An American surfer has died after being attacked by a shark on the Pacific coast of Mexico only four days after another man was killed by a great white shark off California.

Marine biologists are now asking if the attacks are an astonishing coincidence — it has been 30 years since a fatal shark attack on Mexico's Pacific coast and almost 15 years since anyone was killed by a shark off Southern California — or if another factor is at work.

Some argue that thriving seal populations are attracting sharks to tourist areas, especially during March and April when female sharks arrive in the warmer waters between California and Mexico to give birth.

Another theory suggests that the killer shark off California may have been trying to protect a newborn pup. But experts have said that great white females are more likely to eat their offspring than protect them.

In Australia — where a teenage boy was killed last month in a shark attack, the first in two years — at least one expert is warning of a dramatic upsurge in attacks because humans are overfishing the marine life on which the predators feed.

Others argue the opposite: that the sharks themselves are being overfished, resulting in some species, such as the great white, becoming endangered.

Whatever the reason, the two attacks in four days are extraordinarily bad luck for the victims, the most recent of whom was Adrian Ruiz, 24, from San Francisco.

Mr Ruiz was attacked while surfing off the Troncones beach, about a 45-minute drive west from the resort of Ixtapa. He was still alive when he was dragged back to the beach by his friends. He was taken to a nearby naval hospital by a local, where he died from blood loss.

According to a statement from the Mexican authorities Mr Ruiz was bitten on his right thigh, leaving a 15in wound that “reached from the hip to the knee, exposing the femur”.

Witnesses said that the shark weighed as much as 660lb (300kg) and attacked Mr Ruiz when he was about 300 metres from the shore. The American had just arrived in Mexico for a beach holiday with six of his friends.

His death came after that of David Martin, a 66-year-old retired veterinarian, who was attacked and killed by a great white shark off the coast of San Diego on Friday. Mr Martin, a triathlete, bled to death after being bitten on both legs by the shark, which measured 15 feet (4.5m), while he was swimming with fellow triathletes about 150 metres offshore.

Witnesses said the shark lifted him out of the water, possibly thinking that he was a seal, before retreating after a single bite.

More than 27 kilometres (17 miles) of coastline in San Diego were closed to members of the public over the weekend as helicopters patrolled the waters.

In spite of the two cases, shark attacks are extremely rare. Last year the only fatal attack was in New Caledonia, in the southwest Pacific, according to the Florida Museum of Natural History's international shark attack file.

There were 71 other reported attacks worldwide last year, up from 63 in 2006. Attacks on the Atlantic coast, particularly in Florida, are far more common than they are on the Pacific coast.

A study by the Harvard Centre for Risk Analysis showed that a person's chances of being killed by a shark in any given year are about 1 in 280 million, compared with a 1 in 6,700 chance of being killed in a car accident.

This means you would have to swim in the ocean 112 times a day for it to become as dangerous as making a single car journey to the supermarket.

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