Posted by . on October 09, 2003 at 18:47:36:
The mammals may surface too quickly when frightened by sound waves from military sonar, causing internal damage, scientists say.
By Kenneth R. Weiss
Panicked by the sound waves emitted by powerful sonar, whales and dolphins fleeing to the ocean's surface may succumb to "the bends," according to an international team of scientists looking into mysterious die-offs of marine mammals that have occurred in the wake of military exercises at sea.
Writing in today's issue of the journal Nature, the scientists theorize that the frightened mammals surface too quickly, causing compressed gas bubbles to burst inside them like fizz from an uncorked champagne bottle and damage their internal organs.
The scientists report finding tissue damage in livers, kidneys and other organs of whales and dolphins that is consistent with decompression sickness, known as the bends, which has claimed the lives of many scuba divers.
The authors suspect that these air-breathing marine mammals, renowned for their sensitive hearing, react to the extraordinarily loud sounds by surfacing faster than normal.
In such circumstances, compressed nitrogen in their systems would expand rapidly, forming bubbles that could tear the delicate tissue of internal organs. That, in turn, could cause hemorrhaging and death.
The report was based largely on the necropsies of 10 of 14 beaked whales that washed ashore in the Canary Islands in September 2002 after a Spanish-led military exercise involving sonar from warships, submarines and planes from a dozen members of the North Atlantic Treaty Organization, including the United States.
"The beaked whales found in the Canary Islands are not the only stranded cetaceans to provide evidence of bubble-associated tissue injury," the authors wrote. They discovered similar injuries among dolphins and harbor porpoises found dead off England in the last decade.
Whale researcher Ken Balcomb finds the explanation compelling, even though it conflicts with his theory that powerful sonic waves in the water literally shake and tear delicate air-filled tissues in ears and brains, causing bleeding, disorientation and death.
Experts in physics are skeptical of his "resonance effect" hypothesis, questioning whether these sounds could result in sufficient reverberation of tissues to cause them to tear apart.
"Yet everyone agrees that once you have bubble formation, it would be a runaway situation," Balcomb said. "You cannot stop it, just like you cannot put bubbles back into the champagne bottle."
Balcomb, senior scientist at the Center for Whale Research in Friday Harbor, Wash., spent a decade studying beaked whales in the Bahamas, where U.S. naval ships using powerful sonar swept through the area in March 2000, leaving 16 dead and injured whales in their wake.
An extensive study by the National Marine Fisheries Service attributed the mass die-off to high-intensity sonar, concluding that it caused bleeding around the inner ears and trauma to the brain and auditory system. Yet it remained unclear whether the whales were fatally injured by the sounds or whether they were simply so disoriented that they drowned or died of stress and overheating after beaching themselves.
Necropsies of the beaked whales in the Canary Islands found similar hemorrhaging in the ears and brain, said Antonio Fernandez, professor of pathology at the University of Las Palmas on Grand Canary Island and one of 18 Spanish and British coauthors of the Nature article.
Fernandez said researchers also found that expanding nitrogen bubbles had damaged kidneys, livers, lungs and other organs, suggesting "that naval sonar could induce a condition similar to decompression sickness." More research was needed, he said.
Beaked whales and some types of deep-diving dolphins have been the most common casualties associated with deployment of military sonar. The most recent case came in May, when 16 dolphins, mostly Dall porpoises, were found dead after a U.S. warship cruised along the U.S.-Canadian border north of Puget Sound.
Beaked whales are known to feed at depths of 3,300 feet. When they surface for air, they descend immediately to 90 feet for 12-minute intervals, Balcomb said. He likened this to the "safety stop" practiced by human divers to allow nitrogen to escape safely from the bloodstream.
Panic could disrupt this practice, he said. "This is a plausible theory, as is the resonance [shaking] theory. They could be working together here. Either way, we are seeing this consistent pattern of dead animals after military sonar is being used all over the world."