To Catch a Killer Shark

Story by Chris Baraniuk

First was the French tourist, killed while swimming off St. Martin in December 2020. The manager of a nearby water sports club raced out in a dinghy to help, only to find her lifeless body floating face down, a huge gaping wound where part of her right thigh should have been. Then, a month later, another victim. Several Caribbean islands away, a woman snorkeling off St. Kitts and Nevis was badly bitten on her left leg by a shark. Fortunately, she survived.

Soon after the fatal incident in December, Eric Clua, a marine biologist at the École Pratique des Hautes Études in Paris, France, got a phone call. Island nations often ask for his help after a shark bite, he says, “because I am actually presenting a new vision. … I say, ‘You don’t have a problem with sharks, you have a problem with one shark.’”

Human-shark conflicts are not solely the result of accidents, confusion, or happenstance, Clua argues. Instead, he says there are such things as problem sharks: bold individuals that may have learned, perhaps while still young, that humans are potential prey. It’s a controversial stance, but Clua thinks that if it’s true—and if he can identify and remove these problem sharks—it might dissuade authorities from taking even more extreme forms of retribution, including culls.

Though culls of sharks in the wake of human-shark conflict are becoming less common and are generally regarded by scientists as ineffective, they do still happen occasionally. One of the last major culls took place near Réunion, an island in the Indian Ocean, between 2011 and 2013, resulting in the deaths of more than 500 sharks. Even that wasn’t enough for some—four years later, a professional surfer called for daily shark culls near Réunion.

And so, in the immediate aftermath of the French tourist’s death in St. Martin, when one of Clua’s contacts called to explain what had happened, he recalls insisting, “Go, just go there on the beach. … I want swabbing of the wounds.”

After both that bite and the one that occurred one month later near St. Kitts and Nevis, medical professionals collected samples of mucus that the shark had left behind on both victims’ wounds. These samples were sent off for analysis, though it took weeks for the results to come back. But as Clua and colleagues describe in a study published last year, that DNA analysis did eventually confirm that the same tiger shark was responsible for both incidents.

Even before the DNA test was complete, however, forensic analysis of the teeth marks left on the St. Martin victim, and of the tooth fragment collected from her leg, suggested the culprit was a tiger shark (Galeocerdo cuvier) roughly 3-metres (10-feet) long. Knowing this, Clua and colleagues set out to try to catch the killer.

During January and February 2021, Clua and his collaborators hauled 24 tiger sharks from the water off St. Martin and analyzed a further 25 sharks that they caught either around St. Barts or St. Kitts and Nevis. Because both of the women who were bitten had lost a substantial amount of flesh, the scientists saw this as a chance to find the shark responsible. Each time they dragged a tiger shark out of the water they flipped it upside down, flooded its innards with water, and pressed firmly on its stomach to make it vomit. A shark is, generally, “a very easy puker,” Clua says. The team’s examinations turned up no evidence of human remains.

Clua and his colleagues also took DNA samples from each of the tiger sharks, as well as from dead sharks landed by fishers in St. Kitts and Nevis. None of those samples matched the DNA swabbed from the wounds suffered by the two women. But the team hasn’t given up. Currently, Clua is waiting for DNA analysis of mucus samples recovered from a third shark bite that happened off St. Martin in May 2024. If that matches the DNA from the earlier bites, Clua says, that would suggest it “might be possible” to catch the culprit shark in the future.

That some specific sharks have developed a propensity for biting people is controversial among marine scientists, though Lucille Chapuis, a marine sensory ecologist at La Trobe University in Australia, isn’t entirely sure why. The concept of problem animals is well established on land, she says. Terrestrial land managers routinely talk about—and are forced to contend with—problem lions, tigers, and bears. “Why not in a fish?” asks Chapuis. “We know that fishes, including sharks, have amazing cognitive abilities.”

It’s worth noting, however, that even definitions of problem bears can differ. One 2020 study found that people in Alberta sometimes disagree about what constitutes a problem bear, or how one should manage such an individual. In the United States, debates have sometimes raged over whether or not a wild bear that has killed someone ought to be killed.

Yet when I reached out to a range of marine scientists to get their opinion on Clua’s ideas, some rejected the concept of problem sharks outright. One refused to read Clua’s latest papers, saying they did not believe in problem sharks before adding, “I am not interested in that.”

Clua is aware that his approach is divisive: “I have many colleagues—experts—that are against the work I’m doing.” A 2022 story in The New York Times covering some of Clua’s earlier research delved into the divide. The biggest pushback is from scientists who say there is no concrete evidence for the idea that there are extra dangerous, human-biting sharks roaming the seas. And merely talking about problem sharks, they say, could perpetuate the idea that some sharks are hungry, human-eating monsters like the beast from the wildly unscientific movie Jaws.

Clua says the monster from Jaws and his definition of a problem shark are completely different. A problem shark is not savage or extreme; it’s just a shark that learned at some point that humans are among the things that it might prey on. Environmental factors, as well as personality, might precipitate or exacerbate such behavior. Despite dismissals from some peers, Clua says two of his recently published papers contain results that support his hypothesis.

Besides the tiger shark that struck off St. Martin and St. Kitts and Nevis, Clua’s 2024 study also detailed the case of another tiger shark involved in multiple bites in Costa Rica. A third case focused on an oceanic whitetip shark in Egypt that killed a female swimmer by biting off her right leg. The same shark later attempted to bite the shoulder of one of Clua’s colleagues during a dive.

Toby Daly-Engel, a shark expert at the Florida Institute of Technology, says the genetic analysis connecting the same tiger shark to two bite victims in the Caribbean is robust. However, she says such behavior must be rare, and she strongly criticizes the idea that sharks are biting people with any intentionality. “They’re just opportunistic. I mean, these things eat tires.” 

Diego Biston Vaz, who curates fishes at the Natural History Museum in London, England, also praises Clua’s work, calling it “really forensic.”  Though he, too, emphasizes it should not be taken as an excuse to demonize sharks. “They’re not villains; they’re just trying to survive,” he says.

Chapuis adds that the small number of animals involved in Clua’s recent studies mean the research does not prove beyond doubt that problem sharks are real. Plus, while some sharks might learn to bite humans and treat them as prey, she questions whether they would continue to do so long term. Biting people, Chapuis says, is not likely to reward sharks. People tend to defend themselves well and, given there are only a few dozen unprovoked shark bites recorded around the world each year, she says there just isn’t the data to support the idea that even the boldest sharks are actually benefitting from biting people.

Plus, Clua’s plan—to capture so-called problem sharks and force them to meet justice—is unrealistic, says David Shiffman, a marine conservation biologist based in Washington, D.C. Even if scientists can prove beyond a shadow of a doubt that a few specific sharks are responsible for a string of incidents—“which I do not believe he’s done,” Shiffman adds—Shiffman believes that actually finding those sharks is not viable.

Daly-Engel agrees that there are complicating factors that would make tracking down and catching a culprit shark difficult. Unlike stereotypical human serial killers, who often return to the scene of their crime, for instance, Daly-Engel says sharks typically swim far away from a feeding site once they’ve eaten.

Any money and resources used to track down problem sharks would be better spent on preventative measures such as lifeguards, who could spot sharks approaching a busy beach, says conservation biologist Catherine Macdonald from the University of Miami in Florida. While identifying and removing a problem shark is better than culling large numbers of the animals, she urges people to answer harder questions about coexisting with predators. “For people who don’t want to risk interacting with sharks, I have great news,” she says: “Swimming pools exist.”

Clua, for his part, intends to carry on with his research. Currently, he’s working with colleagues on St. Martin to swab shark bite wounds when they occur, and to track down potential problem sharks. 

When I ask Clua whether he has ever experienced a dangerous encounter with a large shark himself, he says that over his 58 years of diving it has happened, but only once. He was spearfishing off New Caledonia. Poised underwater, waiting for a fish to appear, he turned his head. “And there was a bull shark coming [toward] my back,” Clua says. 

Right then, he got the feeling that he was about to become prey. But there was no violence. Clua looked at the bull shark as it turned and swam away.