Professional surfer Dusty Payne of Hawaii was in the middle of his heat when he says he saw something in the water.
"I've seen dolphins before and it wasn't a dolphin; it was the biggest fin I've ever seen in my life, coming straight at me and the waves, they're terrible; I wasn't going to stay out there and get bit by a shark," Payne said.
"Nobody knows for sure what it was; there are a lot of dolphins at this time of the year and they're big dolphins too, it easily could have been a dolphin," Golden Gate National Recreation Area ranger George Durgerian said.
Even so, a lifeguard ranger on a jet ski is patrolling surfing areas along the beach.
Surfers know the shark threat too well. There is concern after a surfer was bitten by a 9-foot shark on Saturday in Monterey County. Rangers have asked people to stay out of the water there the rest of the week.
Back at Ocean Beach, Kelly Slater, the 10 time world champion, could clinch his 11th title. Close on his heals is Adriano de Souza, ranked No. 3 in the world.
The competitors go three at a time in half hour heats. Judges take their top scores for two waves.