Panicked by the sound waves emitted by powerful sonar, whales and dolphins fleeing to the surface of the ocean 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 the journal Nature today, 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 damaging 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, that has claimed the lives of many human scuba divers.
The authors theorize that the air-breathing marine mammals, renowned for their sensitive hearing, react to the loud sounds by surfacing faster than they normally would.
In such circumstances, compressed nitrogen in their systems expands rapidly, forming bubbles that can tear the delicate tissue of internal organs. That, in turn, can cause hemorrhaging and death.
The report is based largely on necropsies of 10 of 14 beaked whales that washed ashore in the Canary Islands in September 2002 after a Spanish-led military exercise that involved sonar from warships, submarines and planes from a dozen NATO members, 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 found similar injuries among dolphins and harbor porpoises found dead off the coast of England in the past decade.
Whale researcher Ken Balcomb finds the explanation compelling, even though it conflicts with his theory that powerful sonic waves in the water shake and tear delicate air-filled tissues in the ears and brains of sea mammals, causing bleeding, disorientation and death.
Balcomb, of the Center for Whale Research in Friday Harbor, spent a decade studying beaked whales in the Bahamas where Navy ships using sonar swept through the area in March 2000, leaving dead and injured whales in their wake.
The most recent case came on May 5 when several porpoises were found dead after the USS Shoup, an Everett-based destroyer, tested its sonar on a cruise between Vancouver Island and San Juan Island. The National Marine Mammal Laboratory in Seattle is conducting necropsies on some of the porpoises to determine why they died.
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