Is there such a thing as a ‘problem shark’? Plan to catch repeat biters divides scientists
First was the French tourist, killed while swimming off Saint-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 gaping wound where part of her right thigh should have been. Then, a month later, another victim. Several Caribbean islands away, a woman snorkelling 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, 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 or happenstance, Clua says. Instead, he says there are such things as problem sharks: bold individuals that may have learned, perhaps while still young, that humans are 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.A shark killed a man at Long Reef beach in Dee Why, Sydney, on 6 September, 2025. Photograph: Dean Lewins/AAPThough culls of sharks after human-shark conflict are becoming less common and are generally regarded by scientists as ineffective, they do still happen. One of the last big culls took place near Réunion, a French island in the Indian Ocean, between 2011 and 2013, resulting in the deaths of more than 500 sharks. Even that was not enough for some – four years later, a professional surfer called for daily shark culls near the island.And so, in the immediate aftermath of the French tourist’s death in Saint-Martin, when one of Clua’s contacts called to explain what had happened, he recalls telling them: “Just go there on the beach … I want swabbing of the wounds.”After that bite and the one that occurred a month later, medical professionals collected samples of mucus that the shark had left behind to send 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, the DNA analysis confirmed that the same tiger shark was responsible for both incidents.Even before the DNA test was complete, however, analysis of the teeth marks left on the Saint-Martin victim, and of the tooth fragment collected from her leg, suggested the perpetrator was a tiger shark (Galeocerdo cuvier) roughly 3 metres (10ft) long. Armed with this knowledge, Clua and his colleagues set out to catch the killer.During January and February 2021, Clua and his team hauled 24 tiger sharks from the water off Saint-Martin and analysed a further 25 sharks that they caught either around St Barts or St Kitts and Nevis.Eric Clua and his colleagues took DNA samples from nearly 50 tiger sharks to try to find one that had bitten two women. Photograph: Courtesy of Eric CluaBecause 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 matched the DNA swabbed from the wounds suffered by the two women.But the team has not given up. Clua is now waiting for DNA analysis of mucus samples recovered from a third shark bite that happened off Saint-Martin in May 2024. If that matches samples from the earlier bites, Clua says, that would suggest it “might be possible” to catch the culprit shark in the future.For people who don’t want to risk interacting with sharks, I have great news – swimming pools existCatherine Macdonald, conservation biologistThat 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, is not entirely sure why. The concept of problem animals is well established on land, she says. Terrestrial land managers routinely contend with problem lions, tigers and bears. “Why not a fish?” asks Chapuis. “We know that fishes, including sharks, have amazing cognitive abilities.”Yet having gleaned a range of opinions on Clua’s ideas, some marine scientists rejected the concept of problem sharks outright.A tiger shark. Some scientists fear that merely talking about problem sharks could perpetuate the preconception of human-eating monsters. Photograph: Jeff Milisen/AlamyClua is aware that his approach is divisive: “I have many colleagues – experts – that are against the work I’m doing.”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. Merely talking about problem sharks, they say, could perpetuate the idea that some sharks are hungry, human-eating monsters such as 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 it might prey on. Environmental factors, as well as personality, might trigger or aggravate such behaviour.Besides the tiger shark that struck off Saint-Martin and St Kitts and Nevis, Clua’s 2024 study 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.Pilot fish follow an oceanic whitetip shark. A woman was killed when an oceanic whitetip bit off her right leg in Egypt. Photograph: Amar and Isabelle Guillen/Guillen Photo LLC/AlamyToby 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 behaviour must be rare. “They’re just opportunistic. I mean, these things eat tyres.”Diego Biston Vaz, curator of fishes at the Natural History Museum in London, also praises Clua’s work, calling it “really forensic”. He, too, emphasises it should not be taken as an excuse to demonise 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 problem sharks are real. Plus, while some sharks might learn to bite humans, she questions whether they would continue to do so long term. 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 is no data to support the idea that even the boldest sharks benefit from biting people.Plus, Clua’s plan – to capture problem sharks and bring them to justice – is unrealistic, says David Shiffman, a marine conservation biologist based in Washington DC. Even if scientists can prove beyond doubt that a few specific sharks are responsible for a string of incidents – “which I do not believe he has done”, Shiffman adds – he thinks finding those sharks is not viable.Any resources used to track down problem sharks would be better spent on preventive measures such as lifeguards, who could spot sharks approaching a busy beach, says Catherine Macdonald, a conservation biologist at the University of Miami in Florida.While identifying and removing a problem shark is better than culling large numbers, 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.”Identifying and removing a problem shark is often regarded as better than culling large numbers. Photograph: Humane Society International/AAPClua, for his part, intends to carry on. He’s working with colleagues on Saint-Martin to swab shark-bite injuries when they occur, and to track down potential problem sharks.Asked whether he has ever experienced a dangerous encounter with a large shark himself, Clua says that in 58 years of diving it has happened only once, while spear fishing off New Caledonia. Poised underwater, waiting for a fish to appear, he turned his head. “There was a bull shark coming [toward] my back,” he says.He got the feeling at that moment that he was about to become prey. But there was no violence. Clua looked at the bull shark as it turned and swam away.This story was originally published in bioGraphic, an independent magazine about nature and regeneration from the California Academy of Sciences.
Some experts think a few sharks may be responsible for a disproportionate number of attacks. Should they be hunted down?First was the French tourist, killed while swimming off Saint-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 gaping wound where part of her right thigh should have been. Then, a month later, another victim. Several Caribbean islands away, a woman snorkelling 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, 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.’” Continue reading...
First was the French tourist, killed while swimming off Saint-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 gaping wound where part of her right thigh should have been. Then, a month later, another victim. Several Caribbean islands away, a woman snorkelling 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, 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 or happenstance, Clua says. Instead, he says there are such things as problem sharks: bold individuals that may have learned, perhaps while still young, that humans are 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 after human-shark conflict are becoming less common and are generally regarded by scientists as ineffective, they do still happen. One of the last big culls took place near Réunion, a French island in the Indian Ocean, between 2011 and 2013, resulting in the deaths of more than 500 sharks. Even that was not enough for some – four years later, a professional surfer called for daily shark culls near the island.
And so, in the immediate aftermath of the French tourist’s death in Saint-Martin, when one of Clua’s contacts called to explain what had happened, he recalls telling them: “Just go there on the beach … I want swabbing of the wounds.”
After that bite and the one that occurred a month later, medical professionals collected samples of mucus that the shark had left behind to send 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, the DNA analysis confirmed that the same tiger shark was responsible for both incidents.
Even before the DNA test was complete, however, analysis of the teeth marks left on the Saint-Martin victim, and of the tooth fragment collected from her leg, suggested the perpetrator was a tiger shark (Galeocerdo cuvier) roughly 3 metres (10ft) long. Armed with this knowledge, Clua and his colleagues set out to catch the killer.
During January and February 2021, Clua and his team hauled 24 tiger sharks from the water off Saint-Martin and analysed 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 matched the DNA swabbed from the wounds suffered by the two women.
But the team has not given up. Clua is now waiting for DNA analysis of mucus samples recovered from a third shark bite that happened off Saint-Martin in May 2024. If that matches samples 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, is not entirely sure why. The concept of problem animals is well established on land, she says. Terrestrial land managers routinely contend with problem lions, tigers and bears. “Why not a fish?” asks Chapuis. “We know that fishes, including sharks, have amazing cognitive abilities.”
Yet having gleaned a range of opinions on Clua’s ideas, some marine scientists rejected the concept of problem sharks outright.
Clua is aware that his approach is divisive: “I have many colleagues – experts – that are against the work I’m doing.”
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. Merely talking about problem sharks, they say, could perpetuate the idea that some sharks are hungry, human-eating monsters such as 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 it might prey on. Environmental factors, as well as personality, might trigger or aggravate such behaviour.
Besides the tiger shark that struck off Saint-Martin and St Kitts and Nevis, Clua’s 2024 study 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 behaviour must be rare. “They’re just opportunistic. I mean, these things eat tyres.”
Diego Biston Vaz, curator of fishes at the Natural History Museum in London, also praises Clua’s work, calling it “really forensic”. He, too, emphasises it should not be taken as an excuse to demonise 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 problem sharks are real. Plus, while some sharks might learn to bite humans, she questions whether they would continue to do so long term. 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 is no data to support the idea that even the boldest sharks benefit from biting people.
Plus, Clua’s plan – to capture problem sharks and bring them to justice – is unrealistic, says David Shiffman, a marine conservation biologist based in Washington DC. Even if scientists can prove beyond doubt that a few specific sharks are responsible for a string of incidents – “which I do not believe he has done”, Shiffman adds – he thinks finding those sharks is not viable.
Any resources used to track down problem sharks would be better spent on preventive measures such as lifeguards, who could spot sharks approaching a busy beach, says Catherine Macdonald, a conservation biologist at the University of Miami in Florida.
While identifying and removing a problem shark is better than culling large numbers, 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. He’s working with colleagues on Saint-Martin to swab shark-bite injuries when they occur, and to track down potential problem sharks.
Asked whether he has ever experienced a dangerous encounter with a large shark himself, Clua says that in 58 years of diving it has happened only once, while spear fishing off New Caledonia. Poised underwater, waiting for a fish to appear, he turned his head. “There was a bull shark coming [toward] my back,” he says.
He got the feeling at that moment that he was about to become prey. But there was no violence. Clua looked at the bull shark as it turned and swam away.
This story was originally published in bioGraphic, an independent magazine about nature and regeneration from the California Academy of Sciences.