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Sonar causes (temporary) deafness in dolphins

One unlucky bottlenose

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Exposure to military sonar may cause temporary hearing loss in bottlenose dolphins, scientists at the Hawaii Institute of Marine Biology have found.

Tests on a captive dolphin indicate exposure to sonar at high, prolonged levels can cause temporary deafness for as long as 20 to 40 minutes — but only if the dolphin stays close to the source of the sonar for several minutes.

In the past decade, numerous beachings of whales, dolphins, and porpoises have been blamed on navies pinging sound pulses into the oceans and listening for the echoes to detect enemy submarines and mines. In particular, mid-frequency active sonar has been fingered as injuring marine mammals and causing some to run aground.

A paper published in the Brit journal Biology Letters today, scientists have reported the first controlled study on the effects of sonar in dolphins.

One very lucky dolphin in an open-water pen was fitted with suction cup sensors to monitor its brainwaves as it was made to listen to recordings of real naval mid-frequency sonar played progressively louder.

When the pings reached 214 decibels underwater* and were repeated, the data showed the dolphin had become temporarily deaf. Other sensors showed the dolphin's breathing rose significantly when the sonar was switched on.

Hearing was typically restored after 20 minutes, but sometimes took as long as 40 minutes. Aran Mooney of the University of Hawaii said the deafening results are akin to a "rock-concert effect" for the mammal because the real-world equivalent of the results would require a dolphin to be just 40 meters from the source of the sonar and remain there for about 2.5 minutes. That's an unlikely scenario for a wild animal, the paper reports, unless there's some unusual conditions involved. For example, sound traps like underwater caverns on the sea bed or a layer of warm water at the oceans surface could cause sonar to bounce around a smaller area.

This could explain why dolphin beachings are relatively rare compared to how often mid-frequency sonar is used in military exercises.

The paper notes additional studies are needed to see how the sonar affects wild animals that aren't used to noise experiments. ®

*203dB above the water would instantly ruin your hearing at best. Under water, it's roughly the equivalent of 170dB in the air (like a gunshot 1 meter away — but that's still loud as hell).

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