Imperial Beach lifeguards posted warnings for surfers and swimmers when experts with the nonprofit Shark Research Committee examined a photograph and identified it as a great white.

City News Service says the shark is estimated to be 16 to 18 feet long.

Imperial Beach is south of San Diego.

Lifeguard Capt. Robert Stabenow says a group of surfers standing at the end of Beach Avenue saw dolphins scatter and a shark dorsal fin appear at about 9 a.m. Tuesday. The shark was about 150 yards offshore.

Stabenow says the photograph confirmed it was a credible great white sighting and he hopes surfers stay out of the ocean for a few days.