Military exercises blamed for whales stranded on beach

Military exercises involving blasts of underwater sound could be the reason why whales and dolphins become stranded on beaches, a study has found.

Military exercises involving blasts of underwater sound could be the reason why whales and dolphins become stranded on beaches, a study has found.

The blasts from active sonar, used to detect submarines, may cause dolphins and whales to suffer a form of decompression sicknesssimilar to "the bends" suffered by deep-sea divers.

Scientists believe they have found evidence to explain why many strandings of whales and dolphins appear to coincide with naval exercises involving active sonar.

Marine mammologists discovered signs of decompression sickness during post-mortem examinations of whales which beached themselves in the Canary Islands four hours after the start of a Spanish naval exercise in September 2002, where active sonar was used.

Antonio Fernandez, professor of pathology at the University of Las Palma de Gran Canaria, who examined 14 beaked whales found beached in the area where the exercise took place, said the link was circumstantial but strong.

"The detailed examination of the mass-stranded beaked whales in the Canaries in 2002 suggests that naval sonar could induce a condition similar to decompression sickness," Professor Fernandez said. "More research is needed to confirm this mechanism and to determine what level of sound can induce this process in exposed whales and dolphins."

The study, in the journal Nature, also involved government-funded British scientists who have performed more than 2,500 post-mortem examinations on sea mammals stranded off Britain over the past decade.

Paul Jepson, of the Institute of Zoology in London and a member of the Anglo-Spanish team, said: "The link between military sonar and stranded sea mammals has been established. Our study suggests a potential mechanism."

Decompression sickness happens when dissolved gases in the tissues of a diving mammal come out of solution because the animal rises to the surface too quickly. The dissolved gases, mostly nitrogen, form bubbles which can press against nerves and joints to cause severe pain and tissue damage. Dr Jepson said examination of stranded sea mammals in Britain showed that they can experience the effects of decompression sickness, challenging the notion that cetaceans (whales and dolphins) cannot suffer from the bends.

In some beached sea mammals, gas cavities had formed to such an extent in their livers that the texture of the dissected organs looked like aerated chocolate, Dr Jepson added. "A small number of stranded animals had gas bubbles and associated tissue injuries.

"Although decompression sickness was previously unheard of in marine mammals, we concluded that a form of marine mammal decompression sickness was the most likely cause."

The scientists have ruled out bacterial infections or other post-mortem changes to the corpses as the possible causes of the tissue bubbles. But an unresolved question is: how does active sonar causes decompression sickness? The most obvious answer is that the sudden loud noises, greater than any natural underwater sound, may startle whales and dolphins, causing them to surface too quickly.

Beaked whales, such as bottlenose and sperm whales, dive to great depths and normally take many minutes to surface, slowly allowing dissolved gases in their bloodstream to be released into the lungs. Anything that interferes with this carefully evolved behaviour may increase the risk of decompression sickness, leading to disorientation and stranding.

Another possibility, proposed by Dorian Houser, of the US Navy marine mammal programme in San Diego, is that sound waves from active sonar interfere with the way diving sea mammals safely store dissolved gases.

Dr Houser has proposed a mathematical model of how sonic blasts have a direct impact on dissolved gases by vibrating microscopic bubbles in the bloodstream, causing them to expand to a size that can cause tissue damage.

The work, which was largely theoretical, found the diving behaviour of beaked whales, which gulp a large amount of air before a dive, makes them more vulnerable to this effect because levels of super-saturated nitrogen in their bloodstream can more than triple by the end of a dive.

Other sea mammals, such as seals, breathe out before a dive, which allows their lungs to collapse at a shallower depth. This helps prevent nitrogen building in the bloodstream while they are underwater.