A zinc-and-graphite hook attachment repels sharks without bothering the tuna

Every year, millions of sharks die on longline fishing hooks meant for tuna and swordfish. They are caught, hauled aboard, and discarded - dead or dying. The industry calls it bycatch. For shark populations that reproduce slowly and mature late, it is an existential threat. For fishermen, it is wasted bait, damaged gear, occupied hooks, and lost revenue from the fish they were actually trying to catch.

Stephen Kajiura, a professor of biological sciences at Florida Atlantic University, has spent years working on a solution that is almost absurdly simple. His device attaches to a standard fishing hook. It uses zinc and graphite to generate a weak electric field in seawater. Sharks hate it. Tuna do not notice.

Exploiting a sense that only sharks possess

The principle behind Kajiura's deterrent is biological, not mechanical. Sharks possess an electrosensory system - clusters of jelly-filled pores called ampullae of Lorenzini that detect minute electrical fields in the water. They use this system to find prey, navigate, and sense their environment. No bony fish - not tuna, not swordfish, not any of the commercially valuable target species on a longline - has anything comparable.

When zinc and graphite contact seawater, they form a galvanic cell - essentially a simple battery. The resulting electric field is too weak for humans or bony fish to detect, but it overwhelms sharks' exquisitely sensitive electroreceptors. The effect is aversive: sharks avoid the hook.

Field tests have shown that the device can reduce shark catch rates by up to 69%. The target catch - tuna, swordfish, and other commercially valuable species - remains unaffected because those fish simply lack the sensory apparatus to detect the field.

A $2.5 billion market with a bycatch problem

The global longline fishing gear market was valued at approximately $2.5 billion in 2023 and is projected to reach $4.1 billion by 2032. That growth is driven by rising seafood demand, expanding fishing fleets, and increasing investment in technologies that improve efficiency. Shark bycatch reduction fits squarely into that trend - it is both an environmental improvement and an economic one.

When a shark takes a hook meant for tuna, the fisherman loses on multiple fronts. The bait is consumed. The hook is occupied, sometimes for hours, unable to catch its intended target. The shark may damage the line or the gear. Hauling and releasing a large shark poses safety risks to the crew. In U.S. waters, regulations require that most shark species caught as bycatch be released, meaning the fisherman gets no economic value from the catch that consumed their resources.

Kajiura's device addresses all of these problems simultaneously. If sharks avoid the hooks entirely, bait stays available for target species, gear stays intact, crew stays safer, and shark mortality drops.

From lab prototype to commercial fishing boats

The device is currently patent-pending, and FAU's Office of Technology Development has awarded Kajiura $15,000 through its Innovation Pilot Award Program to advance the technology from prototype toward commercial readiness. The funding supports CAD modeling, precision CNC milling of refined prototype units, and - critically - field testing with active commercial fishing operations.

That last element matters. Shark deterrent devices have been proposed before, but many failed the practicality test. Some were too expensive per hook. Others were too complicated to deploy on a working longline, which might carry hundreds or thousands of hooks. Others wore out too quickly in corrosive seawater. Kajiura's design is deliberately simple: two metals, no batteries, no electronics, no moving parts.

"We're excited to continue testing the device with commercial fishermen, gather their feedback, and refine the design," Kajiura said. The iterative approach - building, deploying, getting feedback from the people who will actually use the product, and refining - is designed to avoid the common failure mode of academic inventions that work in the lab but not on the water.

What the 69% number does and does not mean

The 69% reduction in shark catch rates comes from field trials, which is encouraging - lab results often fail to replicate in open-ocean conditions where currents, temperature, and animal behavior introduce variability. But several questions remain.

The durability of the galvanic cell in extended deployments has not been fully characterized. Zinc corrodes in seawater. How many deployments a single device withstands before the electric field weakens below the deterrent threshold will determine its cost-effectiveness for commercial use.

The 69% figure is also a peak reduction. Actual performance will vary with shark species (different species have different electrosensitivity), water conditions (temperature and salinity affect the galvanic cell's output), and deployment configuration. Consistent, peer-reviewed data across multiple fisheries and conditions would strengthen the case.

Whether the deterrent effect habituates over time - whether sharks that encounter the field repeatedly learn to tolerate it - has not been tested in long-term studies. Habituation has limited the effectiveness of other sensory deterrents, including acoustic devices used for marine mammal bycatch reduction.

The device reduces bycatch but does not eliminate it. The remaining 31% or more of shark encounters still result in capture. For critically endangered species where every individual matters, additional measures would still be needed.

Still, a cheap, simple, passive device that cuts shark bycatch by more than half while improving catch efficiency for target species is a genuinely useful contribution to fisheries management. The approach is grounded in solid biology - sharks have electrosensory systems, bony fish do not - and the early field data supports the concept.