The Canary Island strandings occurred four hours after Spanish-led, international military exercises commenced there. Mid-frequency active sonar was deployed as part of training to secure the Strait of Gibraltar, a militarily strategic location.
Environmentalists say incidences of marine mammal strandings have sharply increased since this mid-frequency sonar technology was deployed. Environmentalists are now actively opposing testing of a new technology, called low-frequency sonar, which they say would significantly expand the geographic range the sound travels.
Joel Reynolds, a senior attorney and director of the marine mammals program for the Natural Resources Defense Council in Los Angeles, California, says the new research in Nature "is another reason why it is important to regulate the proliferation of active sonar systems around the world."
This August a U.S. District Court judge in California ordered the U.S. Navy to negotiate with environmental groups on when, where, and how it tests the low-frequency sonar.
Gentry with the National Marine Fisheries Service said that the low-frequency sonar is quite different than mid-frequencyoperating at 100 to 300 hertz instead of 2,000 to 10,000 hertzand there is no evidence that low-frequency sonar has caused any strandings.
"The problem we know about is mid-frequency," he said. "There is no firm basis for extrapolating from one group to another."
Paul Jepson, a wildlife epidemiologist at the Zoological Society of London, who is the lead author of the Nature study, said the findings highlight a need for naval and marine mammal scientists to collaborate in order to advance the understanding and mitigation of the impact of active-sonar equipment on cetaceans, particularly beaked whales.
"In order to mitigate the impact of sonar on cetaceans, we need to know where these species are and when they are there," said Jepson.
Gentry said that the fisheries service and the U.S. Navy recognize that there is a cause and effect relationship between the mid-frequency sonar and beaked whale strandings and the agencies are changing the way they use mid-frequency sonar to mitigate the impacts on beaked whales.
"In general, we are very well aware of these strandings and the association with military sonar," he said. "We know we have to be cautious in setting guidelines."
While the fisheries service is not clear why the sonar causes the whales to strand themselves, Jepson and colleagues write in Nature that the sonar likely either causes a behavioral change in the whales such as rising too rapidly, or there is a physical effect of the sonar on bubble precursors in nitrogen-saturated tissues.
This physical effect of sonar on whales was proposed by Dorian Houser of the Navy Marine Mammal Program and colleagues in San Diego, California, in 2001.
Houser and colleagues devised a mathematical model that shows low-frequency sound waves can rapidly compress and then expand microscopic bubbles of gas in the tissue. Each sound wave causes the bubble to absorb more and more of the gas dissolved in the bloodstream, eventually making the bubbles big enough to rupture tissues.
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