Thousands of sharks near shore are part of an annual migration of blacktip sharks moving into feeding grounds near Palm Beach, Fla. 
 
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 Hunter Mason (l.) and Olivia Murat, on spring break from the University of Indianapolis, relax on the beach Tuesday in Destin, Fla. Thousands of blacktip sharks are brushing past Florida's spring break beaches, raising alarm among swimmers and lifeguards. 
 
 Thousands of four- to five-foot blacktip sharks are brushing past America's premier spring break beaches, raising alarm among swimmers and lifeguards but also drawing new attention to a stunning natural phenomena. 
 
 An annual migration of a distinct population of sharks begins off the tip of Florida and moves into an area off Palm Beach in February and March, where the sharks congregate close to shore as the offshore shelf narrows – a prime spot for the small baitfish that the sharks feed on. 
 
 The sharks are drawing attention for their natural showmanship, as well. They're often seen making spinning jumps out of the water. And thanks to the clear water and white sands of south Florida, they're easy to spot and view, especially for lifeguards from their elevated chairs. 
 
 During the annual migration, "they hang onto the coast and follow the baitfish, which makes them super-easy to see, especially when they get brought together in a tight group," says Shari Tellman, a researcher at Florida Atlantic University's Elasmobranch Research Laboratory, or "the shark lab." 
 
 Ms. Tellman and other researchers have been tracking the blacktip shark migration by Cessna airplane since 2011, trying to understand how climate change – specifically warmer average water temperatures – may affect the migration and the dynamics of the ecosystem. 
 
 "We're looking to see whether [climate change] might shift their migratory pattern north and whether we could lose them as apex predators [in this area], and also whether regulations as far as shark fisheries are concerned are actually working." 
 
 A report from the journal Marine Policy estimates that humans may be killing more than 100 million sharks a year. Tellman says the migration appears to contain fewer animals this year, though the cause of that is unknown. 
 
 Blacktips account for 20 percent of unprovoked shark attacks in Florida, and beachgoers are taking precautions. But blacktips are seen as being less of a threat to swimmers and surfers than tiger sharks, which sometimes prey on the blacktips. 
 
 Florida had the largest number of unprovoked shark attacks in 2012, a total of 25. None were fatal. ||||| A pod of spinner sharks is seen through a wave Tuesday morning at Midtown Beach. 
 
 By Alexandra Seltzer 
 
 Palm Beach Post Staff Writer 
 
 Two area beaches were temporarily closed today because of sharks spotted near the shore. 
 
 The waters off the Boynton Inlet were closed to swimmers for about two hours this afternoon and the South Inlet in Boca Raton was closed for about an hour, Palm Beach County Ocean Rescue Lt. Brian McManus said.
– Thousands of visitors are swarming to beaches in South Florida, but not the kind welcomed by the chamber of commerce: sharks. Their annual migration up the East Coast is taking place later than usual this year, coinciding with the start of spring break season, reports ABC News. Several beaches had to close earlier this week, reports the Palm Beach Post, though all were back open as of today. Researchers at Florida Atlantic University have counted 15,000 sharks so far, most within 200 yards of shore. The most common are blacktip and spinner sharks, about 4 to 5 feet long. But it's not all gloom and doom: "The sharks are drawing attention for their natural showmanship, as well," writes Patrik Jonsson at the Christian Science Monitor. "They're often seen making spinning jumps out of the water. And thanks to the clear water and white sands of south Florida, they're easy to spot and view, especially for lifeguards from their elevated chairs."