Environment

Mass dolphin strandings blamed on military sonar

by Amy Judd | April 8, 2009 at 12:44 pm

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Mass dolphin strandings in some parts of the world are being blamed on military sonar, as the animals become temporarily deaf for about 40 minutes after sonar has passed through the region.

Hearing is important to dolphins and whales as without it they can panic and become disorientated.

The Hawaii Institute of Marine Biology conducted some tests on dolphins in captivity, and although strandings can be a natural part of life for dolphins and whales, sonar can increase the chances of it happening. However, dolphins and whales should be able to swim away fast enough when sonar comes into the region.

To induce deafness in the Atlantic bottlenose dolphin, Tursiops truncatus, the sonar device would have to be loud, close and would need to last for at least two minutes.

This should give the animals plenty of time to escape but in some circumstances noises can be caught in “underwater sound traps”, Aran Mooney, of the University of Hawaii, said.

If a layer of warm water is trapped over a layer of cold water, sound can become trapped between them, as the sound will bounce off the cold water instead of going through it. If a dolphin or a whale swims through that part, they could release the sound between it. This could also happen with underwater mountains and ravines.

In the Bahamas in March 2000, 16 Cuvier’s beaked whales and Blainville’s beaked whales and a spotted dolphin beached during a US navy exercise in which sonar was used intensively for 16 hours.

The big question is what causes the animals to strand themselves as if they have the chance to swim away from the sound, why don't they?

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