Lifeguards closed a two-mile stretch of water off Mission Beach after spotting an 18-inch dorsal fin today.
The dorsal fin was seen around 12:30 p.m. near Tower 15, prompting lifeguards to close the waters one mile in each direction, Lt. Nick Lerma said.
Lerma said the size of the fin indicates that it could belong to a shark that is at least 10 feet long.
Lifeguards searched the ocean with the help of a San Diego Fire-Rescue helicopter but no other sightings were reported, Lerma said.
The area will remain closed for the rest of today but will be open tomorrow morning, he said
