Many South Florida beaches were temporarily closed on Thursday when tens of thousands of sharks were seen swarming near the shores.

It's not unusual to spot sharks in Florida's waters, but the stunning event comes during the height of Spring Break, when more people than usual, including college students, flock to the state's sandy beaches.

Fortunately this is just a once-a-year occurrence, when the sharks migrate north, triggered by a change in the water's temperature.

The annual migration typically begins much earlier, in January or February, before peak beach season, shark researcher Steve Kajirua told TCPalm.com.

Florida researchers say about 15,000 sharks were seen less than 600 feet from shore, ABC News reports.

It's not just one species either. Researcher Derek Burkholder tells Discovery News that the cluster "consists primarily of blacktip and spinner sharks, with hammerhead, bull, lemon and tiger sharks also in the mix."

Both blacktip and spinner sharks are typically less than 10 feet in length and are not considered dangerous to humans. Blacktips are responsible for about 16 percent of unprovoked attacks in Florida and spinners have been responsible for 13 unprovoked attacks worldwide, according to the International Shark Attack Profile.

Sharks don't actually like the taste of human flesh, so they aren't built to attack people, but they can bite which is why beaches were shut down as a precaution.

The timing of the migration coincides with a new study that found 100 million sharks are killed each year, mostly due to overfishing. So we shouldn't get the idea that the shark population is suddenly booming — we've just gotten better at taking aerial pictures, Mahmood Shivji, director of Save Our Seas Shark Center USA, tells Discovery.

The AP captured incredible video of the migration. Watch below: