Swimmers at South Beach, State Beach and Tashmoo were told to get out of the water yesterday by Edgartown and Tisbury public safety officials, following the sighting of a shark close to shore. The shark sighting off South Beach was confirmed later by a private airplane flying over the beach.

There also was one report of two sharks being seen late yesterday morning at the eastern end of the Joseph A. Sylvia State Beach, in an area also known as Bend in the Road Beach, but there are serious questions about that report.

“I believe the sighting at South Beach,” said Edgartown harbor master Charlie Blair. “I am not buying the report at Sylvia State Beach. I don’t believe it. I spent two and a half hours out at State Beach and I didn’t see anything.

“But we err on the side of caution. We got the people out of the water,” Mr. Blair said.

Mr. Blair said a fin of a shark was spotted on the water at South Beach at 9:30 a.m. about 70 yards offshore. The report was made by a lifeguard.

Swimmers were immediately advised to get out of the water.

An hour later, a pilot from Classic Aviators, Geof Smith, flew from Katama Airpark and saw what was believed to be the same shark. The animal was identified as a great white shark.

Alyssa Fitzpatrick, manager of the Classic Aviators, said Mr. Smith was piloting a biplane when he and his passenger spotted the big fish. Ms. Fitzpatrick said Mr. Smith estimated that the great white shark measured between 15 and 17 feet in length.

Mr. Blair said: “When a shark is sighted by a reliable witness, in this case it was a lifeguard, we get the people out of the water. We post the signs at the entrance to the beach and we tell the people there has been a shark sighting.”

“Our lifeguards are not going into the water to save anyone from a shark attack,” Mr. Blair said.

Mr. Blair said he has serious questions about the sighting of the shark at the Bend in the Road Beach. “A fisherman came up to the guards and claimed to have seen two great whites offshore,” Mr. Blair said. That had happened at 10:30 a.m.

Tisbury harbor master Jay Wilbur said the town’s director of public works, Fred LaPiana, closed the ocean side of Tashmoo Beach to swimming around noon after he learned of the shark sighting through the Dukes County communications center. The town continued to allow swimming inside Lake Tashmoo.

“Better safe than sorry,” said Mr. Wilbur, who discussed the situation with Mr. LaPiana prior to the closure.

The harbor master said the town plans to reopen the Tashmoo ocean beach this morning, barring any more sightings.

Dennis Arnold, a beach patrol officer for the Edgartown park and recreation department, said shark sightings have increased this year. They are tied to the rising population of seals in local waters.

“There are seals on the beaches every day,” Mr. Arnold said. “We see the seals body surf onto the beach.”

Mr. Blair said he has been following the seal population in these waters since last winter, concerned that this day might come. He said reports earlier this summer of great white sharks being seen off Menemsha is part of that story. “Wherever you have seals you have sharks,” Mr. Blair said.

Lisa Capone, a spokesman for the state Executive Office of Energy and Environmental Affairs, said late yesterday: “We were notified by the Coast Guard and made arrangements with a private plane to go over the areas looking for sharks. At this point we have seen none.”

Mr. Blair said that anyone who sees a shark should call the county communications center at 911. “Any photographs would really help. We tried really hard to get photographs today,” Mr. Blair said. “This helps us identify the shark.”

The state maintains a Web site for shark sightings: masharks.com.