Beaches situated along South Florida have again re-opened to the swimming public after they were forced to close up earlier this week due to a 15,000-strong shark invasion.

As of 9 a.m. ET, at least three beaches in Palm Beach County have started accommodating again swimmers on Thursday after declaring the coast is now clear from the sharks that had been spotted swimming near shore, according to the Palm Beach Post.

Beach patrons reported seeing thousands of sharks earlier in the week around the beaches of South Inlet in Boca Raton, Gulfstream Park close to Boynton Beach as well as Ocean Reef in Singer Island.

Although shark invasions are not uncommon this time of year off South Florida, the county decided to still close the beaches to prevent possible loss of lives to the swimmers.

The months of January and February are migration months for most sharks in the area. Every winter, they go to the area to congregate and feed, according to Steve Kajiura, a shark researcher for Florida Atlantic University.

"Our data has shown that the bulk of the migration occurs in January and February," Mr Kajiura told TCPalm.com. "But it may be a little behind this year due to the warmer weather and water temperatures."

This year's shark invasion counted 15,000, of which most were less than 200 yards from shore.

The predator invaders were identified as the blacktip sharks (Carcharhinus limbatus) and spinner sharks (Carcharhinus brevipinna).

But authorities warned that intermittent beach closures are still possible.