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A new study in Biology Letters confirms what marine biologists have long suspected: loud sonar can cause temporary deafness in dolphins, possibly explaining some mass strandings. The study, using a captive dolphin in a controlled experiment, found that sonar at high prolonged levels could even lead to slight behavioral changes.
The researchers subjected a captive-born Atlantic bottlenosed dolphin to increasingly loud sonar while monitoring the mammal’s brainwaves. When the sonar reached 203 decibels and repeated, the brainwaves showed that the dolphin stopped responding to the sound, i.e. experienced deafness. It took usually twenty minutes, but up to forty, for hearing to return.
“What we found was if you play sound you can cause temporary hearing loss. The sounds have to be surprisingly loud and they have to be repeated over an extended period of time – two to three minutes.” Aran Mooney, one of the researchers from the University of Hawaii told The 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.”
Mooney believes the study confirms that loud sonar may be the cause of some mass dolphin strandings, especially in mountainous underwater areas where underwater sound could be harder to escape since it bounces back and forth.
However, the researchers also stress that since the sonar was played on a captive dolphin in controlled circumstances, more research is needed to uncover how the sounds may play differently for wild dolphins. For one thing that subject dolphin, which was born in captivity, was accustomed to man-made sounds.
Last November, the US Supreme Court found in favor of the navy by lifting some sonar restrictions which were meant to protect marine mammals. The US Supreme Court argued that the public interest was best served by allowing military training exercises to go forward unfettered even at the risk of harming dolphin and whale populations.
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