It was a perfect spring morning for an ocean swim.With the sun shining in a clear blue sky, Dave Martin and his triathlon training group swam past the surfers at Tide Beach on their regular Friday course through cool glassy waters about 150 yards out to sea.Somewhere below, a shark -- presumed to be a great white -- was lurking. It struck around 7 a.m., charging at Martin from below and lifting him vertically out of the water, both legs in its jaws, its serrated teeth slicing deep, fatal gashes."They saw him come up out of the water, scream 'shark,' flail his arms and go back under," said Rob Hill, a member of the Triathlon Club of San Diego, who was running along the beach when the attack happened.Martin, 66, was rescued by two swimmers 20 yards ahead who raced back and dragged him to shore in a little cove shielded by 50-foot bluffs. A lifeguard truck took Martin up to a lifeguard station on the bluff where he was pronounced dead at 7:49 a.m.Martin, a retired veterinarian, was the first shark fatality in San Diego County since 1994.Authorities immediately closed eight miles of popular beaches for 72 hours, heading into a warm weekend. Red emergency helicopters flew over the blue swells trying to track the shark, though experts said the chances of finding it were slim."The shark is still in the area. We're sure of that," said Joe Kellejian, mayor of Solana Beach, a quiet suburb of 13,000 people, million-dollar homes and a median annual household income above $100,000.Martin's family members visited the lifeguard station in small groups, emerging in tears, before his body was transported to the county medical examiner's office. A man who identified himself as Martin's son answered the telephone at Martin's home a few blocks from the beach but declined to comment on the attack.A shark expert who examined Martin's body said sharks mistake humans for seals or sea lions. They attack with a single disabling charge and then retreat while their target bleeds to death."It's just very bad luck for that one man," said Richard Rosenblatt, a professor emeritus of marine biology at the Scripps Institution of Oceanography in San Diego.Rosenblatt said he believed the bite pattern on Martin's legs indicated the shark was almost certainly a great white that may have been 12 feet to 17 feet long. Female sharks sometimes come to Southern California waters to pup, he said.The attack was unusual because it took place over a sandy bottom, Rosenblatt said. Sharks typically attack over rocks, which provide better camouflage.Surfers were astonished.Scott Bass of Encinitas, an editor at Surfer Magazine, was paddling when the attack happened but didn't see it. Helicopters flew overhead, announcing, "There's been a fatal shark attack. Go in immediately.""It was totally surreal," said Bass.Shark warnings were posted as far north as Carlsbad and as far south as San Diego.Friends and acquaintances wandered down to the beach as word of the death spread. Martin, a Solana Beach resident since 1970, was well-known to neighbors.He trained with much younger athletes. The club's median age is 36, ranging from 7 to 83, according to its Web site.The club boasts 1,700 members and welcomes all comers. Bike rides are Tuesday, Wednesday, Thursday and Saturday. Sunday is for running. Monday and Friday are for ocean swims."He was down here all the time," said fellow triathlete Hill. He said club members had been swimming there for at least six years and had never seen a shark.Hill said Martin was mangled when he was brought in to shore. "The flesh was just hanging," he said.Rosenblatt called the bite "really quite clean and massive."Ira Opper of Solana Beach saw Martin's body arrive at the lifeguard station. His "burly and athletic" frame had a black wetsuit that was shredded on both legs beneath blood-soaked gauze bandages.Paramedics worked on Martin for at least 20 minutes before he was declared dead, Opper said.Surfers reported seeing a stranded seal pup on the beach Friday before the attack. Lifeguard Craig Miller said he did not know whether there had been pods of seals or sea lions in the kelp beds nearby.Sharks feed on seals and sea lions, making areas where those animals live more vulnerable to attacks.The last fatal shark attack along California took place on Aug. 15, 2004, in Mendocino County at Kibesillah Rock, according to the state Department of Fish and Game. The victim was a man diving for abalone with a friend.On Aug. 19, 2003, a woman swimmer was killed by a great white at Avila Beach in San Luis Obispo County on the central California coast.The last fatal shark attack along San Diego County was in April 1994. However, some experts question the cause of death of 25-year-old Michelle Von Emster. Many believe Von Emster might have died or drowned in the water and sharks, or other fish, fed on her body, according to Rosenblatt.Overall, shark attacks are extremely rare. There were 71 reported worldwide last year, up from 63 in 2006. Only one attack, in the South Pacific, was fatal, according to the University of Florida.The university's International Shark Attack File has counted an average of 4.1 people killed by sharks annually worldwide in the last seven years.Sharks are highly migratory, making it unlikely that Friday's attacker poses additional risk to swimmers, said George Burgess, a biologist at the university. Still, other sharks may lurk."It's not any more dangerous than it was yesterday or the week before," Burgess said. "The reality is when you enter the sea it's a wilderness experience. There are animals out there that can and do occasionally do harm to us."Sharks feed on seals and sea lions, making areas where those animals live more vulnerable to attacks, Burgess said.