With its mouth open, ready to bite, the shark approached a tempting chunk of opelu baitfish at the end of a pole, then veered away at the last second.
A piece of charged metal attached to the pole gummed up the animal’s electronic sense, biologist John Wang explained as he watched the shark on film.
Wang and colleagues at the National Oceanic and Atmospheric Administration have been experimenting with metals that release electrons into sea water. Their goal is to reduce the incidental catch of sharks in Hawaii’s swordfish and tuna longline fisheries.
At the heart of the project is a shark’s sixth sense, its ability to detect minute magnetic fields, not unlike how humans can feel a television’s electric field close up through the hair on their arms.
Tunas and swordfish do not have an electric sense, said Wang, associate researcher at the Joint Institute for Marine and Atmospheric Research, operated by the University of Hawaii and NOAA.
So the researchers are looking for a way to disrupt the sharks’ feeding behavior and reduce bycatch without harm to the animals or loss to the commercial species, he said.
Millions of sharks are caught unintentionally worldwide, resulting in serious economic, biological and environmental costs, Wang said.
Furthermore, each shark caught is one less hook for tuna or swordfish and can result in damage, loss of fishing gear and possible injury to crew handling sharks, he said.
Removing sharks, “apex predators,” in large numbers from the ecosystem also harms the overall marine community, Wang said. “It is like pulling a lion out of the Serengeti (wildlife sanctuary).”
Researchers are also seeking a way to protect sharks, which increasingly face extinction.
A growing number of shark species are winding up on the World Conservation Union’s endangered species list, said Richard Brill, research scientist at NOAA’s Northeast Fisheries Science Center in Virginia. Because they grow slowly and are slow to reproduce, they are easily overfished, Brill said.
Wang participated in a study, led by Brill in Virginia, looking at the effects of metal on captive sharks in research tanks, and he has been experimenting with metal to deter free-swimming sharks on the North Shore. Wang works with the Pacific Islands Fisheries Science Center of the National Marine Fisheries Service in Honolulu, which funded Brill’s study.
Sharks and rays, part of the class of cartilage-skeletoned fish known as elasmobranchs, have what is known as ampullae of Lorenzini — organs that can detect minute electric fields.
Sharks use electroreception to find prey and mates and to navigate, said Wang, saying that they can detect the earth’s magnetic field to find north.
“It’s one of the cooler things in the world,” he said. “How can they migrate thousands of miles from one discreet place to another? It’s an exquisite sense they have. Amazing.”
Electronic devices are available to deter sharks, but they are relatively expensive and too big to use on fishing gear, Wang said.
He and his colleagues experimented the past year in Haleiwa Harbor with baitfish attached to ends of wooden poles either with a piece of lead or a piece of electropositive metal.
They collaborated with Hawaii Shark Adventures, using its underwater cage to observe and film the response of Galapagos and sandbar sharks.
In 77 trials, 58 sharks chose bait attached to lead rather than the bait associated with metal, Wang said. “They were avoiding that.”
Brill, who earned his doctorate from UH’s John A. Burns School of Medicine, said in a telephone interview he believes the metal overloads the shark’s electroreceptors.
“What we think is happening would be like flashing a bright light in your eyes and you kind of flinch back,” he said.
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