SANTA ROSA, Calif. — The great white shark that bit a surfer two weeks ago off the Sonoma County coast was about 18 feet long and weighed 2 tons, judging by teeth marks left in her surfboard, according to a shark expert.

"This was a very large shark," said Ralph Collier of the Shark Research Committee in Los Angeles, which was asked by the Sonoma County Sheriff's Department to review the attack.

Collier said a shark that large, roughly the size of a Chevrolet Suburban, "is not an infrequent visitor to our coast."

"It is not unusual for us to see an animal this large cruising near the beach, looking for prey," he said.

Collier said reports of encounters between sharks and surfers, divers and kayakers are on the rise, which he attributes to an increase in the shark population since it was protected by the state 11 years ago.

In addition, he said, more people are going into the water and they are being more alert than in the past.

"Surfers would look but not see. They were more interested in seeing the set of waves," Collier said. "Now if they see a shadow moving, they will stop and look at it. You have a higher sensitivity."

There have been four shark attacks off California's coast this year, three of them in the past two weeks.

On Oct. 19, Megan Halavais, 20, was bitten on the back of her leg while surfing at Salmon Creek. She suffered serious, but not life-threatening, injuries and has since been released from the hospital.

On Wednesday, a surfer at Maverick's Beach near Half Moon Bay was knocked off his board by a white shark that grabbed it, leaving a tooth behind. On Oct. 21, a surfer was bitten while surfing at the mouth of the Klamath River just south of Crescent City.

The other attack was in La Jolla on Aug. 24.

While attacks are rare, Collier said sightings and people's being bumped by sharks are almost a daily occurrence.

"This is the time that people get on the water. The sea is lovely now, and the sharks are in the area looking for the yearling elephant seals, which are rich in fat," said Peter Klimley of Petaluma, a UC Davis professor who wrote "The Secret Life of Sharks."

Klimley said there are no data on how many great white sharks there are, but the guess is in "the 10s to 1,000."

The Monterey Bay Aquarium is sponsoring a study by UC Davis and Stanford University to tag sharks and produce a population estimate.

Since 1952, when records started being kept, there have been 111 attacks on the West Coast in which a white shark has bitten a person, and 10 fatalities.

Sixty percent of the attacks were at sites of previous attacks. There were five attacks off Salmon Creek Beach, six near the Farallon Islands and nine off Tomales Point.

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The attack on Halavais left a 19-inch, crescent-shaped imprint of 12 teeth on the bottom of her board, which is half the number of teeth a shark has in its lower jaw.<

Collier said that would indicate a shark that was 18 feet long and weighed 4,000 pounds, making it one of the largest adults ever seen.

Collier said he doesn't believe the shark mistook Halavais for a seal, either, which would have caused the shark to launch a much more violent attack. Rather, he said, it didn't know what Halavais was.

"The shark will give an initial investigatory bite to determine if it is something they are interested in, and they release it and swim on if it's not," Collier said. "We are not a normal prey. If we were, I would not talk to any survivors. There is no way a 200-pound man is going to fight off a 2-ton shark. It would kill us on an initial strike."

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