Nauset Beach in Orleans was closed to swimmers for an hour Sunday morning after a lifeguard spotted the fin of a great white shark in the water, the town harbormaster said.

The shark, which appeared to be about 12- to 13-feet long, was seen swimming south about 150 yards offshore at 10:15 a.m., Orleans Harbormaster Dawson Farber said.

The lifeguard who saw it could not tell whether there was a tag on its dorsal fin, Farber said.

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Swimmers were allowed back into the water at 11:15 a.m.. No additional sightings had been reported as of late Sunday morning.

Cape Cod’s first shark sighting of the season came Friday, when state biologists located a tagged 13-foot-long female great white off the coast of Chatham.