Sonar can induce deafness and behaviour changes in dolphins, but only at prolonged, high levels.

In a rare controlled experiment, researchers from the University of Hawaii, Kaneohe, used an actual mid-frequency naval sonar signal and a captive dolphin to shed some light on this contentious issue.

Many believe sonar, particularly more powerful military sonar, can damage marine mammals and lead to mass stranding (see: Sonar does affect whales, military report confirms). Due to the problems of running controlled experiments on dolphins and whales, firm evidence on this issue has been difficult to come by.

Now Aran Mooney, Paul Nachtigall and Stephanie Vlachos show sonar can induce temporary hearing loss. They also found subtle shifts in behaviour, such as respiration.

“The sounds have to be surprisingly loud and they have to be repeated over an extended period of time – two to three minutes,” says Mooney (Times).

“In that time you would expect them to swim away as fast as possible. They have to be within 40 metres of a ship, but when you have certain oceanographic conditions it’s hard for the animals to get out of the way.”

However, the dolphin used in this study was well trained and in captivity. “We definitely showed that there are physiological and some behavioural effects, but to extrapolate that into the wild, we don’t really know,” Mooney told AFP.

The research is published in Biology Letters.

Previous coverage

Ocean noise is no cocktail party for marine mammals – December 04, 2008

Save the whales . . . or the country? – October 09, 2008

Truce reached in sonar vs whales case – August 13, 2008

Have sonar tests claimed another cetacean victim? – February 22, 2008

Image: “Bottlenose dolphin during a hearing test after a sonar exposure” / T. Aran Mooney