PARIS: Very loud, repeated blasts of sonar can cause a dolphin to temporarily lose its hearing, according to an investigation into a suspected link between naval operations and cetacean strandings.
Numerous beachings of whales, dolphins and porpoises have occurred over the past decade, prompting a finger of blame to be pointed at warship exercises.
A theory is that the mammals' hearing becomes damaged by the powerful mid-frequency sonar used by submarines and surface vessels, prompting the creatures, which themselves use sound for navigation, to become disoriented.
A paper published in the British journal Biology Letters yesterday provides the first lab-scale investigation into this idea, although its authors stress it does not provide proof that warship sonar is to blame.
Marine biologists led by Aran Mooney at the University of Hawaii exposed a captive-born, trained Atlantic bottlenose dolphin to progressively louder pings of mid-frequency sonar.
The experiment took place in open water pens at the Hawaii Institute of Marine Biology and in the presence of the dolphin's trainer.
The scientists fitted a harmless suction cup to the dolphin's head, with a sensor attached that monitored the animal's brainwaves.
When the pings reached 203 decibels and were repeated, the neurological data showed the mammal had become deaf, for its brain no longer responded to sound.
The deafness, though, was temporary and the dolphin was not hurt in the experiment, said Mooney.
The hearing was typically restored after 20 minutes, and its loss only occurred after the dolphin was exposed to five rounds of noise. Each round comprised a block of three pings, with 24 seconds between each block.
Other sensors showed that the dolphin's breathing rose significantly when the sonar was turned on.
"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 said.
"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 is not used to it, he would move around, and the ship itself is moving in a different direction."
On the other hand, a cetacean that sought to escape a persistent loud sonar may not easily find an escape route, said Mooney.
"In the ocean, sound doesn't attenuate in a normal fashion. Sound can sometimes get trapped at the surface, in layers called thermoclines, at the top 100 meters or so," said Mooney.
"Maybe in those conditions it's more difficult to get away from the sound to a quieter area."
Further work is needed to figure out what happens at lower sound levels from sonar and at greater distances to see how cetaceans respond, he said.