It remains one of the great mysteries of marine science: Why do whales and other marine mammals strand themselves, swimming into shallow waters and washing ashore to die?
Decades of research show that many of the strandings are caused by age-old maritime hazards: collisions with ships, infections from parasites, starvation and old age.
But scientists have a new suspect these days: Navy sonar.
Whales, dolphins and other marine mammals use echolocation - a kind of natural sonar -to detect predators, hunt for food, find mates, keep track of offspring and orient themselves in a dark and murky world.
"These are acoustic animals, they use sound the way we use vision to orient ourselves and find their way," said Brandon Southall, an expert on marine mammal bioacoustics with the National Oceanic and Atmospheric Administration.
Some marine mammal experts say that Navy sonar, which generates intense sound waves that bounce off objects to reveal their location, confuses the animals and disrupts normal ability to navigate.
Sonar-related strandings are occurring more often as the Navy increases its use of sonar in coastal areas. Since the 1980s, it has caused the stranding of dozens of marine mammals, said the Natural Resources Defense Council (NRDC), a California environmental group.
Navy officials say that sonar - by itself - does not automatically cause whale strandings.
"The idea that there's a cause and effect relationship is tenuous at best," said Capt. Mark Boensel, director of environmental readiness for the Chief of Naval Operations.
Navy sonar isn't the only potential problem. Oil company geologists blast ocean beds with high powered air guns to find deep-sea oil deposits. About 80,000 commercial ships, fishing boats and other craft ply oceans every day, sending waves of sound into the depths.
"The oceans are extremely noisy places," said Mardi C. Hastings, a scientist in the Office of Naval Research. Hastings spoke last month at an NOAA-sponsored gathering on marine mammals and noise at the National Aquarium in Baltimore.
At Congress' urging, the U.S. Marine Mammal Commission has set up an advisory panel to find ways to reduce the impact of noise created by all three sources. The group is holding hearings and will issue a report in the spring.
