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Why sharks are joining kids in Florida for spring break

Thousands of sharks, heading north after migrating to the south for winter, prompted beach closures along South Florida's Atlantic coastline. NBC's Brian Williams reports.

By Jennifer Viegas

The thousands of sharks that emptied Florida beaches this week are later than usual, but their purpose sounds like the usual spring break itinerary: gulping down food, having sex and seeking warmth, according to shark experts.

The dramatic aggregation consists primarily of blacktip and spinner sharks, with hammerhead, bull, lemon and tiger sharks also in the mix, according to Derek Burkholder of the Guy Harvey Research Institute and Nova Southeastern University.

"During this migration, tens of thousands of sharks are moving up the coast to waters further north where they will spend the summer months before heading back down here for the winter," Burkholder told Discovery News. "The sharks can be found very close to shore -- within a few feet -- as they follow baitfish."

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George Burgess, director of the Florida Program for Shark Research based at the Florida Museum of Natural History, likens the mass grouping to people lined up to get into a football stadium. He said that sharks are actually backed up, trying to get in the warm water stream.

"Nookie is going on too," he said. "Sharks tend to mate during this time."

Both he and Burkholder remarked that the migration is happening later this year.

"Sharks are temperature dependent. The ups and downs of global climate change can affect water temps," Burgess explained.

Humans, he said, are no different, since we can tolerate some climates more so than others.

"That's why a bunch of people live in Florida, but not at the North Pole," he deadpanned.

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A "swarm" of blacktips sharks, and others, has filled the waters off of Florida for spring break.

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The later migration coincided perfectly with this year’s spring break, when numerous college students head to beach towns to relax and party. Florida is the most dangerous state in the United States, in terms of shark attacks, and such encounters are at a 12-year high now.

The sharks in the swarm are known to bite people, but usually these are “hit and run” attacks, Burgess said. Researchers actually document these attacks to track the movement of the big shark congregation as it moves northward.

The good news is that "humans are not on the sharks' normal menu," Burkholder said, adding that it's "almost always a case of mistaken identity, especially in areas where the water is murky or near dusk or dawn, where the shark bites a swimmer thinking they are their normal fish prey."

"This is also the reason that most bites from these species are not fatal attacks," he added. "When the shark realizes its mistake, it lets go and does not continue the attack."

He and Mahmood Shivji, director of both the Guy Harvey Research Institute and Save Our Seas Shark Center USA, believe the swarm is misleading, though, since it can give the false impression that shark populations are booming.

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"Just because we now see swarms of sharks does not mean there are more sharks," Shivji, who is also a professor at the Oceanographic Center, explained. "The reason we are now aware of them is because people are increasingly taking aerial pictures, which was not done until a few years ago. With all the sharks being overfished, it could very well be if such pictures were routinely taken 15 years ago, we might have seen swarms 2-3 times as big."

Nevertheless, swimmers and other recreational water users would do well to take heed. Burgess said the migration would be heading to places like the Chesapeake Bay, waters off of North Carolina, and parts of New Jersey. Some sharks might follow waterways from East to West, such as going deeper into the Gulf of Mexico.

"If you've been in such waters before, you have probably been 10 to 15 away from a shark and just didn't know it," Burgess said.