Electric pulses drastically cut number of sharks caught by accident

Grey reef sharks (Carcharhinus amblyrhynchos) in Bora BoraAn electric pulse-emitting, pinkie-finger-sized device attached to fishing hooks appears to dramatically reduce the accidental catch of sharks and rays in initial field tests. The technology, called “SharkGuard”, could enable fishers to harvest target species like tuna while limiting unintentional bycatch deaths of sharks around the world.

Since 1970, the global abundance of sharks and rays has declined by 71 per cent. A major cause of this has been accidental snags on longline fishing hooks, which consist of a horizontal main line with hundreds or thousands of hooks trailing below.

The SharkGuard device hitches a ride on this same fishing gear and is affixed a few centimetres above a hook. The device – about the size and shape of a AA battery – works by emitting a pulsing electric charge that creates an electromagnetic field around the SharkGuard’s position.

Sharks and rays have electrosensory organs in their skin that detect subtle changes in underwater electric fields. So, the aim of the pulsing device is to “overwhelm the senses” says Phil Doherty at the University of Exeter in the UK. Like a human standing too close to a speaker blasting music, “it’s overstimulating”, he says.

To test the technology, Doherty and his colleagues deployed SharkGuard on two fishing vessels off the coastline of southern France over 11 different trips in the summer of 2021. Each vessel was equipped with 22 longlines with more than 9000 hooks. On both ships, half of the 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. SharkGuard reduced the bycatch of blue sharks (Prionace glauca) and pelagic stingrays (Pteroplatytrygon violacea) by 91 and 71 per cent, respectively. Bluefin tuna (Thunnus thynnus), which lack electrosensory organs, appeared unbothered.

Doherty was impressed by how well the device performed in the field.

“I was quite shocked, especially at the blue shark… I mean, that’s huge.”

Though SharkGuard shows initial promise, the results should be taken with caution, says Nicholas Dulvy at Simon Fraser University in Canada who was not involved in the work. The device may have varying effectiveness between different shark or ray species, as each has a unique configuration of electrosensory organs.

“It would be really interesting to see how this works in, say, hammerhead and silky sharks,” says Dulvy.

Outfitting a longline fishing vessel with SharkGuard costs about $20,000 and the devices have a battery life of 65 hours, something developer Fishtek Marine is currently working to extend. Still, the upfront cost and effort could be offset by allowing fishers to haul in more of the target species.