navy researchers took a chemical approach, mixing and matching 38 compounds to create “Shark Chaser.” It looked like cake mix, and smelled like rotten shark, as Joseph Caputo reported for Smithsonian back in 2009.  During the Vietnam War, military divers were issued this copper acetate powder and black dye cloaking dye, neither of which worked.

Nevertheless, chemical tactics have been employed over the years. Some recipes took inspiration from fish that repel sharks with chemicals, but others drew from more culinary inspirations — Julia Child said her "first big recipe" was for shark repellant for the Navy made out of black dye and copper acetate held together by water-soluble wax. None seem to have been all that effective.

Modern researchers have (mostly) ditched the chemicals and opted for a magnetic strategy. The secret to shark perception seems to lie in specialized holes on their snouts called the ampullae of Lorenzini. According to the going scientific theory, these help sharks pick up on electromagnetic fields underwater, allowing them to track prey and navigate.

Most repelling products marketed today aim to overwhelm their ability to sense these magnetic fields. Some emit electronic pulses that essentially shock the shark’s senses, as Christopher Beam explained for Slate in 2009. Others, like Sharkbanz bracelets, simply use magnets. Positively charged metals can produce a similar electric current underwater. To ward off a shark, “it doesn’t take a very strong magnet, maybe just 10 to 50 times the strength of the Earth’s field,” Eric Stroud, a chemist whose conservation group SharkDefense pioneered this strategy, told Owens at Hakai.

Yet, testing these products and proving their efficacy is tough. For one, it would endanger human subjects, Owens explains. In 2012, the Australian government tried to test the effectiveness of some electronic repellants by sticking them on seals. Sometimes they worked; sometimes the sharks ignored the electronic shield. In a 2008 test, a shark ate a repellant device, writes Beam.

Last year, South Africa began testing an electronic fence to keep great white sharks from popular beaches with some success. But for individual divers and swimmers, the verdict is still out on shark repellents, magnetic or otherwise.

Helen Thompson | | READ MORE

Helen Thompson writes about science and culture for Smithsonian. She's previously written for NPR, National Geographic News, Nature and others.