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Navy Sonar May Mimic Killer Whale Sounds
18 March 2011 4:42 pm
Sonar drives beaked whales long distances from their favorite deep-water habitats, according to the first study conducted during actual U.S. Navy exercises. The finding could explain why these whales sometimes end up in dangerously shallow water where they could strand. It also suggests that the level of sonar that the Navy considers safe may be too high.
Blainville's beaked whales belong to a mysterious family of long-snouted whales that prowl kilometer-deep ocean canyons, often far from land. And yet, beaked whales often turn up stranded shortly after the intense sonar exercises the Navy uses to train sailors to detect silent enemy submarines. During one such event in 2000, six beaked whales died on beaches in the Bahamas following Navy testing. Some researchers have hypothesized that sonar noise scares whales into dangerous dive patterns, causing disorienting bends-like symptoms that could throw them off course and into unfamiliar shallow water. But solid evidence for sonar's effects on whale behavior has remained elusive, in part because these whales spend so little time at the surface that charting their behavior is difficult. Previous studies have also played back sonar recordings rather than tracking the effects of actual Navy exercises.
So in the new study, animal behaviorist Peter Tyack of the Woods Hole Oceanographic Institution in Massachusetts and colleagues enlisted the Navy's help. The researchers set up at the Atlantic Undersea Test and Evaluation Center in the Bahamas, where the Navy trains sailors in sonar use. With a set of underwater microphones, they listened for the "click trains" of Blainville's beaked whales—signature sets of clicks that the animals use to home in on squid and other favorite prey in the murky depths of the sea.
Initially, some two dozen whales were foraging on the Navy's test range, according to the clicks. But as the sonar exercises began, the clicks started to disappear, suggesting that the whales cut short their hunting and swam kilometers away from the sound. Once the exercise stopped, the whales returned to the range within a few days, probably because the range is a prime feeding ground.
Tyack and his colleagues also attached satellite tags to several whales, aiming to find out how loud the sonar was when it reached them and to trace their dive patterns. In separate experiments, the team played recordings of sonar simulating that used by Navy ships, general noise similar in frequency to naval sonar, and killer whale calls. Tyack hypothesized that because of the similarity in frequency, whales might mistake sonar for the vocalizations of killer whales, their most dangerous predator. And indeed, the tagged whales responded similarly to both sounds, swimming away from the source and staying deep, only gradually rising for a breath—a tactic whales often use to avoid predators. "For a beaked whale, being deep is safe," Tyack says. "Killer whales and human activities are mostly near the surface."
Still, Tyack says, these whales are accustomed to Navy sonar, so even though they move away from the sound, they probably don't panic. But whales that have never heard the sound might. These naïve whales could bolt, possibly getting trapped in shallow water or diving in dangerous patterns that could give them the bends.
The team, which reports its findings this week in PLoS ONE, also found the whales broke off their foraging dives at sounds of about 140 decibels—a level the Navy considers safe. "This study is a good foundation to start to say that perhaps those levels are high in some cases," says Terrie Williams, a mammalian physiologist at the University of California, Santa Cruz. She points out that most of what we know about whale strandings comes after they occur, providing little information about what led the animals to beach themselves. Williams calls the study a "landmark" in understanding how whales respond to sound: "The question now is, 'When does that response become detrimental to the point of stranding?' "