City News ServiceLifeguards closed a two-mile stretch of water off Mission Beach Thursday after spotting an 18-inch dorsal fin.
The sighting was made 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 belongs to a large shark.
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 Thursday but will be open Friday morning, he said.