The World Wildlife Fund states that up to 100 million sharks and rays are caught each year across the globe, both on purpose in targeted fisheries or by accident, as bycatch. As a result of this exploitation, shark populations are in rapid decline and more than one third of species are threatened with extinction. Sharks grow relatively slowly, take many years to mature and produce few young, making them very vulnerable to overfishing.
A new device, known as a “SharkGuard” has now been tested experimentally in the field to see whether it helps reduce the number of sharks and rays caught incidentally on longline fishing gear. The novel device is attached to a baited hook on a line, and it produces a small, pulsing electric field around the bait. The electric signal is detected by the electroreceptors of sharks and rays and this, hopefully, will deter them from taking the bait. The scientists tested the device’s efficacy by comparing the catches on two longline vessels that were fishing for bluefin tuna (Thunnus thynnus) using both conventional hooks and SharkGuard hooks.
The experimental sea trials took place off southern France, during July and August 2021. Eleven separate fishing trips were made involving 22 longlines and a total deployment of 18,866 hooks, half of which carried a SharkGuard device. The results, published in the journal Current Biology, showed that SharkGuard hooks significantly reduced the number of blue sharks and oceanic stingrays caught, while having no significant impact on the catch of bluefin tuna.
More specifically, the catch of blue sharks (Prionace glauca) averaged 6 per 1,000 control hooks, compared to 0.5 per 1,000 SharkGuard hooks, which translated into a decrease in catch per unit effort (CPUE) of 91 percent. An average of 7 pelagic stingrays (Pteroplatytrygon violacea) were caught per 1,000 control hooks, compared with 2 stingrays per 1,000 SharkGuard hooks, representing a decrease in CPUE of 71 percent. These encouraging results indicate that it is possible to reduce the shark and stingray bycatch when using longline fishing gear.
“The main implication is that commercial longline fishing may continue, but it won’t always necessarily result in the mass bycatch of sharks and rays,” said Robert Enever of Fishtek Marine, Dartington, Devon, U.K. “This is important in balancing the needs of the fishers with the needs of the environment, and contributes to national and international biodiversity commitments for long-term sustainability.”
Enever and his colleagues developed the new SharkGuard technology in response to the dramatic declines in shark and ray numbers in recent decades. They based it on shark deterrents that are currently used successfully to protect scuba divers and surfers, and modified it to be used in tuna fisheries, where bycatch of sharks and rays is significant.
Fisheries that target sharks and rays in particular, for fins and meat, are often poorly managed and controlled, and usually lack catch limits. But the added issue of bycatch in other fisheries, targeting primarily tuna and billfish, compounds the problem enormously. Sometimes bycatch is released, but this requires more effort and commitment on the part of the fishers. The researchers note that, in comparison, SharkGuard offers a more comprehensive mitigation solution because it prevents the capture in the first place. If its use were scaled up to the level of whole fisheries, it would mean much reduced interaction between sharks and fishing gear.
Despite these encouraging results, there is some way to go before fisheries can use the SharkGuard mitigation devices efficiently. Firstly, there is the limitation that the batteries in each SharkGuard need to be changed or recharged after every 65 hours of deployment. The developers are working to overcome this barrier, so fishers could “fit and forget” them, while still protecting sharks and other bycatch species.
It’s expected that a full set of induction-charged SharkGuard devices for 2,000 hooks (one longline fishing vessel) could cost around $20,000 and would last 3 to 5 years (~$4-7K per annum) which, they note, is a modest annual cost for most commercial tuna fishing operations.
As consumers, retailers and supply chains become more aware of the impacts of fishing on bycatch species, including sharks and rays, the fishing industry will have to respond to calls for more sustainable and environmentally acceptable capture methods. The researchers encourage fishers who experience high shark and ray bycatch, as well as retail corporates who are intent on improving the sustainability of their supply chain, to seek contact with Fishtek Marine early as sea trials and engineering developments are planned for commercialization.
“There is hope!” Enever said. “Against the relentless backdrop of stories of dramatic declines occurring across all species, it is important to remember that there are people working hard to find solutions. SharkGuard is an example of where, given the appropriate backing, it would be possible to roll the solution out on a sufficient scale to reverse the current decline in global shark populations.”
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