DEERFIELD BEACH, Fla. -

Lifeguards spotted a school of sharks about 20 yards off the shore of Deerfield Beach.

WATCH: Unedited video of Deerfield Beach sharks

A one-mile stretch of the beach was closed about noon Thursday.

"We had a group of them on our south end of the beach that stayed in the area for a good 45 minutes, very close to shore, right off the sand bar," said Deerfield Beach Ocean Rescue Capt. Mike Brown. "On Monday, they were almost on the sand."

A city of Deerfield Beach spokesperson said the school of sharks was seen near tower 1.

George Burgess, the director of the Florida Program for Shark Research, identified them as blacktip sharks. He said they are on their annual migration north for the summer.

Burgess said the species is one of South Florida's most common.

Lifeguards said they also saw spinner sharks.

"Any shark can be a potential danger because anytime they attack humans, they mistake it for trouble," said Brown. "If someone is out in the water splashing, drawing attention to themselves, they think it's an easy meal. Something in distress and the same goes for surfers."

A well-known and sometimes controversial boat captain and shark fisherman known as "Mark the Shark" said this is just nature at work.

"These sharks in Deerfield Beach -- no problem," he said. "They're just here to look for bait, schools of mullet and other fish they can eat on the shoreline."

--"Mark the Shark"