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Loud sonar deafens isle dolphin in study


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POSTED: Thursday, April 09, 2009

University of Hawaii and other scientists are suggesting that sonar used by the military can cause hearing loss in marine mammals, but only when it is extremely loud and close.

Marine biologist Aran Mooney of the Woods Hole Oceanographic Institution and his colleagues at the Hawaii Institute of Marine Biology at Coconut Island reported in the British journal “;Biology Letters”; yesterday that they exposed a 22-year-old bottlenose dolphin, named Boris, to a tape of the sonar sounds of sub “;pings”; from a Puget Sound, Wash., naval exercise—15 sonar “;pings”; over two minutes—and recorded his reaction using a device that measured his brain waves.

An article in the Scientific American on Mooney's study said the scientists could temporarily deafen the captive-born, trained Atlantic bottlenose dolphin if the sounds were top-volume (203 decibels or more) and unleashed relatively close to his pen in Kaneohe Bay.

“;We had to expose him to very loud sounds repeatedly,”; Mooney said in the Scientific American article, noting that Boris' 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 said.

“;Even if we know how they react to sound, it doesn't give us a good idea why they end up on the beach. We may never really know that answer.”;

For years, the Navy has been fighting critics who charge marine mammals' hearing becomes damaged by the powerful mid-frequency sonar used by submarines and warships causing them to become disoriented.

In response, the Pacific Fleet, in a written statement, saying it was “;still reviewing the report.”;

But the Navy noted that Mooney's research indicates that higher decibel levels are required to cause a shift in hearing than in the Navy's more conservative models.

“;This would mean that the Navy may have vastly overestimated impacts of mid-frequency active sonar on marine mammals in its environmental planning documents,”; the statement said. “;Navy biologists look forward to reviewing the paper more completely, existing body of scientific knowledge.”;

In a 2005 Puget Sound incident, a pod of whales apparently lost its way and washed ashore following naval training. Until now, no one had tested the actual impact of the sub “;pings”; on marine mammals.

The new 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, Mooney said. “;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. It should be relatively easy to avoid problems.”;