Navy sonar blamed for death of beaked whales found washed up in the Hebrides

Anti-submarine sonar may have killed a group of whales found dead in the Hebrides in one of Britain's most unusual strandings, scientists believe.

Five Cuvier's beaked whales, a species rarely seen in British waters, were discovered on beaches in the Western Isles on succeeding days in February. Another animal from a related species was discovered at the same time.

Experts consider such a multiple stranding to be highly abnormal. They calculate, from the state of the carcasses inspected that the whales died in the same incident out in the Atlantic to the south and west of Britain, and then drifted towards the Scottish coast over two or three weeks.

The main suspect in the case is sonar, as it is known that beaked whales are highly sensitive to the powerful sound waves used by all the world's navies to locate underwater objects such as submarines.

Groups of beaked whales have been killed, with sonar suspected as the direct cause, several times in recent years; well-documented incidents include anti-submarine exercises in Greece in 1996, the Bahamas in 2000 and the Canary Islands in 2002. In 2003, an American judge banned the US Navy from testing a new sonar after a court case brought by environmentalists to protect marine life.

Britain's Whale and Dolphin Conservation Society has now submitted a Freedom of Information request to the Ministry of Defence over the Hebridean strandings, with the aim of finding out if any Royal Navy activity coincided with the possible location and timing of the whales' deaths. So far, the MoD has provided no answers, but it is possible that other navies might have been involved.

The 21 species of beaked whale include some of the world's most rarely seen mammals; they are also the deepest-diving air-breathing animals. A Cuvier's beaked whale set the record for a deep dive two years ago: 1,899 metres, or 6,230ft, beneath the surface, holding its breath for an astonishing 85 minutes.

The animals use these deep dives to forage, but when sonar gets involved, their remarkable habit may be their undoing. One theory is that the whales are so distressed by the intensely loud sound waves that they return too quickly to the surface, and in doing so, fatally suffer "the bends" – the formation of nitrogen bubbles in the blood which can kill human divers.

The Hebridean strandings began when a Cuvier's beaked whale was found dead on the beach at Saligo Bay, Islay, on 2 February; three days later another washed up on nearby Machair beach. On 7 February a third was found further north, on the island of Tiree, and the following day a fourth carcass, probably a beaked whale but washed away before it was identified, was found on neighbouring Mull. On 12 February a fifth animal was found further north at Gobhaig on Lewis. A Sowerby's beaked whale was also found, at Benbecula, to the south of Lewis, the day before.

One man deeply concerned at the deaths is Professor Ian Boyd, the director of Britain's Sea Mammal Research Unit, based at the University of St Andrews. Professor Boyd is the chief scientist on an American project investigating beaked whales' sensitivity to underwater sound.

"The beaked whales have a problem with military sonar, and these strandings on the west coast of Scotland are very suspicious," he said. "The chances of them happening through natural causes are really quite small. It's likely that the animals died together in a single event, and also, it's quite likely that ... they were not the only ones which died." He added: "We don't yet have the evidence to make a direct connection with anti-submarine exercises, but there are enough examples of events like this to make it likely."

Professor Boyd asked the Proudman Oeceanographic Laboratory in Liverpool to model the whales' possible drift to find the location of the initial incident.

Secretive creatures

*The beaked whales, so-called because of their "snout", are the second-largest family of the cetaceans (whales and dolphins) but less is known about them than almost any other group of large mammals; some are known only from carcasses and have hardly ever been seen alive. They broke into the headlines in January 2006 when one of the 21 beaked whale species, a northern bottlenose whale, swam up the Thames into London. It died on the barge that was transporting it to possible freedom. The beaked whales are capable of deep vertical dives to 6,000ft and more to hunt for prey by echo-location.