Shark warning signs went up Sunday afternoon along the sands of the beach town 20 miles north of San Luis Obispo.

The shark sighting near the pier was determined to be "credible," according to Cal Fire, and San Luis Obispo County lifeguards posted signs. There were no details about the size of the shark.

It wasn't clear if swimmers and surfers would be allowed in the water or simply warned. There was a spate of shark sightings in July along Avila and Pismo beaches to the south.

In August 2003, swimmer Deborah Franzman was attacked by a great white shark off Avila Beach.

Meanwhile, a Humboldt County surfer was still counting her blessing Monday morning.

Sue Snyder was sitting on her surfboard Thursday when the great white knocked her off from behind, said Ralph Collier of the Van Nuys-based Shark Research Committee, which tracks shark attacks along the Pacific coast.

Snyder began kicking the shark, which left a 16-inch bite and teeth fragments in her board, according to other witnesses in the water at the time of the attack.

"The guys said it bit my board -- that is what knocked me off the board and onto his back," Snyder, a 52-year-old Safeway clerk who has Save the Sharks bumper stickers on her car, told the Santa Rosa Press Democrat. "I was kind of like in a fetal position, with my arms out. I felt his whole body as he swam by me. Then I kicked him, and whether he felt it or not, he didn't come back."

Based on the bite marks in Snyder's board, Ralph Collier of the Shark Research Committee in Van Nuys estimated the shark was 14 to 15 feet long and weighed about 3,000 pounds.

"She was lucky," Collier said told the newspaper. "When you look at the capability of what an animal can do in a predatory attack, it could have been very injurious to her."

Shark run-ins are becoming more common in Northern California as more people take to the water, said Jess Bareilles, owner of the Greenhouse Surf Shop in Arcata.