In a world-first discovery, researchers have found an electrical shark deterrent used at Cocos (Keeling) Islands was effective at reducing the number of fish taken off fishing hooks by sharks – a process known as depredation.
The study, led by The University of Western Australia's Dr Jonathan Mitchell and published in CSIRO's Marine & Freshwater Research, is the first of its kind globally to document the results of testing a deterrent to reduce depredation.
Dr Mitchell, an Adjunct Research Fellow from UWA's School of Biological Sciences, said shark depredation was a serious issue affecting fisheries in Australia and the results provided an important step forward in developing effective and practical mitigation methods to reduce shark depredation.
"Having their prized fish eaten is frustrating to fishers, exacerbating negative perceptions of and actions towards sharks," he said.
"For commercial fishers especially, it adds to the cost of their business with less fish to sell and increased time, fuel and gear costs."
Study co-author and UWA Adjunct Research Fellow Victoria Camilieri-Asch said shark depredation resulted in more target fish being killed, which might otherwise have been released due to bag and size limits.
"It can also affect the sustainability of certain fisheries," Ms Camilieri-Asch said.
This study tested the effectiveness of an electrical shark deterrent device (RPELX) in reducing shark depredation during line fishing for demersal fish species.
The device works in a similar way to those available for surfers and divers by creating a powerful electrical field to overwhelm sharks' electrosensory system but is specifically designed for line fishing practices.
The device clips on to the fishing line 50cm above the hook and creates an electrical field around the hooked fish as it is retrieved..
Across 51 fishing sessions at Cocos (Keeling) Islands off Western Australia in areas with high densities of grey reef sharks, 262 fish were caught, with results indicating that the deterrent reduced the probability of depredation by 63 per cent while also reducing shark bycatch and loss of fishing gear.
Dr Mitchell said in addition to using shark deterrents, there were several modifications to fishing methods that could be used to reduce shark depredation.
"These include moving location frequently and fishing in more remote areas away from other boats, using electric reels or handlines to retrieve fish faster, using jigs and lures instead of bait, avoiding dumping fish waste near fishing spots to reduce odour stimulants in the water, and switching target species," he said.
"While each of these measures individually won't stop shark depredation, using them in combination can add up to a meaningful reduction and enable more successful fishing outcomes.
"Further work is needed to continue refining these measures, developing and testing new deterrents and understanding the context and drivers of shark depredation in more detail. Ultimately, scientists and fishers must continue working together to reduce shark depredation as much as possible."
