A recently completed Recreational Fishing Initiative Fund-funded study into devices that can reduce or prevent shark depredation (bite-offs) is providing encouraging results for commercial and recreational fishers.
After hundreds of hours of testing at a range of locations between the Abrolhos and the Montebello Islands, the study found that the three tested devices were effective at reducing depredation by approximately 65 per cent overall.
Dr Gary Jackson, Principal Research Scientist at the Western Australia Department of Primary Industries and Regional Development (DPIRD), said that while each of the three devices tested were based on different operating systems – one based on electrical field, one based on magnetic field, and one based on acoustics – all had a similar purpose.
While there are more than 100 species of sharks recorded in commercial and recreational catches around the WA coast, this study identified around 10 shark species, mostly from the whaler family that were regularly involved in depredation.
Testing involved line-fishing in waters up to 50 metres depth in an experimental design. Underwater cameras were used to record the interactions and behaviour of sharks when the different devices were deployed.
During the study, the team shot more than 180 hours of video footage in WA's northern waters to provide invaluable insights into shark behaviour. The footage has increased understanding of shark biology and behaviours under a range of conditions.
"The study successfully tested the effectiveness of devices," said Dr Jackson. "We now know that the devices do provide a level of deterrence, but like many research projects, we now find ourselves asking more questions than the ones we set out to answer.
"Through this study our understanding of shark depredation behaviours has substantially increased. But now we're asking, 'Why are these things happening?' and considering possible relationships between different findings, which will require further research."
The research identified a number of simple ways fishers can reduce the incidence of shark depredation, without a deterrent device. One simple method is to move one's fishing location regularly, so sharks aren't attracted to the area as fish are caught.
Fishers are also advised to avoid areas known to be depredation hotspots and not to clean fish at sea. Fish waste should instead be brought back to land for composting.
Rapid retrieval was also found to be effective and can be achieved through the use of high-speed reels or electric winches, but this also increases the risks of barotrauma in deep water. The study also found reduced depredation when not using burley or dumping fish offal, as this is a dinner bell for sharks.
Is a northern shark fishery sustainable?
Western Australia's northern shark fishery, which operated between Exmouth and Broome, was closed in 2005 as a result of overfishing of the temperate shark resource including species such as sandbar and dusky sharks.
The closure was part of a 20-year recovery plan for the shark resource. However, with a new stock assessment due in 2022, the option to reopen this fishery sooner is currently being deliberated.
While some shark stocks are showing signs of healthy recovery, others, including sandbar and dusky sharks, which breed in the waters of the northern shark fishery, may still be rebuilding.
"On one hand there may be potential for increased commercial shark fishing in the future," Dr Jackson said. "However, we need more specific research to understand how WA's shark populations are recovering. Once the resource has recovered, we can look at a range of options to support future commercial fishing operations."
Changing underwater environment
The variables of nature added a further degree of difficulty to the study. Dr Jackson said that on some days the team would interact with dozens of sharks at different depths, while on other days there were none.
"On top of this you have marine environments being impacted by climate, so some of our northern shark species are moving further south. We are still learning about shark biology in these changing conditions."
Adding a further challenge for the future, the researchers found it is not just sharks chasing the fish.
"Our video cameras also captured large cod taking fish from the line as well, which further demonstrates the complexity of the deterrent debate."
More facts
DPIRD is currently developing a fact sheet on shark depredation and deterrents. This will be available online in the near future at www.fish.wa.gov.au.
