Surfers and kayakers often take advantage of summer and fall weather along the California coast, but that bracing dip in the murky Pacific can also be a rather chilling gamble against a ghastly fate.

That's because on the West Coast those seasons belong to the great white shark.

The ferocious predators are out there now patrolling the coastline in what experts believe are increasing numbers.

"There seems to be more sharks in the water, but we don't have enough evidence yet to support that," said John McCosker, a shark expert and chairman of aquatic biology at the California Academy of Sciences in San Francisco. "There have certainly been more sightings."

Kayakers were attacked by great whites on two separate occasions last month, pushing the number of verified great white shark attacks on humans in California to 102 since 1952. Numerous sightings of the giant predators have been recently reported, including the August mauling of a sea lion by an 18- to 20-footer off Pacifica.

The vast majority of shark attacks in California occur between July and November, McCosker said.

"The reason is there are a lot of seals and sea lions in the area at this time of year," said McCosker, who, with Robert Lea, has verified, analyzed and documented every great white attack on humans in the state. "The salmon are returning to the Sacramento River, the sea lions are following them and the sharks are following them. Meanwhile, the surfers are doing their best to imitate shark food."

Breaking new ground

A landmark study released last year by Stanford University determined that this region's genetically unique white sharks begin returning from the deep ocean to Northern California in August and begin leaving in December.

The area where the sharks gather - from the south end of Monterey Bay out to the Farallon Islands and across to Bodega Head - is known as "The Red Triangle," where McCosker said half of the state's shark attacks have occurred.

Eleven people have been killed by sharks off the California coast, the majority of them surfers, since the first documented human attack 58 years ago. The body of a probable 12th victim was never found, so he isn't counted. McCosker said there were no verified attacks before then, probably because previously very few people surfed or went into shark-infested areas for recreation. American Indians evidently knew not to go in the water during shark season, he said.

Surfers are by far the most common human victims, but the last two attacks in California have been on kayakers.

On Aug. 2, Duane Strosaker was attacked by a 15-foot or larger great white as he kayaked 5 miles out from Gaviota State Beach in Santa Barbara County.

"It attacked from my left side, with its head coming up from the water," Strosaker wrote in a blog. "Its mouth wrapped half way around the hull ... Its head was huge, and I remember seeing its eye and a hole on the side of its head, as well as its gray skin."

Strosaker pushed his paddle against the shark's head, but was afraid to hit the bloodthirsty creature out of fear it would thrash and capsize his boat.

"The whole time the shark was latched onto the kayak with my foot inside I was screaming like a little girl," he wrote. "After the longest 10-15 seconds in my life, the shark gently let go of the kayak and slid back into the water."

The same thing happened to Adam Coca of Pinole on Aug. 14. He was paddling off Pigeon Point in San Mateo County when a shark rocketed upward into the nose of his kayak, flipping it over and sending him thrashing into the water. Coca desperately held out his paddle to ward off the brute. It took a bite of the paddle and then swam away, Coca told authorities.

Another kayaker was attacked in 2007 in almost the exact same location, McCosker said.

Great white sharks, known scientifically as Carcharodon carcharias, grow as long as 20 feet and weigh 3 tons. The biggest one ever recorded was caught in 1939. It was 21 feet long and weighed 7,300 pounds, McCosker said.

Favor cool waters

The toothy beasts live worldwide in cool coastal waters and have a well-developed sense of smell and eyesight. They have an innate ability to sense changes in water pressure and electrical pulses, helping them find prey.

Female white sharks typically visit the Gulf of the Farallones in alternate years, suggesting that their migration pattern is tied to a two-year reproductive cycle. Five tagged sharks were tracked into San Francisco Bay using acoustic tags during the shark study, but there has never been a reported attack in the bay.

Nobody knows why sharks have recently been attacking kayakers. It may simply be because there are more paddlers out there.

McCosker said it would be foolish to think that a kayak is adequate protection, as the first documented great white attack on a kayaker in 1989 showed in gruesome detail. The female kayaker in that case was found floating in the ocean near Malibu. She had a large bite on her left thigh, defensive wounds on her hands and no water in her lungs, suggesting she died from her injuries and not from drowning. Her male companion and his kayak were never found, McCosker said.

The most recent reported sighting of a great white was Aug. 30, when sightseers spotted one attacking a sea lion about 150 yards off Linda Mar Beach in Pacifica.

Fighting helps little

The study of shark attacks by McCosker and Lea concludes that punching sharks in the nose, grabbing the gills or otherwise fighting the animal probably has little effect. Rather, humans' best defense appears to be that their flesh or neoprene is probably distasteful to sharks.

The statistics on attacks do not include the numerous bumpings and brushings of surfers' boards. McCosker also notes that some people who were reported as missing at sea or drowned and not recovered may have actually been consumed by white sharks.

The bottom line, McCosker said, is that right now is not the safest time to swim, surf or kayak in the ocean, especially near the Farallon Islands, Año Nuevo, Point Reyes and Tomales Bay. It is significantly safer to do those things between January and July, when the sharks live in the deep ocean near Hawaii.

Those die-hards who must go in the water in the fall should take a buddy with them, he said. That's because after taking a bite and tasting unappetizing human flesh instead of tasty blubber, a shark often leaves in disgust. But if it does stay to eat, it usually waits for its prey to bleed out.

"If you are attacked, the white shark is not going to consume you until you are dead," McCosker said. "If you don't have anyone to take you back in, you will probably bleed to death. If you have someone to take you to shore, there is a very high probability of survival."