UNDERWATER explosions and sonar tests may be giving whales and dolphins the equivalent of the bends.
The US Navy regularly booms out sonar signals across the ocean in a bid to track submarines, and conducts controlled explosions under water. Commercial shipping, seismic exploration for oil, and scientific experiments that use sound to measure ocean temperature add to the cacophony (New Scientist, 1 March 1997, p 30).
Biologists have long suspected that noise pollution is disrupting the hearing and behaviour of these inhabitants of the deep, compelling them to beach themselves. Now researchers believe that it could cause haemorrhaging and decompression sickness.
When whales and dolphins dive, nitrogen gets squeezed out of their lungs and into the bloodstream, saturating the surrounding tissue. The longer and deeper they dive the more dissolved gas accumulates in their bodies. As they surface, cetaceans exhale, washing this potentially dangerous build-up of nitrogen bubbles from their blood.
But Dorian Houser of the Navy Marine Mammal Program in San Diego and his team have devised a mathematical model which shows that low-frequency sound waves can interfere with a whale's ability to safely store the gas.
Sound waves rapidly compress and then expand microscopic bubbles of gas in the tissue, they say. With each cycle, each bubble absorbs more and more of the gas dissolved in the bloodstream. Eventually, the bubbles become so big that they can rupture tissues or block blood vessels.
They may also crush nerves, leading to classic symptoms of decompression sickness such as joint pain and disorientation.
Houser reviewed studies looking at the levels of dissolved gas in diving whales and dolphins. He found that the diving behaviour of beaked whales, such as bottlenose and sperm whales, makes them particularly susceptible, as nitrogen levels have often quadrupled by the end of a typical dive.
That may explain why beaked whales seem to beach themselves more often than other species in areas with high levels of naval activity, he says.
Darlene Ketten of the Harvard Medical School has also discovered that loud blasts, like those produced by military shells, can damage the heart, lungs, liver and spleen of dolphins, as well as the most sensitive organ, the ear.
She exposed the carcasses of beached dolphins to controlled underwater blasts. "We're seeing classic symptoms of blast lung and gut haemorrhage," she told a meeting of the Society for Marine Mammalogy in Vancouver last week. Smaller individuals are particularly at risk, she says.
The experience of one veteran whale researcher seems to back up the idea. Ken Balcomb of the Center for Whale Research in Washington state told the conference that in just one day in March, 16 whales and dolphins washed up around the island of Abaco in the Bahamas.
All the whales showed signs of unusual haemorrhaging. Sure enough, Balcomb later discovered that the US Navy had been conducting exercises in the area the day before.
Author: James Hrynyshyn
More at: Journal of Theoretical Biology (vol 213, p 183)
New Scientist issue: 15TH December 2001
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