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Whale deaths linked to the bends

By Anne McIlroy

Science reporter -The Globe and Mail

9th October 2003

Researchers have found evidence that military sonar is killing whales and dolphins around the world by driving them to the surface too quickly, resulting in a fatal case of what divers call the bends.

Scientists have known for several years that whale strandings and deaths are connected with the use of naval sonar, which uses loud sound to detect submarines, but they haven't established why.

A team of British and Spanish researchers studied 14 beaked whales that died in September last year after beaching in the Canary Islands near the site of a Spanish-led international naval exercise.

The whales became stranded four hours after mid-frequency sonar was used in an antisubmarine drill. The researchers found gas bubbles in the whales' internal organs similar to those found in scuba divers who surface too fast and get decompression sickness.

"This is definitive proof of the extent to which high-intensity sonar can cause physical changes or damage that contribute to the stranding and beaching of whales," said Joey Reynolds of the Marine Mammal Protection Project at the Natural Resources Defense Council in the United States.

"We didn't know the mechanism of injury. This suggests that whales get decompression sickness," the NRDC official said.

Up until now it was widely believed that decompression sickness, also known as the bends, did not affect whales. Decompression sickness occurs when divers surface too quickly. Nitrogen gas that has dissolved in their blood under pressure expands rapidly, forming bubbles that clot or damage blood vessels.

In humans, decompression sickness causes pain, itching, dizziness and chest pain. Untreated, it can cause paralysis or death.

The British and Spanish researchers who examined 10 of the 14 whales that died in the Canary Islands. They found that the whales' livers were full of gas bubbles, and that some blood vessels had exploded.

They believe that the sonar exercise might have caused the whales to rise to the surface too quickly. The alternative explanation, they argue, is that sonar actually causes gas bubbles to form.

"Both explanations are intriguing. Both are scary," said Hal Whitehead, a marine biologist at Dalhousie University in Halifax.

He said he can see both scenarios. Sometimes human beings do stupid things when they believe their lives are in danger, such as running into a busy street after escaping a house fire, he said. Whales and dolphins might react the same way in a similarly stressful situation.

There is also a theory that loud noises might cause gas bubbles to form in deep-diving marine mammals, he said.

Low-frequency sonar that has been tested by the U.S. Navy generates sound as high as 160 decibels -- loud enough to cause permanent hearing damage in humans -- as far away as 160 kilometres.

Signs of severe damage to the eyes and ears have been found in whales that died during past sonar exercises. The British and Spanish researchers examined the whales relatively soon after they died, which might explain why they found the bubbles.

Researchers have also speculated that the loud noises disorient marine mammals.

Paul Jepson, of the Institute of Zoology in London, said the findings, published in today's edition of the journal Nature, need to be taken into account in regulating the use of naval sonar.

In August, the NRDC won a legal victory in its campaign to limit the U.S. Navy's plan to deploy a new, high-intensity sonar system around the world.

Federal Judge Elizabeth Laporte blocked the deployment of the new sonar and ordered the navy to negotiate limits on its use with conservation groups who had launched the lawsuit.

Negotiations have begun but are confidential, Mr. Reynolds said.

In March, 2000, seven whales died on beaches in the Bahamas after the U.S. Navy used active sonar in the area. At the same time, the region's entire population of beaked whales disappeared.

Beaked whales, which dive deeper than most other whales, also swim in Canadian waters. Dr. Whitehead, who studies the northern bottlenose whale off the coast of Nova Scotia, said he is worried about the damage that might be caused by increasing seismic exploration in the area.

He said the sonar used by the Canadian naval ships isn't as powerful as the low-frequency sonar employed by the U.S. Navy.

Cetaceans under pressure

A new study published in Nature states that there are links between some mass strandings of cetaceans and the use of military sonar. The article presents evidence of tissue damage from the formation of gas bubbles in stranded cetaceans, challenging the view that such mammals do not suffer from decompression sickness.

Whales and dolphins mentioned in the study:

Harbour porpoise: 1.9 m

Common dolphin: 2.6 m

Risso's dolphin: 4.3 m

Blainville's beaked whale: 4.9 m

Gervais' beaked whale: 5.5 m

Cuvier's beaked whale: 7.0 m

Understanding decompression sickness

Decompression sickness takes place when sudden pressure changes in the environment cause gases that are dissolved in the blood and tissues to form bubbles. Here's what happens:

When a diver is several hundred feet under water (high pressure) the blood can contain more dissolved nitrogen than at sea level (lower pressure).

If a diver returns to the surface too quickly, there is not time for the nitrogen to leave the blood. Tiny gas bubbles form in the blood where they can block arteries and cause other serious problems.

A diver should rise gradually, stopping at specific depths, to allow the extra nitrogen to gradually dissipate from the blood.

Back at the surface, all of the extra nitrogen has safely left the body.