Sound and Fury
Whale deaths blamed on sonar have triggered a heated debate about man-made noise in the sea
The din below. Indeed, scientists concur, there's a lot that isn't known about whale acoustics. Giving a whale a hearing test, says Ketten, "is not a trivial problem." So they have had to make assumptions. One of them is reassuring: Whales' ears cannot be preternaturally fragile because the environment in which they evolved is a naturally noisy one. Winds riffling the surface, raindrops, and snapping shrimp all contribute noise beneath the waves. Lightning strikes produce the loudest sounds in the ocean, as high as 260 decibels at the source. Many of the big whales themselves produce astonishingly loud noises. Some baleen whales produce low-frequency calls that can reach 180 decibels or more, intense enough at close range to bruise human lung tissue.
Yet whale biologists also assume that most cetaceans can hear--and thus can potentially be harmed by--midfrequency sonars like the one used in the Bahamas. That's because it's generally thought that animals' hearing is tuned to the same frequency range at which they vocalize, and most cetaceans, from great whales to dolphins, emit at least some midrange sound. Indeed, whalers have long known that midrange pings can impel frightened whales to run at the surface instead of diving. By the same argument, blue, finback, humpback, and other baleen whales, which make low-frequency sounds, might be most vulnerable to the very low "booms" emitted by the Navy's new, long-range sonar system, called SURTASS LFA, for "surveillance towed array sensor system low frequency active."
The Navy, however, says it is encouraged by studies it funded in 1997 and 1998, which exposed blue, finback, humpback, and gray whales off the coasts of California and Hawaii to signals from a test array. The sonar has a remarkable range. Under the right water conditions, its signal will travel hundreds of miles before dropping below 120 decibels, which many scientists thought might be the threshold at which the sound would begin disrupting whale behavior. Yet the whales turned out to be relatively tolerant of the signals.
In one experiment, the array was placed a mile and a quarter from the shore, in the middle of the whales' historical migration corridor. When the sonar began broadcasting at 180 decibels, the whales deviated from their path, but by only a mile or so. When the test array was placed outside the corridor, about 3 miles from the shore, the whales stayed right on track. In another study, singing humpbacks exposed to signals between 120 and 150 decibels sang longer songs instead of quitting the area. "Yes, we saw reactions," says Christopher Clark, director of the bioacoustics lab at Cornell University, "but not all the time. And when we did see them, the animals seemed to resume their normal activities within tens of minutes and a couple of miles [of the signal source]."
To Clark, what's known so far suggests sonar will not be the "death knell of the oceans." But the problem of noise is bigger than sonar, he says. Sonar is part of a man-made "acoustic smog" that could be threatening marine mammals in ways that are not fully understood. One major contributor is the air guns and drilling rigs of the offshore oil industry. "I can't hear blue whales off the Grand Banks anymore because there's so much seismic exploration in Canadian waters," Clark says, "It's `ka-bam, ka-bam, ka-bam' every 11 seconds, 24 hours a day, for days on end."
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