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Veterinarian-turned-shark-expert Eric Clua knows how rare it is for a shark to attack a human. While dogs kill some 30,000 people annually, only 100 shark attacks are documented worldwide each year, and fewer than 15 percent of these are fatal. Still, he wants to understand why sharks attack when they do.
Scientists have long believed that sharks bite humans because sharks encounter humans in their ocean homes. They mistake surfers and swimmers for prey like seals or turtles, the theory goes. In some places, such as La Réunion island in the Indian Ocean, this perspective has encouraged the practice of mass culling of sharks to prevent attacks.
But when Clua first began documenting shark attacks, he began to question this assumption. In 2009, he spoke with a young surfer in the French overseas territory of New Caledonia who had watched his friend get fatally attacked by a great white shark. “He told me, ‘it was like the shark was playing with my friend. He was moving around my friend and biting him,’” says Clua. This didn’t sound like a random encounter by a confused shark. Rather, it seemed this particular shark was testing whether a human could make an easy meal.
Clua wondered: What if individual sharks weren’t all the same? What if only a select few individuals within a population—the boldest and most daring—were willing to experiment with unfamiliar prey, occasionally leading them to bite humans? After all, among some terrestrial predators such as tigers and leopards, a select few “problem individuals” are thought to be disproportionately responsible for attacks on humans. “I am convinced that instead of getting mistaken, we are just in the presence of bold animals that are exploring and testing new prey,” says Clua, who is based in French Polynesia, at the Center for Insular Research and Observatory of the Environment, a French research unit.
“It was like the shark was playing with my friend.”
Some scientists have dismissed the notion of “problem sharks” as unlikely, a remnant of the kind of pop-cultural demonizing of the animals that began with films like Jaws from the 1970s. Because shark attacks are so rare, and others are rarely there to document the particulars, it’s difficult to prove that an individual shark is responsible for repeat attacks. But in a new study in Conservation Letters, Clua and his colleagues present the very first evidence for his theory, by documenting three sharks that have been responsible for repeat attacks. The findings shine a new light on shark personality and suggest indiscriminate culling may not be an effective method of reducing shark attacks.
Clua’s data were sourced through a combination of luck and determination. Medical staff were able to swab the wound of a female swimmer who was fatally wounded by a tiger shark bite in 2022 off of St. Martin, an island in the Caribbean. They did the same for a snorkeler who later narrowly survived an attack more than 50 miles away. The DNA testing results seemed to match: Both were victims of the same predator.
A second problematic tiger shark, likely responsible for two bites that injured divers near Costa Rica’s Cocos Island, was identified by meticulously interviewing boat captains and divers and analyzing photos and videos. All descriptions pointed to a female tiger shark dubbed “Lagertha,” who has a distinctive white spot on her fin and had long shown behavior toward humans that could be described as inquisitive. And in Egypt’s Red Sea, the third shark—an oceanic whitetip shark thought to be involved in several attacks—was identified by examining several years’ worth of photos taken by local divers. Three sharks is a pretty small sample size from which to draw firm conclusions, but it’s a start.
Clua believes that problem sharks are not “rogue” predators who develop a taste for human flesh or enjoy attacking people, bloodthirstily and deliberately hunting humans. Rather, they are just naturally bold individuals with an inclination to take risks. In contrast to risk-averse sharks that shy away from humans, these daring individuals are willing to try novel prey alongside their regular diet, a behavior that could have evolved as a strategy to cope with fluctuations in natural prey populations.
“I am convinced that bold animals are exploring and testing new prey.”
Still, even a bold shark must overcome its fear of people; some might abort an attack on a human if the water is too clear to provide sufficient camouflage, while others may hesitate if there are multiple people in the water. But, if bold sharks successfully gain a meal from killing a human, that behavior may be reinforced, leading to repeat attacks. Clua reckons many attacks are perpetrated by “bold” individuals, estimating that repeat offenders may be responsible for up to half of all attacks.
Marine behavioral ecologist Johann Mourier of the University of Montpellier in France, finds Clua’s data convincing in demonstrating that problem sharks exist, noting that the idea is in line with a growing body of evidence that sharks have different personalities. However, the number of attacks these problem sharks are responsible for remains a mystery. “I am convinced that some shark incidents are due to specific individuals, but some may just be due to random encounters between sharks and humans,” Mourier writes in an email.
For Clua, problem shark behavior suggests a new strategy for preventing attacks. He and his colleagues call for efforts to systematically fish sharks, without killing them, to collect DNA and attach tags or notch their dorsal fins for easy identification. Then, if a shark bites a person, DNA can be swabbed from the victim’s wound to identify the culprit, which can be selectively targeted and killed.
Mourier isn’t convinced this solution could be widely implemented. For one, “you need at least a few hours and even days to get the results of the DNA extraction and identification, so during that time the shark is likely to be far away and the chance to find it again is very low in most cases,” he writes.
Whether or not his approach can work, Clua hopes he can help reduce fear of sharks. “They are not instinctive and dumb animals. They are clever animals,” Clua says. “They are very prudent, and we are lucky of that.” 
*An earlier version of this article misspelled Johann Mourier’s first name. The error has been corrected.
Lead image: karelnoppe / Shutterstock
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