Town of Hempstead lifeguards spotted the 6-foot shark about 25 yards off Lido West Beach just before 11 a.m.
"There was fin in the area, the shark looked about 6 to 7 feet, it was circling, lifeguard Frank Falcone said.
The shark was apparently circling in the sandbar r, only about 25 feet from the shore.
Falcone was one of five lifeguards on the beach who spotted the shark during a routine training exercise.
As a result, swimmers were pulled out of the water along a four-mile stretch of shoreline including Lido, Point Lookout, and Nickerson Beach.
They were allowed back in later that afternoon, but only into waist-or-knee-high water.
Extra lifeguard patrols were out on jet skis and surfboards.
"It's the hottest day of the year and now we have sharks down here in the waterways, not ideal circumstances for anybody, but we want residents to know our lifeguards are trained in the identification of the sharks. We have all of the precautions necessary," Town of Hempstead Supervisor Don Clavin said.
This is the 20th confirmed shark sighting at Nassau beaches so far this summer, more than triple what was recorded by this time last year.
"They've had positive identifications of sharks at Jones Beach, at Long Beach and yesterday, there were four positive identifications of sharks in the Rockaways," Clavin said.
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