Accidental Shark Catches Have Been Dramatically Reduced By Electric Pulses

The ocean roared around the tuna fishing boat, waves crashing against the hull of the vessel as fishers clad in water-proof clothes strained to pull their catch onto the deck.

“Empty!” one screamed over the sounds of nature, as another wave rocked them sideways. The metal hook was quickly cleared away, lest someone step on it. The next hook was firmly lodged into the jaw of a huge tuna, and behind it, another animal they hadn’t meant to catch: a shark. With a swear, the fisher quickly tried to retrieve it from the animal’s mouth, but it was firmly lodged between rows of teeth.

A few more yanks, wiggles, and what felt like an eternity later… it was out! The fisher quickly dumped the animal overboard, praying it would make it with just a sore jaw. This was a common occurrence for not just this fishing boat, but fleets all over the world – and many hoped there would be an answer to this problem. Thankfully, initial field tests have found that a pinkie-finger-sized electric pulse-emitting device attached to fishing hooks drastically reduces the accidental capture of sharks and rays. By using “SharkGuard,” fishers can catch tuna while limiting bycatch deaths of sharks around the globe.

Good news, since the most recent global International Union for Conservation of Nature (IUCN) Red List of Threatened Species assessment of sharks estimated that over one third of species (37%) are threatened with extinction (i.e., considered Critically Endangered, Endangered, or Vulnerable). Recently, a team of experts from around the world assessed 31 shark and ray species and found a decline by 71 percent in global abundance since 1970, a period that saw a doubling of fishing pressure and a tripling of shark and ray catches.

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

Sharks and rays have electrosensory organs in their skin that detect subtle changes in underwater electric fields. 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 this in mind, aim of this new pulsing device is to “overwhelm the senses,” explains lead author Dr. Phil Doherty at the University of Exeter. He likens it to a person standing too close to a speaker blasting music in a concert – “it’s overstimulating” and can make one avoid that experience again.

To test the technology, Doherty and his colleagues set out to deploy the SharkGuard device on two fishing vessels off the coastline of southern France where each vessel was equipped with 22 longlines with more than 9,000 hooks. On both ships, half of these thousands of hooks were secured with a SharkGuard device in an alternating pattern to see if there was a difference in shark bycatch between pulsing and non-pulsing hooks. The experiment took place across over 11 different trips in the summer of 2021 and the results left the authors impressed by how well the device performed in the field: it reduced the bycatch of blue sharks (Prionace glauca) and pelagic stingrays (Pteroplatytrygon violacea) by 91 and 71 per cent, respectively. Yet bluefin tuna (Thunnus thynnus), which lack any sort of electrosensory organs, appeared unbothered.

“I was quite shocked, especially at the blue shark… I mean, that’s huge,” says Doherty. The blue shark has been called the most frequently caught shark in the world, partially due to their range, long migration patterns, and vulnerability to fishing pressure. Found around the world in both temperate and tropical waters, they’re caught extensively in high seas targeted fisheries due to their transoceanic migrations; this results in many fisheries having access to them.

Though SharkGuard shows initial promise, some scientists bring up a number of concerns. First, there’s the cost: outfitting a longline fishing vessel with SharkGuard has a pricetag of around $20,000, something many might not be willing to fork over upfront. The battery life isn’t that long either (65 hours), although that is something developer Fishtek Marine is currently working to extend. Still, the upfront cost could be offset by allowing fishers to haul in more of the species they are actually targeting.

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