Three decades have passed since the movie Jaws sent terrified bathers scrambling out of the ocean. But as any beach lifeguard knows, there's still nothing like a gory shark attack to stoke public hysteria and paranoia.

Two deaths in the waters off California and Mexico last week and a spate of shark-inflicted injuries to surfers off Florida's Atlantic coast have left beachgoers seeking an explanation for a sudden surge in the number of strikes.

In the first four months of this year, there were four fatal shark attacks worldwide, compared with one in the whole of 2007, according to the International Shark Attack File at the Florida Museum of Natural History in Gainesville.

'The one thing that's affecting shark attacks more than anything else is human activity,' said Dr George Burgess of Florida University, a shark expert who maintains the database. 'As the population continues to rise, so does the number of people in the water for recreation. And as long as we have an increase in human hours in the water, we will have an increase in shark bites.'

Some experts suggest that an abundance of seals has attracted high numbers of sharks, while others believe that overfishing has hit their food chain. 'I'm not saying it doesn't happen, but it's a convenient excuse,' Burgess said. Another contributory factor to the location of shark attacks could be global warming and rising sea temperatures. 'You'll find that some species will begin to appear in places they didn't in the past with some regularity,' he said.

New Smyrna Beach, Florida, is called the shark attack capital of the world. It has had more recorded incidents per square mile than any beach on Earth. So far this year there have been 10 attacks on surfers, including three in three days last week, although officials say most of the wounded were able to make their own way to hospital.

'It's more like a vicious dog bite, half a dozen stitches, a few bandages, that sort of thing,' said Scott Petersohn, a captain with the Volusia County Beach Patrol, which covers 47 miles of coastline including New Smyrna Beach.

'The sharks that inflict the most damage here, the black tips, can be about two or three feet long. There are some bigger ones along our coast, tiger sharks and bull sharks, but there's a sustainable food supply for them. People are not on the menu for sharks.'

At Solano Beach, California, where 66-year-old David Martin was killed last week by a great white shark estimated to be 4.5 metres long, and off the Mexican coast near Acapulco, where 25-year-old American tourist Adrian Ruiz fell victim to a suspected tiger shark, there were conflicting claims.

Meanwhile, the wildlife protection group Wildcoast has accused the Mexican authorities of 'international shark hysteria' over the slaughter of at least 10 near the beach at Troncones on the Pacific coast where Ruiz died. A navy spokesman said a 200-metre line with baited hooks was set up to catch any sharks threatening the beach.

'They more than likely had nothing to do with the attack. Since sharks are threatened in Mexico, this is the worst type of vengeance security imaginable,' said Aida Navarro, the group's wildlife conservation programme manager.

'It's the equivalent of stepping on to the plains of the Serengeti when you step into the water,' Burgess said. 'It's not like a swimming pool. This is a wilderness experience and with it comes a certain amount of risk.

'What's needed is some kind of system to prevent people and sharks coming together in a dangerous way.'