Spring Break Shark Swarm: Thousands of Sharks Shut Down Florida Beaches Sharks migrating up the east coast has spring breakers out of the water.

March 7, 2013  -- A huge swarm of sharks that shut down beaches in Florida is migrating up the East Coast in a display that, while stunning, has spring breakers staying out of the water.

Tens of thousands of the predators -- mostly blacktip and spinner sharks -- are now coming to shore, and towards swimmers, during their annual migration north.

"We saw something moving in the water and everybody was saying, 'ahh! sharks!,'" one witness in Palm Beach, Fla. told ABC News.

Craig Pollock, a lifeguard supervisor in Palm Beach, said that sharks for the most part don't disturb the area beaches.

"We don't have a sandbar. A lot of times when we have a sandbar the sharks stay off of the shore a little further," he said.

Shark sightings are not uncommon for South Florida beaches.

"Every year we expect annual shark migration to come through this area," Pollock said.

But the migration from Florida to North Carolina usually starts and ends sooner -- well before Florida's prime beach season. But that's not the case this year.

Researchers at Florida Atlantic University say they have now counted some 15,000 sharks. Most of them were seen less than 200 yards from shore.

"It's the beauty of living in Florida," beachgoer Laura Salerno in Palm Beach said. "It's also the danger."

As a precaution, many beaches are on high alert today, with double red flags waving to keep swimmers out of the water, at least for now.

"People really need to heed these warnings because thank god it's a public beach, and they have lifeguards and they have these warnings," beachgoer Elizabeth Horowitz said. "Sharks are not to be reckoned with."

Blacktip sharks only account for 20 percent of unprovoked attacks in Florida. But during this migration, people there aren't taking any chances.