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Hearing test. A bottlenose dolphin listens to Navy sonar.

T. Aran Mooney

Sonar Doesn't Appear to Deafen Dolphins

Sonar has long been blamed for harming dolphins, whales, and other underwater creatures that use sound to navigate, but it may not be as dangerous as previously believed. In the first controlled study in dolphins, biologists show that, although sonar can cause temporary hearing loss, marine mammals have to either be very close to the source or in an area of the sea where the sound can get trapped to be hurt by it.

Marine conservationists first began to worry about the negative effects of sonar more than 20 years ago. There have been a few instances of demonstrable harm. In 2000, for example, the U.S. Navy admitted that its sonar exercises led to six beaked whales beaching themselves in the Bahamas. Since then, researchers have reported at least five more cases of whales and dolphins running aground near sonar installations. However, the Navy argues that sonar is essential for exercises that protect national security and has successfully fought the restrictions demanded by environmental groups (ScienceNOW, 12 November 2008).

It's not clear how naval sonar affects marine mammals. Some studies indicate that it may interfere with the animals' own internal sonar. Sonar may also affect the way some animals dive and even cause decompression sickness--called "the bends" in divers--if they surface too quickly (ScienceNOW, 14 December 2007). But these studies used computer models or assessed animals after they beached themselves, says T. Aran Mooney, a marine biologist at the Woods Hole Oceanographic Institution in Massachusetts.

Mooney and colleagues at the University of Hawaii, Kaneohe, tried for a more specific experiment--one that would directly test the effect on sonar on dolphin hearing. The researchers placed electrodes on the head of a bottlenose dolphin housed in an open-water pen. They then played midfrequency sonar pings recorded from a naval mission that coincided with whales and dolphins washing up on a beach in Puget Sound, Washington, in 2005.

Those sounds had to be played loudly and repeatedly to induce temporary hearing loss, Mooney says. What's more, in the wild, a dolphin would have to be 40 meters from a ship's sonar source and remain there for 2.5 minutes to be adversely affected--an unlikely scenario when any animal is faced with uncomfortably loud noise, the team reports online today in Biology Letters. However, more work is needed to see how the sonar would affect wild animals that are not used to noise experiments, says Mooney.

So how to explain the whales and dolphins beached after naval training? Mooney says sonar could still be the culprit but only when extremely unusual conditions exist in the ocean. For example, a layer of warm water at the ocean's surface could cause sonar sounds to ricochet, he says. "In these circumstances, the animal can be exposed to sound for some time." In addition, in cavernous areas of the seabed, sounds could bounce off the walls and concentrate in a smaller area.

Other researchers are divided over the findings. Brandon Southall, an ocean acoustics expert at the National Oceanic and Atmospheric Administration in Silver Spring, Maryland, says the study "will provide critical information to policymakers and regulatory bodies in predicting and mitigating impacts on marine mammals." However, Michael Jasny, a policy analyst for the Natural Resources Defense Council in New York City, says sonar can also increase stress levels in marine mammals and change their behaviour--concerns that the current study doesn't address. "Marine noise ... is probably having its greatest impact on marine mammals at a sublethal, subhearing loss level," adds Naomi Rose, a marine mammal scientist at the Humane Society International in Washington, D.C.