Flordia: Florida's ever-popular Spring Break resorts have attracted some extra visitors to the beach this year - thousands of blacktip and spinner sharks on their annual migration north.

Officials in Broward and Palm Beach counties on Florida's south-east coast have closed several beaches to swimmers as a precaution, although marine biologists say that there is little risk to humans.

"The sharks are not predatory, at least as far as humans are concerned, and in clear water are far more likely to just swim away," Shari Tellman, of Florida Atlantic University's Elasmobranch Research Laboratory, told the Guardian.

"The only real issue would be if the water's murky and there's case of mistaken identity, or a shark just bumps into you, because they have very rough skin. When they are together in a big group there's always the possibility of larger sharks being nearby, but there's only a small chance of any problem."

Tellman's team of researchers have been tracked the migrations from the shore and the air for two years and say the sharks will be gone from the Florida coast by next month to spend the summer months in the warm Atlantic waters off Georgia and the Carolinas.

She likened them to Florida's famous human 'snowbirds', visitors from the colder northern states and Canada who spend the winter enjoying the more tropical climes in the south.

"They come down in the late fall when it starts getting colder, they're very visible here during the winter hanging around and enjoying the warmer temperatures, and in the spring they turn around and start heading back north again," she said.

"It's their annual migration pattern. It looks spectacular but it's part of their normal behaviour."

One research team tracked a pod of 4-5,000 of the sharks a close to the shoreline by the Jupiter Inlet on Wednesday.

Lifeguards say the beaches will reopen as soon as most of the sharks have moved on.