These fishing line devices keep sharks and rays from becoming bycatch in longline fisheries, new research suggests.
Every year, more than 20 million sharks die because they are accidentally caught by fishermen who were actually after tuna, for example. At least a quarter of the affected shark species are therefore listed as endangered species. Bycatch is therefore a major problem. The company Fishtek Marine now thinks it has found the solution: a device that uses electrical pulses to keep sharks away from fish hooks. Researchers at the University of Exeter tested the gadget and published their findings Current Biology.
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Scare off sharks
Shark deterrents have been around for a long time. Sometimes divers and surfers wear these gadgets to protect themselves from attacks. But can they also protect the sharks themselves against bycatch from longline fishing? Fishtek developed the SharkGuard especially for this purpose. This device is attached to the fishing line and creates a small electric field around the fishing hook.
Sharks and rays have sensitive senses in the skin of their snouts that allow them to pick up electrical fields. With these so-called ampoules of Lorenzini they can, for example, find their prey or orient themselves on sea currents. If the predatory fish come too close to a SharkGuard, the ampoules become overstimulated by the electric field. The animals get scared and flee from the hooks. At least that is the intention.
No effect on tuna?
But does it really work? The researchers, led by Philip Doherty, put it to the test last summer. They made a total of eleven trips with two fishing boats and used a total of 22 long lines and more than 18,000 hooks to fish for bluefin tuna. The number of by-catches on fishing lines with and without SharkGuard was tallied.
The devices were found to reduce the number of blue sharks caught by 91 percent and the number of stingrays by 71 percent. Good news, you would say. Especially since there was almost no difference between the number of tuna caught on the fishing lines with and without the devices. So the tunas didn’t seem to be affected by SharkGuard, something fishermen can no doubt appreciate.
Benevolence of fishing industry
“Using techniques to reduce shark and ray bycatch in tuna and swordfish fisheries is always a trade-off between the effectiveness of the technique to deter sharks and the application for anglers to use the technique without compromising the catch,” says marine biologist and shark expert Guido Leurs of the University of Groningen.
“The device described here sounds promising,” he continues. “However, the total catch of blue whales in this study was very low (a total of 27, ed.). This may have other causes, but it will influence the willingness of the fishing industry to use the technique.”
Fisheries expert Leo Nagelkerk of Wageningen University agrees. “Because the total tuna catch was so low, I have not yet found convincing evidence that there was no effect of SharkGuard on the target fish.”
Not a panacea
You can also ask yourself whether a device on the fishing line will make a big difference, Nagelkerke continues. “The fishermen caught 27 bluefin tuna against no fewer than 75 blue sharks and 270 (!) rays. As interesting as the potential of SharkGuard is, it must be very effective in protecting these animals, which are very sensitive to fisheries, against the fishing hook. So I don’t expect it to be a panacea.”
In addition, according to both Leurs and Nagelkerke, there are some other practical matters that need to be improved to make the technique more practical for fishermen. For example, one of the limitations is that the battery must be replaced regularly. Fishtek wants to tackle this problem by charging SharkGuard by, for example, pulling the fishing line. Leurs: “And as the researchers also indicate: the fishing industry must be actively involved in further development in order to make SharkGuard a possible success.”
Bronnen: Current Biology, Cell Press via EurekAlert!, University of Exeter via EurekAlert!
Image: © Robert Enever