Sonar, whale deaths linked
2003-10-09 07:16
Paris - Military sonar may be to blame for catastrophic deaths of whales, prompting the deep-diving mammals to ascend so quickly that, like divers who decompress too fast, they suffer a mortal attack of "the bends", a study says.
Powerful naval sonar has already been implicated in the stranding of whales and dolphins, perhaps by disorienting them, but this research is the first to yield clinical evidence as to exactly how the animals could be affected.
British and Spanish marine pathologists carried out autopsies on 10 out of 14 beaked whales that were stranded on the beaches of Fuerteventura and Lanzarote in the Canary Islands in September 2002.
No sign of disease
The mammals beached about four hours after a Spanish-led naval task force nearby began using mid-frequency sonar as part of an anti-submarine exercise.
Tissue dissection showed that the whales' livers and other internal organs were filled with gas bubbles, and smaller blood vessels had been literally blown apart from inside. There was no sign of any disease.
"These lesions are consistent with acute trauma due to... bubble formation (within the body) resulting from rapid decompression," the authors, led by Paul Jepson of London's Zoological Society, report in Thursday's issue of Nature, the British science weekly.
Two possibilities
Decompression sickness is caused by surfacing too fast from a depth. Instead of remaining dissolved in the blood, nitrogen gas expands quickly, forming bubbles that can clot or breach blood vessels.
The study suggests two possibilities for the phenomenon.
One is that the sonar's radiowaves are so powerful at close range and at great depths that they excite the compressed nitrogen nuclei in certain tissues, causing the gas to expand.
Alternatively, they postulate, the sonar confuses the whales' sense of depth, causing them to surface dangerously quickly.
Previous evidence about sonars has been largely anecdotal, seeing only a statistical link between nearby naval exercises and mass strandings.
It has given rise to the theory that extremely loud sonars, especially in the low-frequency range, may disrupt whales' hearing, internal compass and the communications on which they depend for mating.