Shark sightings closed beaches in Florida as the sharks' winter migration interferes with the state's prime beach season, officials said. Tens of thousands of sharks, mostly blacktip and spinner sharks, are approaching the shore in their annual migration from Florida to North Carolina, ABC News said Thursday. Shark sightings are not uncommon in South Florida, but this year's migration comes later in the winter than usual, and many beaches are on high alert, ABC said. Researchers at Florida Atlantic University reported they have counted about 15,000 sharks, most of them less than 200 yards from shore. Lifeguards closed three public beaches in Palm Beach, Fla., County Wednesday after dozens of sharks were spotted nearby, the Palm Beach, Fla., Post reported. "They've been here every day since December," said Ocean Reef Park lifeguard Eddie Green. "They just like our beach for some reason."