In recent years, the U.S. Navy has come under fire because of training exercises involving sonar that whale-lovers charge is deafening marine mammals and, in some cases, leading to their deaths by disrupting their communications and sending them astray. New research suggests that sonar does cause hearing loss, but only when it's extremely loud and extremely close.

Anecdotal evidence abounds of links between sonar training and beachings. For instance, a pod of whales apparently lost their way and washed ashore in the Puget Sound, Wash., in the summer of 2005 following a naval training. But until now, no one had tested the actual impact of the sub "pings" on marine mammals.

Marine biologist Aran Mooney of the Woods Hole Oceanographic Institution and his colleagues report in Biology Letters that they exposed a bottlenose dolphin with an  electro-encephalogram (EEG) strapped on his head to a tape of the sonar sounds from that same Puget Sound exercise—15 sonar "pings" over two minutes—and measured his reaction.

Their findings: they could temporarily deafen Boris the dolphin if the sounds were top-volume (203 decibels or more) and unleashed relatively close to his open Pacific Oean pen off the coast of Hawaii.

"We had to expose him to very loud sounds repeatedly," Mooney says, noting that Boris's hearing returned within 20 minutes or so. "The animal would have to be relatively close to the sonar source, the equivalent of 40 meters [131 feet] from the Navy ship."

And it's not clear exactly how sonar would lead dolphins and whales to beach themselves, even if they lose their way because of temporary hearing loss, Mooney says. "Even if we know how they react to sound, it doesn't give us a good idea why they end up on the beach," he says. "We may never really know that answer."

Such sonar-induced strandings may occur in relatively rare special acoustic circumstances, such as when the ocean's underwater topography allows for strong echoing or water conditions prevail that don't let the sound diminish over distance as it would normally. "No one knows what beaked whales hear," Mooney says. "Maybe they're exceptionally sensitive to sounds."

Other research has shown that some whales' hearing range has diminished from 1,000 miles (1600 km) in 1940 to just 100 miles (160 km) today,  due to increasing noise pollution in the ocean. And the U.S. Navy in its own environmental assessments has admitted that its sonar use may permanently damage whales.

The new experimental data will allow those deciding the appropriate noise level under the sea, such as the National Marine Fisheries Service, to better understand what might help avoid any such situations. "It shows us that these sounds do have to be relatively loud and the animals close and that leaves a lot of room to mitigate the situation," Mooney says. "It should be relatively easy to avoid problems."

Image: A. Mooney