Understanding Shark Depredation: A Growing Challenge for Australian Fisheries

Shark depredation, the phenomenon where sharks bite or remove fish from fishing lines or nets before fishers can land them, has become a significant issue in Australian waters. This human-wildlife conflict not only frustrates recreational anglers but also imposes economic burdens on commercial operations. In regions like Western Australia, where recreational fishing is a cultural staple, reports indicate that up to 52% of fishers have experienced at least one shark encounter annually. Rates can vary widely, from 9.1% in some recreational fisheries to as high as 38-42% of trips in hotspots like Ningaloo Marine Park and Exmouth Gulf. Globally, depredation affects fisheries sustainability by increasing target species mortality—fish that might otherwise be released under bag or size limits end up dead—and leading to gear loss.

In Australia, the problem is exacerbated in areas with high shark densities, such as coral atolls like Cocos (Keeling) Islands. Local fishers there dub sharks 'cucut,' reflecting their pervasive impact on daily catches. Economic losses include direct costs from lost fish and gear, estimated at around A$100 per day for some operations, alongside indirect hits like increased fuel use from repeated trips and reduced tourism appeal for charter services.

UWA Researchers Pioneer RPELX Deterrent in World-First Study

Researchers from the University of Western Australia (UWA) have made headlines with a groundbreaking study demonstrating a 63% reduction in shark depredation using an innovative electrical deterrent called RPELX. Led by Dr. Jonathan D. Mitchell, an Adjunct Research Fellow in UWA's School of Biological Sciences and affiliated with the UWA Oceans Institute, the study was published in CSIRO's Marine & Freshwater Research. Co-author Victoria Camilieri-Asch, also from UWA, collaborated with experts from Bond University, Stantec, and Western Australia's Department of Primary Industries and Regional Development.

Dr. Mitchell, whose PhD focused on quantifying depredation in Ningaloo, brings deep expertise to this field. His prior work documented rates around 12% in recreational line fishing there, highlighting spatial variations and behavioral interactions. This latest research marks the first global test of a deterrent specifically for line fishing depredation, addressing a critical gap.

How the RPELX Device Works: Overloading Shark Senses

The RPELX is a compact electrical device clipped onto the fishing line approximately 50 cm above the hook. When activated during retrieval, it generates a 200-volt electromagnetic field around the baited fish, targeting sharks' highly sensitive ampullae of Lorenzini—electrosensory organs that detect prey's bioelectric fields. This overload disrupts the shark's ability to approach closely, deterring depredation without harming the animal.

Unlike surfer deterrents like magnetic bands or wetsuit fabrics, RPELX is optimized for fishing gear, with a 1.1-meter design suited for deepwater line fishing (50-200m depths). Field tests showed it effective against grey reef sharks (Carcharhinus amblyrhynchos), the primary culprits at Cocos, observed in 23 depredation attempts.

In controlled trials at Lord Howe Island, similar devices like RPELX reduced bites by 48-83%, confirming electrical fields' promise over magnets.

Study Methodology: Rigorous Field Testing at Cocos Keeling Islands

Conducted over 11 days in February-March 2024, the randomized experiment involved six local skippers across 51 fishing sessions, yielding 262 demersal fish catches. Treatments alternated between control (no device) and RPELX-on, balanced at 74 events each for analysis. Data was logged via a Survey123 app, capturing depredation (shark removal), bycatch, and gear loss.

Statistical analysis used generalized linear models (GLM), revealing treatment and skipper as key predictors (P < 0.0001). Depredation probability dropped from 94% (control) to 35% (RPELX), a 63% relative reduction. Catch-per-unit-effort dipped slightly (4.3 vs. 7.5 events/hour), but net landed fish rose due to fewer losses. No target species avoidance was noted.

This mirrors Mitchell's Ningaloo PhD, where depredation hit 38.7% of west coast trips, emphasizing the need for such innovations.Explore Mitchell's full publication list.

Key Results and Statistics: 63% Fewer Losses

• Depredation events: 85% control vs. 26% RPELX-on.
• Annual savings at Cocos: ~A$9,828 for six fishers (gear/fuel/fish value).
• Reduced bycatch: Fewer hooked sharks released unharmed.
• No impact on fish CPUE long-term; potential for 983 fewer lost fish/year per group.

These figures underscore RPELX's dual benefits: economic relief and ecological gains by curbing excess mortality.

Photo by Marija Zaric on Unsplash

Economic and Social Impacts on Australian Fishers

Shark depredation costs Australian fisheries dearly. In WA, 77% of surveyed commercial/charter/recreational fishers faced it in five years, with recreational rates up to 41.9% in Exmouth. Nationally, it erodes social amenity, fuels anti-shark sentiment, and hampers sustainability amid rising recreational participation (over 5 million anglers).

At Cocos, an Australian territory, depredation threatens food security and tourism. RPELX could save thousands annually, easing pressures on remote communities. Broader adoption might mitigate national losses, estimated in millions via gear (~A$100/day) and opportunity costs.Marine research jobs at Australian universities like UWA drive such solutions.

Conservation Benefits: Balancing Human Needs and Shark Populations

Reducing depredation minimizes unintended shark bycatch and target fish waste, supporting sustainable fisheries. Grey reef sharks, protected in many areas, benefit from non-lethal deterrence, fostering coexistence. UWA's Oceans Institute emphasizes such interdisciplinary work, aligning with Australia's marine park goals.

Prior UWA-led Lord Howe Island trials showed RPELX slashing bycatch 94% in small samples, reinforcing electrical tech's role. This advances global efforts, as depredation drives illegal shark culling calls.

Dr. Mitchell's Journey: From Ningaloo PhD to Global First

Dr. Jonathan Mitchell's 2018 UWA PhD quantified Ningaloo depredation at ~12%, using baited cameras and genetics to ID species like sicklefin lemon and blacktip sharks. Publications in Marine Ecology Progress Series detail behaviors, informing RPELX design. His work spans JCU collaborations on management reviews, positioning UWA as a leader.

"Having their prized fish eaten is frustrating... it adds to business costs," Mitchell notes, advocating fisher-scientist partnerships.Career advice for marine research assistants.

Complementary Strategies: Beyond RPELX for Maximum Effect

• Frequent spot changes to avoid shark hotspots.
• Electric reels/handlines for rapid retrieval.
• Jigs/lures over bait; target less shark-prone species.
• Avoid dumping fish waste nearby.

Combining these with RPELX yields compounded reductions, per UWA recommendations. Ongoing trials at Lord Howe aim to refine.

Future Outlook: Expanding UWA-Led Innovations Nationwide

UWA calls for broader testing in demersal and pelagic fisheries, alongside behavioral studies on 'learned' depredation. With FRDC workshops highlighting gaps, university research like this paves the way.Read the full UWA study. As climate shifts alter shark distributions, academic insights from Bond and QUT collaborators will be vital.

Australian universities drive marine solutions; explore opportunities at AcademicJobs Australia.

Photo by Willian Justen de Vasconcellos on Unsplash

Why University Research Matters for Sustainable Fishing

Institutions like UWA, with its Oceans Institute, bridge science and practice. Mitchell's trajectory—from PhD to adjunct fellow—exemplifies higher ed's role in tackling real-world issues. Programs in marine biology equip students for such challenges, fostering careers in conservation and fisheries.Research assistant jobs in higher ed.

This study not only cuts losses but promotes ethical shark-human harmony, a win for biodiversity and communities.

Conclusion: A Brighter Future for Fishers and Sharks

UWA's shark depredation reduction study heralds practical progress, slashing losses by 63% via RPELX. As Dr. Mitchell urges, "scientists and fishers must continue working together." For marine enthusiasts, check Rate My Professor for top lecturers, browse higher ed jobs, or seek career advice. Discover university jobs advancing such research—post yours at /recruitment.