For a decade, marine biologists have been trying to unravel the subtle ways sonar may harm whales and dolphins, which hunt and travel using echolocation.
But experts peeling back the role of sound in the marine world are making surprising observations.
Alongside a boom in international shipping and deep-sea oil development, the ocean is growing ever-noisier and scientists are increasingly wary of sound's potential to impact sea life beyond just marine mammals.
Herring and cod appear to alter their swimming patterns in response to noise from ships. Schools of bluefin tuna scatter, with some diving and others rising to the surface. New studies suggest even small bumps in ocean noise may affect everything from damselfish and pollock to octopus and squid.
"The new research has been an eye-opener," said Jason Gedamke, who runs the National Marine Fisheries Service ocean-acoustics program.
The study of human-caused underwater noise pollution is in its infancy. For most creatures it's too soon to say how much is too much.
But the issue is capturing attention at high levels. Among the last acts of the top two science advisers in the George W. Bush White House was a report recommending a decadelong research plan to grasp the "biologically significant effects" of marine noise.
And last year, leading experts on the sea's auditory environment compared the potential harm from ocean noise to Rachel Carson's 1962 plea to curb the use of toxic pesticides.
"Studies on the impact of pesticides on birds ... have curbed the prophesy of a 'silent spring,' " they wrote. More noise research will provide "a better alternative to waiting to see what happens to fish in the dim future of a 'noisy spring.' "
The Bush administration was responding largely to gridlock from fights over sonar's impacts on marine mammals. But it recognized that our understanding of sound in the sea was changing rapidly.
In the last decade, beaked whales washed ashore with bleeding around the brain shortly after exposure to mid-frequency sonar. Researchers figured out that stranded bottlenose and rough-toothed dolphins often were nearly deaf. Dall's porpoises and killer whales were found to alter travel patterns during Navy exercises in Haro Strait.
Environmentalists repeatedly sued the Navy, but the precise science behind the impact on whales was often elusive.
Only this spring did new research finally suggest, for example, that Navy sonar may mimic sounds produced by predatory killer whales. That may drive prey like beaked whales away from feeding areas -- and send them rocketing to the surface, giving them the equivalent of the bends.
But this acoustic war over mid-frequency sonar masked another emerging issue.
At least 800 species of fish hear and produce sounds, either while fighting or competing for food or when courting or spawning. Some, like herring and shad, can detect ultrasound, which may be how they avoid hungry whales. Even sharks, considered poor listeners, follow sounds, perhaps when they resemble noises made by wounded prey.
The noise pollution emanating from shipping lanes has increased more than tenfold since the early 1960s. And while higher-frequency sonar may be harmful to animals nearby, the low-frequency groan from shipping and the deep-sea air guns used to build oil platforms and bridges can travel halfway around the world.
