Sonar oil search blamed for Madagascar whale deaths

Prospectors searching for undersea oil with a noisy high-frequency sonar probably caused the deaths of 75 melon-headed whales that stranded themselves off Madagascar, experts have concluded.

An independent panel of scientists found that sonar surveying by ExxonMobil in late May 2008 led to the sudden displacement of about 100 whales, of which at least three-quarters died.

"This is the first known such marine mammal mass stranding event closely associated with relatively high frequency mapping sonar systems," the report released on Thursday by the International Whaling Commission concluded.

"Earlier such events may have been undetected because detailed inquiries were not conducted."

The researchers described a "highly unusual event" in which whales became stranded in shallow waters in the Loza Lagoon system in northwest Madagascar in May and June of 2008.

The culprit was named as a high-power, 12 kilohertz multi-beam echo-sounder system, or MBES, operated by an ExxonMobil vessel on May 29 about 65 kilometres offshore from the first known stranding.

The five-member scientific review panel said the vessel's MBES was "the most plausible and likely behavioural trigger for the animals initially entering the lagoon system".