A two-mile portion of Mission Beach was closed for the rest of the day Thursday afternoon after a lifeguard reported seeing what may have been a great white shark about 100 yards off shore.
The lifeguard was paddling on a board at about 12:15 p.m. when he saw a dorsal fin about 18 inches long protude from the water near Tower 15, said lifeguard Lt. Nick Lerma.
Based on the size of its dorsal fin, the shark's estimated size is at least 10 feet long, Lerma said.
"This is an experienced lifeguard," Lerma said. "He is a surfer, diver, spear fisherman and so he was able to identify a large shark by the way it was moving and whatnot."
Swimmers were pulled from the water, and several lifeguards searched for the fish from boats and jet skis with assistance from a helicopter.
The two-mile stretch, south of the Mission Beach Jetty and north of Tower 19, will remain closed until Friday.
"It's definitely not the best situation," said tourist Samantha Figueroa, "but better to be told in advance rather than find out on your own."
The Scripps Institution of Oceanography is working to confirm whether the shark was a great white.
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