Several beaches in Palm Beach and along the coast of Florida are closed after beachgoers reported sightings of thousands of sharks only a few feet from the shoreline.

The swarm, identified as blacktips and spinner sharks, are moving north up the East Coast as part of the annual migration. While blacktip sharks are responsible for most shark bites on Florida beaches, there has never been a recorded fatality due to a blacktip shark attack. Blacktips are carnivores who grow to a length of about eight feet and weigh between 70 and 220 pounds.

Shark sightings are a common occurrence at Florida beaches, but swimmers are warned to stay no more than 300 feet away from the shore. Researchers at Florida Atlantic University told ABC News that northern migration of predator sharks generally starts earlier in the year and is complete by the start of the beach season. Those same researchers have counted more than 15,000 sharks during the migration. According to National Geographic, blacktip sharks migrate before summer to the Cape Cod, Massachusetts area while some choose stay put in warmer waters off the southern tip of Florida year-round.

While many beaches are on high alert, several beaches further south have reopened after the closing earlier in the week. The Palm Beach Post reports that despite low temperatures that will likely keep beachgoers away, several Palm Beach County beaches might reopen if the swarm has moved further north.