Navy sonar unquestionably disturbs beaked whales, concludes a new analysis investigating how underwater sound affects these elusive deep-divers. The results, published online March 14 in PLoS ONE, suggest that the current noise levels deemed risky for beaked whales need to be lowered.
During sonar exercises at the U.S. Navy’s underwater test range in the Bahamas, beaked whales stopped their chirpy echolocations and fled the area, experiments employing a huge array of underwater microphones revealed. Other experiments that exposed tagged whales to increasing levels of sound found that at exposures of around 140 decibels, the animals stopped hunting for food and slowly swam toward the surface, heading north toward the only exit of the deepwater basin known as the Tongue of the Ocean. Current regulations rate underwater exposures of about 160 decibels as disturbing.
“It seems beaked whales may be more sensitive than other species to sound,” says study leader Peter Tyack of the Woods Hole Oceanographic Institution in Massachusetts. “At the very least we may need a special rule for these whales,” he says. “If the criteria are changed they will be more protected.”
Until a few different species of beaked whales started showing up in unusual mass strandings, the animals were understudied and rarely seen. Because the strandings often coincided with nearby naval sonar exercises, scientists suspected sonar was somehow driving these whales to the beach. And strange bubbles in the bodies of some of the whales suggested that sonar might trigger behavior that gave the whales the equivalent of the bends.
But designing experiments that might untangle cause and effect has been extremely challenging, both in tracking the whales and trying not to cause them harm. The new results arose from a major, concerted effort by scientists from the Navy, the National Marine Fisheries Service and several academic institutions.
As it turns out, sonar does seem to spur a behavioral response — to flee. Because such get-out-of-Dodge behavior is unusual for these whales, it may mess with them physiologically as well, says marine conservation biologist Tara Cox, who was not involved in the new work. In the current study, sonar activity was stopped as soon as it elicited a response, so its full effects are not clear.
Researchers had also speculated that that the whales might confuse sonar with killer whale calls and frantically flee this mortal enemy in some way that might be dangerous. But the study found that beaked whales respond to playbacks of killer whale calls at decibel levels much lower than those of the sonar.
Other marine mammals, such as harbor porpoises, are particularly sensitive to sound and react strongly while other species seem unbothered. The new results suggest beaked whales also may be especially sensitive.
“We treat porpoises differently, and now there’s evidence that beaked whales respond differently as well,” says Cox, of Savannah State University in Georgia. Current sound levels deemed safe for beaked whales are not low enough, she says.
The National Oceanic and Atmospheric Administration is reviewing how it assesses the impact of man-made sounds, says Jason Gedamke, who manages the agency’s ocean acoustics program. “This paper … is groundbreaking work,” he says. Additional experiments assessing how several species, including blue and fin whales, respond to sound are being conducted off Southern California and could be used to craft regulations that minimize harm to marine life.