This actually is favorable. First the sound levels are very high and the hearing loss was temporary. “The deafness, though, was only temporary and the dolphin was not hurt in the experiment”. “The hearing was typically restored after 20 minutes, and its loss only occurred after the dolphin was exposed to five rounds of noise.”
Secondly the animal was restrained, but in the wild the dolphin can move away our turn its body. The levels are equivalent to a very high level in the peak of a sonar beam. “The sound levels that we used were essentially the equivalent of if an animal is about 40 metres (yards) from the sonar source” . That would mean a source level of 203 + 32 = 235dB re 1 uPa at 1 meter. But the sound typically reduces as 20 Log (Range) from 1 to hundreds of meters. At 100 m the sound is reduced by 40 dB and at 300 m the sound is reduced by 50 dB.
Powerful sonar causes deafness in dolphins: study AFP/File – Dolphins swim off the coast of the Tuamotu islands in French Polynesia. Very loud, repeated blasts of …
Tue Apr 7, 7:05 pm ET
PARIS (AFP) – 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 on Wednesday 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 only 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 behavioural effects [from repeated, loud sonar], but to extrapolate that into the wild, we don’t really know,” Mooney said in an interview with AFP.
“The sound levels that we used were essentially the equivalent of if an animal is about 40 metres (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.”
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 metres (325 feet) or so.
“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.