A shark sighting offshore closed South Beach to swimmers Tuesday afternoon. The water was opened again for swimming a few hours later.

In an email Edgartown town administrator James Hagerty said a fin was sighted about 20 yards offshore. (An early report that said the shark was sighted 60 yards offshore was later changed.)

Park administrator Jessica McGroarty said the shark was reported to the communications center at about noon. The communications center then notified South Beach life guards. She said it was sighted between Norton Point and the left fork.

Fin was spotted about 20 yards offshore. — Courtesy town of Edgartown

Officials were unable to immediately confirm what type of shark it was.

Brian Jordan, head lifeguard in Edgartown, was called in from a day off when he got the call. He said he and an assistant guard walked on the beach toward Norton Point where they were approached by a woman who had photographed the animal. After seeing the photographs, the guards decided to call swimmers out of the water.

“Just to be safe, we’re going to close the water to swimming,” Mr. Jordan said. A biplane from the Katama Airfield went up shortly after the shark was sighted and confirmed its presence, Ms. McGroarty said. The beach was briefly reopened for swimming at 2:15 p.m. but then promptly closed again at 2:27 p.m. after a second biplane sweep confirmed another sighting, according to chief ranger of the Trustees of Reservations, Rick Dwyer. The water was open again for swimming by 4:20 p.m.

Visitors to the popular South shore beach were not panicked, and many stayed ankle deep in the surf waiting to enter the water again. South Beach was crowded with beach umbrellas as temperatures topped 80 degrees. Heads turned when a biplane flew overhead, followed minutes later by a helicopter.  John Marabello, a beachgoer from Lunenburg, stood along the shoreline awaiting the signal that all was safe.

“It seems like they're everywhere these days. We can't see them. We just have to trust the people in charge,” he said of sharks.

When the first horn alerted swimmers that the water was safe to swim, some eagerly plunged back in while others were more cautious.

No sharks were sighted off the beach last summer, but a hammerhead was sighted off the south shore in 2016.

“It’s stressful, but we have to make the right decision and we made a very safe decision,” head lifeguard Mr. Jordan said.

“They don't usually come this close but their territory is large,” Mr. Dwyer said. “This is all their home.” 

 Will Sennott and Noah Asimow contributed reporting.