Whales blasted by military sonar appear to die of the bends. The finding suggests the use of sound waves to detect submarines under the sea might need to be restricted.
Scientists from Spain and Britain have uncovered the first evidence that cetaceans suffer from the formation of nitrogen bubbles in their vital organs. This is a classic symptom of the decompression sickness suffered by divers who surface too quickly, and can be fatal.
Lesions caused by bubbles were found in 14 beaked whales stranded in the Canary Islands after sonars were used in a major international naval exercise on 24 September 2002. 鈥淥ur findings suggest that naval sonar could be killing whales,鈥� says Antonio Fern谩ndez, a veterinary pathologist from the University of Las Palmas de Gran Canaria.
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The exercise was hosted by Spain and involved naval forces from US, the UK and other European countries. Whales started being washed up four hours after sonars were switched on.
Post mortem examinations revealed bubbles in blood vessels in their brains and livers. Clots of fat were also discovered in blood in their brains, livers, lungs, kidney and other tissues, along with widespread haemorrhaging. These are all characteristic of the acute trauma caused by decompression sickness in humans.
Startle response
Fern谩ndez says the bubbles could have formed because the deep-diving whales, startled by the sonar, surfaced too quickly or changed their diving patterns. This would have caused the nitrogen accumulated in their tissues to come out of solution and create bubbles large enough to block arteries.
Another possibility is that the sound waves increased bubble formation as they passed through tissue. Mathematical modelling has previously suggested that sonar could interfere with the ability of tissue to safely store nitrogen under pressure (麻豆传媒 print edition, 15 December 2001).
Supportive evidence comes from an analysis by London Zoo of 1401 dolphins and porpoises stranded around Britain between 1992 and 2003. Seven, including three deep-diving Risso鈥檚 dolphins, were found to have bubbles of gas in their blood and livers. The researchers say that the lesions found in the cetaceans in Spain and Britain are new to marine mammal pathology.
Marine conservationists are worried. 鈥淭here now ought to be a complete reevaluation of the threat to marine life from noise,鈥� says Mark Simmonds, director of science at the Whale and Dolphin Conservation Society.
In August, the US Navy was barred by a federal judge in California from introducing a new high-intensity sonar system in peacetime because of the risks it might pose to cetaceans.
Journal reference: Nature (vol 425, p 575)