The Key to Saving Humans and Sharks from Each Other? Maybe Magnets

​Interactions between sharks and humans don't always end well. Shark attacks may still be over-hyped in the press, but they still eat a human every year or two. Less publicized is the fact that every day, countless sharks will be hauled up as bycatch and left to die on a ship deck—one study found humans kill as many as 100 million sharks a year. As such, it's probably for the best if our two species just stayed away from each other.

Magnets might help. Sharks are among a host of organisms, including certain bacteria, insects, birds and mammals, that possess magnetoreception, or the ability to detect and respond to magnets. The notion that we might use magnets to repel sharks has garnered scientific interest in recent years. But for a magnetic shark-repellant to be commercially viable, we're going to have to make sure it doesn't repel teleosts—the bony fish we like to eat, and which comprise an $80 billion global industry—as well.

"Whether or not magnetic hooks reduce catch rates of teleost fishes remains largely an open question," Michael Courtney of BTG Research writes in a new paper that reviews what we know about magnetic marine repellents. "Failure to explicitly test for sensitivity to magnetic deterrents creates a real risk of inadvertently altering catch rates of teleost species."

Early efforts to develop shark deterrents were motivated by the simple and understandable fear of, yes, being eaten by a shark. More recently, conservationists started speaking up about the need to reduce bycatch, a widespread problem that's causing shark populations worldwide to plummet. To that end, magnetic hooks, which come at a nominal cost increase, are an attractive option. In 2011, marine scientists published a study showing how magnetized fish hooks can significantly reduce catch rates of at least two sharks, the Atlantic sharpnose and dusky smooth-hound.

But, as the authors of the present review paper point out, much more data is needed to generalize the 2011 study's findings, to verify how bony fish species will or won't be affected.

Magnetic sensitivity typically works in one of two ways: Direct field detection or electromagnetic induction. In direct detection, the organism possess nanometer-sized magnetite crystals. These iron-rich crystal grains act as sensors, orienting themselves to the local magnetic field and relaying a signal to the animals' brain. The famous homing pigeon, as well as a host of other birds and insects, use their magnet-rich flesh to trek across thousands of miles without getting lost.

In electromagnetic induction, animals take advantage of the fact that a magnet moving through a conductive medium like saltwater induces an electric field. Sharks, and other cartilaginous fish, possess a special organ called the ampullae of Lorenzini which allows them to sense electric fields. In theory, this also renders them susceptible to magnets.

That's all well and interesting, but, according to Courtney and his co-authors, there's a bit of a problem here. See, some have taken the fact that sharks possess special, electrically sensitive organs as rock-solid evidence that all sharks respond to magnets, and all through the same mechanism. As Courtney points out in his paper, not all sharks have ampullae large enough to sensitively detect magnetic fields.

What's more, many proponents of shark magnets make another unsubstantiated assumption: That bony fish which lack ampullae can't detect magnets at all. Indeed, there's evidence for direct detection via magnetic crystals in a several familiar fishes, including yellowfin tuna, sockeye salmon, Mozambique tilapia and the common carp.

"The necessary selectivity of magnetic shark deterrents has been assumed though flawed

theoretical reasoning that teleost species cannot detect electric and magnetic fields," Courtney and his co-authors write. Yet, they argue, it'd be relatively easy to conduct a series of field trials that directly test the effect of magnetic hooks on a wide range of commercial fish.

Hopefully, scientists can rustle up the funding for such experiments, and fast. If we want to make a quick dent in our incidental shark kills, magnetic repellants may be one the best options we've got. But a magnetic hook that also drives all the fish away is going to be a really tough sell.