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Increasingly warm waters around the globe due to climate change and El Niño led to the highest number of shark attacks ever recorded in 2015, according to data released this week.
The University of Florida found that there were 98 unprovoked shark attack incidents last year, surpassing the previous record of 88 in the year 2000. The numbers follow an upward trend of attacks that scientists at the the university's International Shark Attack File, which collects and analyzes data on shark attacks, say is due to greater interaction between humans and sharks in the water.
That interaction has increased as air and water temperatures have increased, sending humans into the ocean for more months out of the year in more places, and sending sharks into waters that were previously too cold for them, according to George H. Burgess, curator of ISAF.
"Each year, in theory, we should have more shark attacks than ever because we have more people on earth than ever before," Burgess said. "The equation for shark attacks is one shark plus one human, you have to get them together, so it's an odds game."
Burgess said that the number of attacks could be expected continue to increase steadily as long as the world's population continues to increase, sending more humans into the sea. Shark populations, meanwhile, are either declining or remaining steady in many parts of the world, he said, due to overfishing, which should theoretically help reduce the number of attacks. But complicating factors have prevented this.
The two biggest factors in 2015 were global warming and El Niño, he said, both of which have warmed previously-cool waters enough to make them hospitable to sharks likely to bite.
"Warmer temperatures also affect humans, in the sense that we also have a water temperature preference, so we are more likely to go in the water when the temperature is one we're happy with," he said. "So more people are going into water in more northerly latitudes in the northern hemisphere, and more southerly latitudes in the southern hemisphere, so there's more opportunity for sharks and humans to get together."
Sharks that like tropical water, like bull sharks and tiger sharks, have moved beyond the traditional waters of Florida and up the southeastern coast of the United States to the Carolinas, for instance. There, an increase in population has led to more people going in the water from spring to fall, as air temperatures were also warmer in 2015. The combination leads to more potential encounters between sharks and humans and, thus, more attacks, Burgess said.
Shark attacks tend to cluster in places with large populations of both sharks and people, including tourist destinations, he said.
In more temperate waters, like those off the coast of California and New England, white sharks have been more likely to encounter swimmers.
"We can safely say it won't be highest we'll ever see because as long as human populations continue to rise and climate change continues and periodic events like El Niño happen, that number will go up," Burgess said.
El Niño is expected to end during the spring or summer of 2016, and La Nina, which Burgess described as El Niño's "opposite," is expected to bring cooler water temperatures toward the end of the year.
The United States had the highest number of attacks, with 59 last year, as North America continued its long-term position atop the list of places with the most attacks. But there were no fatalities in North American waters during the year, and only one fatality in the US, in Hawaii.
Florida still leads the nation in terms of the number of attacks each year, with 30 in 2015. The ISAF attributes the high concentration of attacks in Florida's Brevard and Volusia Counties to having two of the most popular beaches in the state, with a huge influx of tourists and surfers using the water each year. North and South Carolina had the next highest numbers of bites, with eight each last year, followed by Hawaii, with seven bites.
Australia and South Africa, which have coastlines similar to the United States in that they range from tropical to temperate and draw large numbers of people to them, also saw high numbers of attacks.
ISAF also noted in its data that its methods for documenting and analyzing attacks has grown more efficient since it bgan tracking the data, which could also lead to the increasing numbers of recorded attacks.
Still, while the number of attacks has increased each year, the likelihood of being killed by a shark is "infinitesimally small," Burgess said. Most shark bites are accidents caused by sharks' poor visibility near the beach and humans' poor swimming abilities. When sharks do take a bite, they often realize their mistake, let go, and leave, Burgess said. Scientists call those mistakes "hit and run attacks," he said.
"A lot of people on the East Coast go in the water in the surf zone, where there's murky water and a lot of physical activity, tidal currents, breaking waves, and so forth. Under those conditions, animals that are making a living in that area have to make quick decisions when going after prey. They very much focus in on movement and splashing and things like that, and when we're in the water we're as about ungainly as anything can be in the sea," he said.
Surfers are still the most likely ocean-goers to receive bites, making up nearly 50 percent of all attacks in 2015. Swimmers, waders, and snorkelers are the next highest-risk groups. There were no attacks on scuba divers in 2015.
"Surfers obviously spend a lot of time in the water, much more than the average bather, and they also go out at times that are not necessarily the best times to be in the water, like dawn and dusk, before and after work," Burgess said. "That's the time period when sharks are most active in feeding."
He called surfing a "provocative" activity because of the amount of splashing and movement caused by feet, hands, and wipeouts.
Burgess noted that both surfers and swimmers are more likely to face other water dangers, including drowning or injury, than they are to face a shark attack. And sharks are more likely to be attacked by humans, he noted. Though there are an average of six deaths per year from shark attacks, there are an average of 100 million sharks killed per year by humans, sometimes accidentally, by fishermen trawling for other large fish, and sometimes on purpose, to sell at a high-price for delicacies like shark fin soup.
ISAF released official recommendations for how to handle a shark attack with its annual report. If attacked by a shark, it notes, use an inanimate object to hit the shark in the nose to temporarily stop the attack, and then flee. If it's not possible to flee, claw at the shark's eyes or gills in hopes of stopping it, the report said.
Follow Colleen Curry on Twitter: @currycolleen