April 8, 2009

Sonar Pings May Cause Temporary Deafness In Dolphins

Military sonar exercises could be to blame for numerous beachings of dolphins and whales due to temporary deafness, scientists suggest in the Royal Society journal Biology Letters on Wednesday.

The study is the first to test a theory that claims mammals can lose their hearing due to the strong mid-frequency sonar used by military submarines and other ships.

These mammals rely on sonar to navigate through the waters, without it, they can lose direction and become stranded.

Aran Mooney at the Woods Hole Oceanographic Institution at the University of Hawaii, led a team of researchers who exposed a trained Atlantic bottlenose dolphin to pings of mid-frequency sonar that continued to become louder.

"With several marine mammal strandings linked to naval sonar activities, concerns are rising about the effects of naval sonar on marine mammal physiology and behavior," researchers wrote.

"In controlled laboratory studies, we show that actual sonar pings can induce at least temporary hearing loss in dolphins."

A sensor was attached to the dolphin's head in order to monitor the mammal's brain activity.

Researchers noted that neurological data showed the dolphin became deaf once the sonar pings reached 203 decibels.

Deafness was temporary, Mooney said, and the dolphin recovered hearing 20 minutes after the experiment.

The dolphin's hearing was only lost after being exposed to five rounds of three sonar pings. Additionally, the mammal's breathing rate also increased while being exposed to sonar, said Mooney.

"We definitely showed that there are physiological and some behavioral effects [from repeated, loud sonar], but to extrapolate that into the wild, we don't really know," Mooney told AFP.

"The sound levels that we used were essentially the equivalent of if an animal is about 40 meters (yards) from the sonar source," he said.

"The animal would have to be there for about two minutes or so" to get the same level of exposure as in the Hawaii experiment.

"That's a pretty long time for an animal to be there. If the sound's pretty loud and the animal's not used to it, he would move around, and the ship itself is moving in a different direction."

"In the ocean, sound doesn't attenuate in a normal fashion," he added. "Sound can sometimes get trapped at the surface, in layers called thermoclines, at the top 100 meters (325 feet) or so.

"Maybe in those conditions it's more difficult to get away from the sound to a quieter area."


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