Sharks Repelled by Metal That Creates Electric Field
for National Geographic News
|May 1, 2008|
A metal mix that reacts with seawater to produce an electric field could help curb the global death toll of sharks caught inadvertently on longline fishing gear.
An alloy of the rare earth metals palladium and neodymium caused captive sandbar sharks to avoid hooked bait, according to a recent study released by the National Oceanic and Atmospheric Administration (NOAA).
"We could see them shudder and then dash away," said study co-author Rich Brill, a NOAA fisheries biologist.
The team hopes that suspending ingots of the metal on fishing gear will help deter wild sharks from longlines meant to catch commercial species such as tuna and swordfish.
The lines snag an estimated 11 to 13 million unwanted sharks each year, contributing to worldwide shark declines. (Related news: "8 Million Sharks Killed Accidentally off Africa Yearly" [April 17, 2007].)
"Being slow growing and slow to reproduce, sharks usually cannot take a lot of fishing pressure," Brill said.
It's also possible that the metal could deter sharks from attacking human swimmers, but the researchers are cautious about this application, as the alloy only works at a distance of about 20 to 24 inches (50 to 60 centimeters).
"If you had a very large bull shark coming toward you, the metal may work," said study co-author Eric Stroud, a chemist from the research company Shark Defense.
"But would you really want to be that close to the shark?"
Sandbar sharks are among the largest coastal sharks in the world, reaching up to 8 feet (2.4 meters) long and weighing up to 200 pounds (90.7 kilograms).
Like other sharks, sandbars hunt by sensing the tiny electrical fields given off by their prey. (See photos of sharks on the prowl.)
Stroud had been studying magnets as potential shark repellents when he noticed the animals' dislike for certain rare earth metals that were being used as controls.
"It was then that we suspected we had an electrochemical repellent on our hands," he said.
Stroud and colleagues tried various different metals and identified a group of alloys, known as misch metals, as likely cost-effective candidates for a shark repellent.
To test how sensitive sharks are to these metals, NOAA's Brill and colleagues studied the behavior of young sandbar sharks in laboratory tanks.
They found that captive sharks were happy to eat baitfish suspended below small lead weights.
When the lead was replaced with the misch metal palladium neodymium, the sharks would only get within 1.6 feet (0.5 meter) of the food.
But after a few days of repeating the trial, the 14 hungry sharks learned to "hold their noses" and take the bait.
"The fact that it worked at all was pretty remarkable," Brill said.
Further tests using a digital camera and scanning software revealed that individual sharks would swim up to lead weights but would always steer clear of the misch metal.
And tests conducted by Shark Defense confirmed that target fish species are not sensitive to the electric fields generated by the metal alloy.
Results of the experiment were presented last month at a NOAA-sponsored shark-deterrent workshop in Boston, Massachusetts.
This summer, Brill and colleagues will set up simulated longlines in inshore estuaries to test whether misch metals will reduce shark catch rates.
"We are close to this becoming an engineering problem, for example, to find out the optimum size and number for the metal pieces," Brill said.
Co-author Stroud points out that metal repellents might not work for every shark, but that the keyword is "reduce."
"If catch rates can be reduced by 25 to 50 percent, that would translate to a thousand sharks saved in one fishery alone in a month," he said.
Sonja Fordham is deputy chair of the World Conservation Union's Shark Specialist Group and director of the Shark Conservation Program at the Ocean Conservancy.
"Research into ways to repel sharks from fishing gear meant for other fish species holds great promise for shark conservation," she said.
"The success of bycatch-avoidance initiatives will depend not only on sound research, but also on cooperation from fishermen as well as political will from fishery managers to impose such measures where they are needed but not voluntarily adopted."
This kind of approach should also translate into financial savings for fishers, NOAA's Brill added.
"Every hook that has a shark hanging on it doesn't have an expensive targeted tuna fish species hanging on it," he said.
"[And] a big shark will cause an awful lot of damage to a longline."
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