A half-dollar-sized metal alloy clamped onto fishing gear protects sharks by creating a mild electrical current.
"This alloy is not meant to protect humans from sharks," said Robert Brill, a scientist at the Virginia Institute of Marine Science who has tested the alloy on several shark species. "This is meant to protect sharks from humans."
While only 4.3 people die of unprovoked shark attacks on average each year, humans kill an estimated 73 million sharks each year. Some sharks are intentionally caught and their fins cut off for soups.
Sharks inadvertently caught on fishing gear meant for tuna, swordfish and other commercially valuable fish species, are known as bycatch.
Clamping a half-dollar size alloy, made from an unspecified combination of rare-earth elements, repels sharks away from bait intended for tuna and swordfish.
When the gear is dropped into the ocean it reacts with salt water to create 1.2 to 1.5 volts of electricity that extends out several feet from the metal.
Sharks use pimple-sized, jelly-filled electrically conductive nodules on their nose, known as ampullae of Lorenzini to detect the beating heart of their prey in murky waters.
The ampullae of Lorenzini are exquisitely sensitive, able to detect electrical currents even in the nanovolt range. So when a shark comes within the metal's 60-cm (23.6-inch) range and suddenly finds as much as 1.5 volts of electricity, it's a surprise to the shark.
"The shark was expecting the electrical impulse from a heart beat, and we are giving it eight or nine times what it was expecting," said Eric Stroud, one of the co-founders of Shark Defense who helped develop the shark repellent metal alloy.
For the shark, Stroud thinks "it's like shining a flashlight into their eyes."
For solitary and slow-moving species the alloy is effective, reducing sand bar shark by-catch by 64 percent.
"If it's a high-speed, visual predator like a great white shark however, then it probably wouldn't be much use," said Stroud.
The effectiveness of the metal also decreases as the number of sharks goes up, says Brill. Alone, a shark or ray might avoid the metal. But as the number of sharks increases, so does the chance that the sharks will venture into the electrical field to take an investigatory nibble or even a full bit.
After 72 hours, half of the metal will have turned into a harmless white precipate and fallen to the ocean floor. The resulting metal is non-toxic and is already found inside the ocean.
The alloy is meant to protect sharks form humans, but Stroud thinks that people have been wearing pieces of the metal to help protect them from sharks as well.
"We don't advocate personal use," said Stroud. "But people do buy them online and modify them for personal use."
For fisherman who use the alloys, it does add another step to the rigging process. The next step for Shark Defense, and its Canadian manufacturer HEFA, is to make hooks out of the metal to make it easier for fisherman to catch the fish they want and avoid the sharks they don't.
"Sharks tend to tear up fishing gear," says Brill. "And every shark on the line means that line doesn't have a valuable tuna or swordfish on the line."