Sonic booms blamed for beached whales

The bones of beached whales show signs of decompression sickness - the "bends" suffered by deep-sea divers - which scientists believe could explain why some sea mammals get stranded en masse.

A study of the skeletons of sperm whales has found that many of them suffered from a type of bone damage normally associated with the bends, when bubbles of nitrogen form in the blood causing pain and distress.

The findings lend support to the theory that loud underwater noises produced during naval exercises using submarine sonar could startle whales and dolphins and cause them to surface more rapidly than they would normally.

For years marine biologists have believed that sea mammals are somehow immune to decompression sickness but the latest study, published in the journal Science, forms part of a growing body of evidence suggesting this is not the case.

Michael Moore and Greg Early of the Woods Hole Oceanographic Institution in Massachusetts found extensive damage, called osteonecrosis, to the bones of stranded sperm whales bones. They suggest that this could support the idea that sea mammals in general are vulnerable to becoming stranded when they are startled into rising too soon from a deep dive.

"The long-held dogma of complete immunity to decompression sickness in marine mammals should be revisited," the scientists say.

"It therefore appears that sperm whales may be neither anatomically nor physiologically immune to the effects of deep diving," they say.

Sperm whales can dive more than two miles in the search for their quarry - giant squid - and any sudden rise to the surface could in theory cause dissolved nitrogen to come out of solution as gas bubbles.

Bubbles forming in the tissues can cause acute pain by pressing against nerves and blocking small blood vessels, resulting in the muscles being starved of oxygen. When this happens in bone, small areas of the tissue can die, leading to the formation of tiny cavities.

Moore and Early studied the bones of stranded whales dating back 111 years and found the sort of cavities associated with the bends. What was interesting was that some of the skeletons obviously predated the use of submarine sonar and seismic explosions for oil exploration.

They suggested therefore that such damage from decompression sickness may be a normal feature of a sperm whale's life, which the animals try to minimise by adopting tactics such as rising slowly to the surface.

Dr Moore said anything that interferes with this evolved behaviour, such as acoustic signals from a submarine or a seismic explosion from a ship exploring for oil, could easily disrupt the carefully-evolved slow ascent.

"If any acoustic stressors were to override normal behaviour, then they may run the risk of getting acute nitrogen problems which could cause pain and potentially strand them," Dr Moore said.

"This study opens the question that acoustic stressors may be impacting the normal physiology of these animals," he said.

Another team of scientists investigating a mass stranding of beached whales off the Canaries in 2002 reported last year that all of the animals showed signs of acute decompression sickness in their soft tissues.

The mass stranding had occurred four hours after a Spanish naval exercise in September 2002 that had involved the use of active sonar - powerful bursts of underwater noise used to echo-locate other submarines.

Paul Jepson of the Institute of Zoology in London, who took part in the study of the strandings in the Canaries, said that discovering signs of decompression sickness in whales suggests a possible reason why strandings are linked to submarine exercises.

"It's still a hypothesis. There are pieces of the jigsaw that are strongly supportive but we don't have everything in the jigsaw that proves it," Dr Jepson said.

- THE INDEPENDENT