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 News :. Bends may be culprit in whale strandings

Gas filled bubbles discovered in the damaged tissues of stranded whales provide new evidence that military sonar can give whales the bends.

An international team of scientists led by Dr P. D. Jepson of the Zoological Society of London report their findings in today's issue of the journal Nature.

The researchers took tissue samples from whales which had died after becoming stranded around the time and place of an international naval exercise in the Canary Islands in September 2002: 'Strandings began about four hours after the onset of mid-frequency sonar activity,' write Jepson and colleagues.

The researchers found gas bubbles in blood vessels and haemorrhages in vital organs - especially the livers - of the animals. Finding no evidence of disease-causing bacteria in the animals, their theory is that the animals suffered a kind of decompression sickness of the kind known to occur in divers using scuba equipment (which has a high amount of nitrogen dissolved under pressure). If divers rise to the surface too quickly, bubbles of nitrogen gas can form in the blood and cause dangerous blockages. This condition was not previously thought to occur in whales and other cetations.

'The cavitary lesions described here are new to marine-mammal pathology,' write Jepson and team.

The theory that sonar can give whales the bends is hotly debated. One possible explanation for how sonar causes gas bubbles is that it startles the animals and causes them to surface too quickly. Another theory is that the sonar may directly cause already-existing tiny nitrogen bubbles in the blood, which would normally be filtered out, to expand and become a problem.

'It's very interesting stuff,' comments Jeff Weir, Executive Director of the Dolphin Research Institute.

However he is cautious about 'jumping to conclusions' as to whether or not strandings have increased because of sonar: 'The difficulty we have with this article is it doesn't necessarily show what's in the bubbles,' Weir told ABC Science Online. 'There isn't anything in here that says these bubbles are nitrogen. They could be oxygen or carbon dioxide.'

'There is certainly a great deal of reason to be concerned,' he says. 'However it would be silly to draw an absolute cause and effect. It's something that needs further investigation.'

Jepson and colleagues agree and say that such tissue samples should in future be compared with samples from control whales not exposed to sonar. Nevertheless, they conclude: 'In a wider conservation sense, our findings need to be taken into account in considering the regulation and limitation of the adverse impact of anthropogenic sonar on citations.'

According to Dr Michale Noad of the University of Queensland, the question is less whether sonar causes problems, but how.

'It's highly likely that sonar is causing damage to marine mammals but what we don't know is whether it's through a direct or indirect effect,' says Noad, who does basic research on the behaviour of marine mammals funded by the US Office of Naval Research. 'Whichever way, it does seem to be linked to the use of military sonar.'

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