This article originally appeared in Mongabay.
Unintentional catch is a big reason that more than a hundred shark species are threatened with extinction. Yet creating a small electric field around fishing hooks using cheap inputs — zinc and graphite — is enough to keep many away, a new study indicates.
In coastal waters off Florida, small zinc-and-graphite blocks rigged next to fishing hooks reduced shark catch by around two-thirds, according to the study, which was published in the Canadian Journal of Fisheries and Aquatic Sciences on Jan. 15.
“This study was part of an effort to reduce the number of sharks that are caught and killed as incidental bycatch in commercial fisheries,” Stephen Kajiura, a professor of biological sciences at Florida Atlantic University and lead author of the study, told Mongabay. “We’re trying to develop a method that will be cheap and effective that the fishermen could use, that would keep the sharks off the hooks but still allow them to catch their target species.”
“It’s no good if it impedes the fisherman’s ability to get what they want,” he added. “And that’s the cool thing about this type of repellent … it only repels sharks and not anything else.”
Sharks and related species are especially electrosensitive. Researchers have for decades tried to take advantage of this to develop devices to keep them off fishing hooks. A 2022 systematic review of the techniques, some of which involved magnets or rare elements, found mixed results.
Kajiura said magnets were impractical, as they stick to any number of objects on a fishing vessel — fishing gear, tools, the boat itself — and that some of the elements previously tested were rare. His zinc-graphite treatment, on the other hand, is nonmagnetic and made of cheap, readily available materials, he said.
Kajiura’s team designed the treatment for use on longline fishing vessels, which drop hundreds or thousands of hooks per set and generally pose the greatest danger to sharks of any fishing gear. The longliners often target tuna or swordfish. Some boats also target sharks, and the extent of this intentional shark catch is poorly reported. Nevertheless, a large proportion of shark catch remains unintentional, and it’s this type of catch that the new work seeks to address.
For the study, which was funded by the U.S. government, the research team placed small, brick-shaped blocks composed of layers of zinc and graphite just above each hook on a fishing line. Each one created an electric field about the size of a beach ball.
Their headline finding came from demersal, or seafloor, fishing in coastal Florida. The research team attached hooks to lines in three configurations: some with a zinc-graphite block, some with a plastic block that was visually similar but had no electric charge, and a final, normal group with no blocks. The hooks next to zinc-graphite blocks caught only 58 sharks, compared with 155 for the group with plastic blocks and 190 for the group with no blocks — reductions of 62.4% and 69.4%, respectively.
The researchers also tried the experiment in two other settings. A trial in pelagic, or upper ocean, offshore Florida waters yielded too few results to draw scientific conclusions, while the treatment just didn’t work in demersal Massachusetts waters. There, nearly every hook, regardless of zinc-and-graphite treatment, had a piked dogfish (Squalus acanthias) when the lines came up.
Kajiura said the team knew the treatment was unlikely to work in Massachusetts because piked dogfish “eat everything” and are “like little vacuum cleaners on the bottom.” Piked dogfish are in the Squalidae family of sharks, whereas nearly every shark caught in the coastal Florida trial was in the Carcharhinidae family — just like most sharks caught by longliners, such as blue sharks (Prionace glauca) and critically endangered oceanic whitetip sharks (Carcharhinus longimanus), Kajiura said.
“If it worked for the nearshore Carcharhinid species … it will also likely work with the offshore Carcharhinid species that are caught [in] the commercial fisheries,” he said in an email to Mongabay.
Eric Gilman, a Honolulu-based marine scientist who works on reducing incidental shark catch and wasn’t involved with this study, said its findings were “robust” and “extremely promising” but more research would be needed to see if the treatment works in other contexts.
“The species of sharks that are predominantly captured in demersal fisheries are distinct from those captured in blue-water pelagic fisheries,” Gilman told Mongabay in an email.
Even within one family, such as Carcharhinidae, there “can be a broad range of heritable traits (such as response to an electrical deterrent),” Gilman said.
Kajiura and two former students are developing commercial applications of the treatment that he said are much more convenient for fishers than the brick-like blocks used in the study. They have a patent pending and are refining the treatment with input from fishers from Florida to New Jersey, he said.
“To get commercial guys to use it, it needs to be slick and easy,” Kajiura said.
Commercial fishers have an incentive to use the treatment because every hook that is shark-free is another hook that could catch a valuable tuna or a swordfish, he added.