Sharks are built to sense electricity. Tiny gel-filled pores around their snouts called the ampullae of Lorenzini allow them to detect weak electric fields produced by struggling prey or even the Earth’s magnetic field. For years, scientists have wondered whether that sensitivity could be used to keep sharks away from fishing gear, reducing bycatch without hurting the catch of target species. And a recent study put this surprisingly simple idea to the test.
The two materials chosen (zinc and graphite) wasn’t done by accident or luck of the draw. When they sit next to each other in seawater, they create a small galvanic electric field, no batteries required! The question was straightforward: would sharks avoid hooks that produced this field? To find out, researchers measured the voltage around zinc and graphite treatments and deployed them on both demersal longlines (which sit on or near the seafloor) and pelagic longlines (which drift higher in the water column). They compared three setups — untreated hooks, procedural controls and hooks fitted with the zinc and graphite pairing.
In Florida, the results for bottom-dwelling sharks were shocking, if you pardon the pun. The catch-per-unit-effort, or CPUE, of demersal sharks on untreated hooks was 67.0. On procedural controls it was 54.6 and on hooks fitted with zinc and graphite, that number dropped to 20.6. That translates to a 62.3 to 69.5 percent reduction in shark catch compared to the controls. For fisheries managers trying to reduce bycatch without shutting down operations, that kind of reduction is a big deal! But when similar trials were conducted in Massachusetts, the CPUE for demersal sharks did not differ among treatments. In fact, it sat at 667.1 regardless of whether hooks were untreated, controls or fitted with zinc and graphite.
In other words, in one region the deterrent worked. In another, it did not. Why?
Part of the answer may lie in shark diversity. Not all sharks are created equal when it comes to electroreception. The Florida trials primarily involved sharks from the order Carcharhiniformes, better known as the “ground sharks.” The largest order of sharks, comprising more than 270 species, they are characterized by five gill slits, movable eyelids that help protect the eyes from injury, two spineless dorsal fins, an anal fin, and a wide mouth lined with sharp teeth positioned behind the eyes. Species include blue sharks, bull sharks, Caribbean reef sharks, bronze whaler sharsk, dusky sharks, hammerheads, tigers and spinner sharks. The Massachusetts trials involved more Squaliformes, or the “dogfish sharks.” This order includes about 126 species found across nearly all marine habitats. They have elongated snouts, short mouths, five gill slits, two dorsal fins, and no anal fin. Some deepwater species are even bioluminescent! Species include cookiecutter sharks, great lanternsharks, Greenland sharks, kitefin sharks, gulpers, Pacific sleeper sharks, and spiny dogfish. The difference in efficacy between these groups suggests that the response to galvanic electric fields may be species-specific. That makes biological sense as the sensitivity of the ampullae of Lorenzini can vary among species depending on ecology, habitat and evolutionary history; acoastal predator that hunts active prey in murky waters may rely heavily on electroreception while a deepwater species living in colder, darker environments might process electric cues differently. If the deterrent field falls within the sensory threshold of one group but not another, you would expect exactly this kind of split result.
But what about the pelagic longlines? Too few sharks were captured to allow for meaningful statistical comparison, a reminder of how variable fisheries data can be. However, one finding stood out: the capture of targeted bony fishes was greatest on hooks with the zinc and graphite treatment. That suggests the electric field did not reduce target catch rates… if anything, it may have had no negative effect at all.
For fishers, that is the million-dollar question. It is one thing to reduce shark bycatch in a controlled study, it is another to do so without sacrificing the species that keep a fishery economically viable. A deterrent that scares off sharks but also reduces tuna or swordfish catch would be a hard sell. A deterrent that selectively reduces shark interactions while leaving target catch untouched is a different conversation entirely.
So where does this leave us?
First, it shows there is not likely a single silver bullet for shark bycatch. Fisheries operate in different oceans, target different species and interact with different shark communities. A tool that works in one place may not translate directly to another. Now that doesn’t mean the tool is useless! It just means it needs to be applied strategically. Second, it raises questions about how we think about shark conservation. Overfishing and bycatch are the leading threats to many shark populations worldwide and international bodies like the International Union for Conservation of Nature regularly list shark species as “Vulnerable” or “Endangered” due in large part to fishing pressure. If a low-tech, relatively inexpensive material pairing can reduce bycatch for certain groups by more than half, it should be considered for standard practice in those fisheries! But t
he authors of the study are clear that large-scale testing, especially in pelagic fisheries, is still needed. Small trials are promising but commercial fisheries operate at massive scales. Gear durability, cost, maintenance and long-term effectiveness all matter. It’s unknown if the zinc will corrode too quickly, if fishers adopt the method if it requires additional handling time, or if sharks might habituate to the electric field over time. The challenge now is figuring out where, for which species and at what scale this simple approach can make a meaningful dent in one of the ocean’s most persistent conservation problems.
Can we use what makes sharks extraordinary to give them a better chance? We’ll see.
