Thousands of sharks near shore of Florida's spring break beaches

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.