Incidental catches of sharks have been drastically reduced thanks to electric pulses

The ocean roared around the tuna fishing boat, the waves crashing against the ship’s hull as fishermen in waterproof gear struggled to haul their catch onto the deck.

“Empty!” one howled over the sounds of nature, while another wave swung them aside. The metal hook was quickly disengaged, lest someone step on it. The next hook was firmly embedded in the jaw of a huge tuna, and behind it was another animal they hadn’t wanted to catch: a shark. With a curse, the fisherman quickly tried to pull it out of the animal’s mouth, but it was firmly stuck between the rows of teeth.

A few twitches, tremors and what seemed like an eternity later…it was over! The fisherman quickly threw the animal overboard, praying it would do so with just a sore jaw. It was a common occurrence not only for this fishing vessel, but for fleets around the world – and many hoped there would be an answer to this problem. Fortunately, early field tests have shown that a device that emits electric pulses the size of a little finger attached to fishing hooks significantly reduces the incidental capture of sharks and rays. By using “SharkGuard”, anglers can catch tuna while limiting accidental shark deaths worldwide.

Good news, as the International Union for Conservation of Nature’s (IUCN) most recent global assessment of the Red List of Threatened Shark Species estimated that more than a third of species (37%) are threatened with Extinct (i.e. considered Critically Endangered, Endangered or Vulnerable). Recently, a team of experts from around the world assessed 31 species of sharks and rays and found a 71% drop in global abundance since 1970, a period that saw a doubling of fishing pressure and a tripling of catches of sharks and rays.

Some fish, like sharks and rays, have young at a slower rate than others, making them more vulnerable to increasing fishing pressure. Sharks are caught in targeted fisheries (their meat, fins, liver oil, skin, cartilage, teeth and jaws) but also caught incidentally, when fishermen target other species such as tuna and sharks. swordfish but accidentally end up with a shark in their net or on their line. Commercial fishermen use five main methods (or types of fishing gear) to catch fish – and longline hooks, which have hundreds or thousands of hooks trailing below a horizontal main line, have been a major shark killer. The small SharkGuard device is attached a few centimeters above a hook where it emits a pulsating electrical charge that creates an electromagnetic field around itself.

Sharks and rays have electrosensory organs in their skin that detect subtle changes in electrical fields underwater. These include electroreceptors known as ampullae of Lorenzini, jelly-filled tubes that open on the surface of their skin, which are extremely sensitive as they can pick up very weak electric fields produced by prey and other animals. With that in mind, the goal of this new pulse device is to “overwhelm the senses,” says lead author Dr. Phil Doherty at the University of Exeter. He likens it to someone standing too close to a loudspeaker playing music at a concert – “it’s too stimulating” and again can avoid that experience.

To test the technology, Doherty and his colleagues set out to deploy the SharkGuard device on two fishing vessels off the coast of southern France, where each vessel was equipped with 22 longlines with more than 9,000 hooks. On both vessels, half of these thousands of hooks were alternately secured with a SharkGuard device to see if there was a difference in shark bycatch between pulsed and non-pulsed hooks. The experiment took place on more than 11 different trips during the summer of 2021 and the results left the authors impressed with device performance in the field: it has reduced bycatch of blue sharks (Prionaceae glauca) and pelagic rays (Pteroplatytrygon violacea) by 91 and 71 percent, respectively. Yet bluefin tuna (Thunnus thynnus), devoid of any sort of electrosensory organs, seemed indifferent.

“I was quite shocked, especially by the blue shark…I mean, it’s huge,” Doherty says. The blue shark has been called the most frequently caught shark in the world, in part due to its range, long migration patterns and vulnerability to fishing pressure. Found worldwide in temperate and tropical waters, they are widely caught in targeted deep sea fisheries due to their transoceanic migrations; as a result, many fisheries have access to it.

Although SharkGuard initially looks promising, some scientists are raising a number of concerns. First, there’s the cost: outfitting a longline fishing vessel with SharkGuard costs around $20,000, which many might not be willing to shell out upfront. Battery life isn’t that long either (65 hours), although that’s a developer thing Fishtek Marine is currently working on its extension. Yet the initial cost could be offset by allowing anglers to catch more of the species they are actually targeting.

What worked for these species of sharks and rays might not work for other species. How 100% effective personal electric shark deterrents are for all species, Nicolas Dulvy at Simon Fraser University in Canada points out that the device may have varying effectiveness between different species of sharks or rays, as each has a unique configuration of electrosensory organs. Dulvy was not involved in this work, but remarked, “It would be really interesting to see how this works in, say, hammerhead sharks and silky sharks.”

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