TV’s celebration this week of everything sharky comes during a summer with a record-high number of shark bites to swimmers in North Carolina. But that does not mean the risk of getting bitten by a shark is getting worse in North Carolina or anywhere elsein the world. In fact, a new study suggests the risk might be dropping globally, just as it has off the California coast.
Visitors there are now 91 percent less likely to be bitten by a great white than they were in 1950, researchers report in a paper set to be published in Frontiers in Ecology and the Environment in the next few weeks. That is despite the fact that actual shark bites have increased, from 0.9 per year in the 1950s to 1.5 per year in the decade from 2004 to 2013.
How does that work? There are now a lot more people in the water. "Even if the number of attacks has increased since the 1950s, the number of people engaging in ocean activities has increased much faster over the same period of time, resulting in a reduction of the individual probability of suffering an attack," says Francesco Ferretti, a researcher at Stanford University Hopkins Marine Station in California and lead author of the study.
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And that's the same pattern playing out in North Carolina. "The population has been going up and the number of people going in the water is always increasing," says Chuck Bangley, a shark researcher at East Carolina University. He says North Carolina is setting near-record numbers for people at the beach this year, in part due to a heat wave that coincided with schools letting out for the summer. According to Visit North Carolina, the state tourism office, more than 6.5 million people visited the state’s coast in 2014, an 18 percent increase in visitation since 2010.
The risk of any shark bite is already incredibly low—far less likely than drowning or many other rare risks, Bangley says. But, “the more people you have going into the water, the better the odds are that something bad is going to happen, whether it's a shark bite or getting pulled under on a riptide.”
Although Ferretti's paper focuses on California, the basic conclusions almost certainly apply throughout the world, given the increase in world population and the even greater increase in commercial and recreational use of the ocean, he says. In their analysis he and his co-authors focused on white sharks because the damage inflicted by their bites is nearly always significant enough to require medical attention, making it more likely that the reporting of bites remained constant from 1950 to 2013.
Meanwhile a 2013 study found, based on global catches and shark mortality, that humans kill an average of 100 million sharks annually across the world.
The researchers used data from the Global Shark Attack File (GSAF), a large, comprehensive active surveillance organization for shark bites across the world. The GSAF, dating back to the 1930s, relies on an interdisciplinary team of on-site researchers, including historians, shark researchers, archaeologists, ER doctors, plastic surgeons and other types of researchers, to gather information on local incidents, investigate them to be sure the incident involved a shark and analyze the factors and species involved.
The scientists also assumed that the likelihood a shark bit a person it encountered remained constant, so it was a matter of analyzing the abundance of humans, the estimated number of sharks and the spatial overlap of both. Ferretti’s team used annual California surfing events and other published data, along with the number of annual diving days of certified scuba divers, to estimate the change in surfers and divers over the years. They used published data about beach tourism and population increases in California coastal communities to extrapolate an estimate of beach visitors and swimmers over the past six decades.
Much research has documented the decline of shark populations across the world, although it's unclear whether that's the case for white sharks in California. But the number of scuba divers, surfers and beachgoers has exploded there over the past six decades.
Ferretti and his team estimate surfers increased by a factor of 125, from 7,000 in 1950 to 872,000 in 2013. Scuba divers increased more than 200-fold, and the number of overall beachgoers tripled, from 53 million in the 1950s to 165 million today. Given the far more modest increase in shark bites over that time, the authors went on to calculate the actual risk of an attack. The rate dropped 2.4 percent per year, translating to a 91.2 percent drop over the full time period.
The risk varies by activities. For swimmers, the risk of a bite in 2013 was one per 738 million beach visits, a drop of 81.5 percent since 1950. For scuba divers, the risk was one in 1.44 million in 2013, a drop of 99.7 percent since 1962. Although the risk for surfers didn’t drop, their likelihood of a shark bite has remained steady at one in 17 million.
The mechanism behind the reduction in shark attacks remains unclear. It’s likely relevant that many species have declined between 50 percent and more than 90 percent over the past several decades worldwide, primarily due to overfishing. In California the recovery of elephant seal colonies that sharks feast on may play a role. (The seals are thought to draw predators away from humans.)
Any focus on shark bites that result in human injury or death should also be viewed in the context of humans’ massive impact on sharks worldwide, largely driven by consumer demand for shark fins. "If you put it in perspective, we're killing 10 million sharks for every one that kills us," says George Burgess, director of the Florida Program for Shark Research and curator of the International Shark Attack File.
Still, Burgess and Bangley both acknowledged that North Carolina's eight attacks in two months is exceptional given the previous record of five in 2001, the same year as the last fatal shark bite. The spike likely occurred because of a confluence of factors. The same heat wave that brought more humans into the water may also have condensed the normal seasonal migration of sea life that passes through North Carolina waters each summer, Bangley says. For example, he explains, the annual migration of menhaden fish, a favorite shark food, appears linked to water temperature, which jumped 10 degrees in a week during the heat wave.
Understanding the factors influencing shark behavior can help humans adjust our behavior to avoid bites, Burgess says. "We're the animals with the brains, they're the ones with the teeth, and we're in their house," he says. "So it's incumbent upon us to adjust our behavioral patterns and not to expect the animals, be they sharks, jellyfish or whatever, to adjust theirs."
A smart practice for humans in the sea includes keeping a wide berth from seal and sea lion colonies and from fishing piers, which tend to attract populations of sharks hanging around for scraps. Avoiding schools of fish can also reduce the risk of a shark bite, especially at times that fish dart quickly or sea birds dive at the surface. "If you see predator–prey interactions in front of you, get out of the water," Bangley says. "When you're in the ocean, that's where the shark is supposed to be. It's a wilderness experience just like walking in the woods where you might encounter a bear."
An attitude of awareness is certainly ecologically wiser than shark culls—such as a $22-million elimination endeavor implemented in 2014 in Western Australia—which end up being destructive for both sharks and humans, Ferretti says. For example, a century-old fishery for bay scallops in North Carolina collapsed when rapid declines in sharks no longer kept a local population of cownose rays in check, thereby leading to an increase in rays that fed on the scallops. No evidence exists to demonstrate that culls actually reduce attacks, Bangley says. But changing human behavior does: Ferretti’s paper found that surfing in Mendocino County in March instead of October and November reduces the risk of an attack 24-fold. And surfers’ risk of a shark bite drops more than 1,500 times if they surf between San Diego and Los Angeles instead of the coast off Mendocino County.In addition, most bites are cases of mistaken identity: Surfers look similar to the pinnipeds that great whites primarily feed on. In North Carolina bull and blacktip sharks are most likely mistaking an errant foot or hand for the foot-long bait fish they usually go after.
More studies conducted elsewhere in the world would help researchers identify patterns of shark bites, learn the preference and behavior of local shark populations and use that knowledge to tweak human behavior. "Our results indicate that investing in increasing and communicating our understanding of the behavior, distribution and ecological role of sharks as well as the factors influencing the risk of shark bites, may ultimately be the most effective way to increase safety of people," Ferretti says. If people learn to avoid being near shark food during feeding times, we become far less likely to end up an as accidental appetizer.