Friday, June 27, 2008

Whales versus Waves


Sound waves, that is. The Supreme Court recently decided to hear the U.S. Navy's appeal of a federal court ruling that banned sonar within 12 miles of the coast, and ordered the immediate termination of high-powered sonar when whales (and other marine animals) have been detected. In light of the sonar-whale dilemma, it might be helpful to run through the basics of underwater acoustics.

Sonar is the process of detecting what already happens to sound waves whenever there is a noise: an echo. As sound bounces off surrounding objects, some of them are reflected back to the noisemaker. The acoustic sound waves in sonar (an acronym for SOund Navigation And Ranging) are transmitted through the water in short pulses, where they travel at frequencies high above what humans can hear.

The reflected wave signals can then be detected and used to infer the distance and relative position of an object. The military uses sonar to detect enemy submarines and other tools of aquatic warfare. Keep in mind that the speed of sound in water can change, depending on the depth, temperature, and even how salty the water is. So sound waves in an ocean will travel at a different frequency than sound waves in a large pool or lake.

The interests on each side of the debate couldn't be any more conflicting. The Bush administrations argues (in the name of national security) that the Navy should be given permission to disregard current environmental laws that protect endangered species, and work themselves into a sonar power frenzy if needed. Plus, they say they already take precautions to protect whales. But the environmentalists say sonar has been proven to harm whales and other endangered marine mammals to the point of death.

While the mechanistic details of how sonar affects whales are unknown, scientists are certain a link between the two exists. Several studies published in Nature over the past couple of year have provided evidence of the adverse effects of
sonar.

It is thought that sonar causes abnormal behavior in whales, causing them to disorient themselves and surface the water too quickly, leading to a condition called decompression sickness. In addition, autopsies performed on whales that died on the Canary Islands hours after a mid-frequency sonar was sent out (as part of an international military exercise), show that there may also be physical effects to sonar. CAT scans showed bleeding around the brain and ears of whales, along with an excess of gas bubbles that filled bodily tissue.

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