By HENRY FOUNTAIN
Sonar has been implicated in some strandings of whales and dolphins, but the mechanism by which acoustic pings might cause disorientation is uncertain.
Researchers at the University of Hawaii tested one possibility: that sonar can deafen marine mammals. T. Aran Mooney, now at the Woods Hole Oceanographic Institution, and colleagues exposed a bottlenose dolphin to naval sonar signals and then determined whether its hearing was affected. The pings, about a half-second long and about one second apart, were recorded in Puget Sound, Washington, just before a stranding in 2005.
As reported in Biology Letters, the dolphin suffered hearing loss for up to 40 minutes. But the effect was seen only with repeated exposure to the sonar pings at very high sound levels. The researchers calculated that a dolphin or other marine mammal would have to be within about 45 yards of a sonar source, and stay within that range for several minutes, for temporary deafness to occur.It seems unlikely that an animal would stay so close to the source of a sound that was causing it discomfort, so sonar-induced deafness may not be much of a factor in strandings. But the researchers suggested that under certain conditions — multiple sonar sources, for example, or undersea features that reflect signals — an animal may not be able to avoid intense exposure.