Study Snapshot: Shark bycatch is a major global problem, with millions of sharks caught unintentionally each year in fisheries targeting tuna, swordfish and other species. Even in U.S. waters, sharks are frequently caught on longlines, and many are discarded dead. Because sharks grow and reproduce slowly, these high bycatch rates threaten already vulnerable populations and disrupt marine ecosystems.
Researchers at FAU’s Charles E. Schmidt College of Science have developed a patent-pending zinc-and-graphite device to address the problem. The metals generate a small electric field that repels sharks from baited hooks while leaving target species unaffected. In Florida field trials, the device reduced shark bycatch by more than 60%. Inexpensive, scalable and practical for fishers, this technology has the potential to dramatically reduce bycatch and support more sustainable fisheries.
For decades, sharks have been the unintended victims of longline fisheries aimed at tuna and swordfish. Rising accidental catches have contributed to population declines and created serious challenges for both conservation and commercial fishing. And the impacts go beyond the sharks themselves – every time a shark takes the bait, hooks are lost to target species, gear gets damaged, costs climb, and crews face added risks when handling or releasing the animals.
Although some gear modifications can reduce bycatch, they often also cut into catches of valuable species, making it hard to protect sharks without putting fisheries at a disadvantage.
To tackle this challenge, researchers at Florida Atlantic University’s Charles E. Schmidt College of Science have developed an innovative, patent-pending shark deterrent. The device works by pairing zinc and graphite in seawater. The zinc reacts with the graphite to produce a small electric field in the surrounding seawater through a galvanic chemical reaction. This electric field can be detected by the sharks, repelling them from the bait without affecting target fish.
To test the efficacy of the zinc/graphite treatment at deterring elasmobranch species, longline fishing gear was deployed to target demersal sharks (live and hunt near the sea floor) off the Florida panhandle and Massachusetts, and pelagic sharks (live and hunt in open water) in the Gulf of America.
The results of the field trials, published in the Canadian Journal of Fisheries and Aquatic Sciences, delivered striking results. In Florida, the zinc/graphite treatment reduced the catch of coastal sharks on demersal longlines by 62% to 70% compared to untreated hooks. The effect was particularly strong for Atlantic sharpnose (Rhizoprionodon terraenovae) and blacktip sharks (Carcharhinus limbatus), two common coastal species.
“Sharks have an incredible ability to sense even the smallest electric fields, and our tests show that this new approach can be used to keep them away from baited hooks,” said Stephen Kajiura, Ph.D., senior author, inventor and a professor in the FAU Department of Biological Sciences. “At the same time, important target species like tuna and swordfish are completely unaffected. What makes this approach so exciting is its practicality – zinc and graphite are inexpensive, widely available, and already familiar to fishers because zinc is commonly used to prevent corrosion on boats. This means it could be adopted quickly and cost-effectively, providing a real solution to reduce shark bycatch while supporting sustainable fisheries.”
Importantly, the treatment did not reduce catches of commercially important fish species. Preliminary pelagic trials suggest swordfish and yellowfin tuna were caught at similar or slightly higher rates on treated hooks, showing the approach could protect sharks without hurting the catch of target species.
The study also outlines practical considerations for real-world use. Because the electric field is strongest close to the hook, each line would need its own zinc-graphite device. The zinc anode slowly wears down, but it’s cheap and easy to swap out.
Shark bycatch is a widespread and pressing problem, both in the United States and around the world. Globally, millions of sharks are caught unintentionally every year in fisheries targeting other species, and some estimates suggest tens of millions fall victim to bycatch annually. In U.S. waters, despite strict regulations, sharks are still caught incidentally on longlines and other gear. Because sharks reproduce slowly and have long lifespans, these high bycatch rates can push populations toward dangerously low levels. The scope of shark bycatch, from small coastal fisheries to large international fleets, makes it a global conservation challenge with serious ecological consequences.
“Our approach could be scaled up to pelagic longline fisheries, where millions of sharks are caught as bycatch annually,” said Kajiura. “Even a 60% to 70% reduction in shark bycatch, like that observed in Florida demersal trials, could have a dramatic impact on global shark populations. The zinc/graphite treatment offers a practical, affordable and environmentally responsible tool for reducing shark bycatch while maintaining commercial catch rates.”
Study co-authors are FAU graduate students Tanner H. Anderson and Kieran T. Smith; co-inventor on the patent application; Cheston T. Peterson, a Ph.D. student at Florida State University; Bryan A. Keller, Ph.D., a foreign affairs specialist at NOAA Fisheries; and Dean Grubbs, Ph.D., a full research faculty and associate director of research at FSU.
This research was supported by the Florida SeaGrant awarded to Kajiura and Grubbs.
The patent-pending device works by pairing zinc and graphite in seawater, creating an electric field that can be detected by the sharks, repelling them from the bait without affecting target fish.
