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".

The sounds would have been "clearly audible over many hundreds of square kilometres of melon-headed whale deep-water habitat areas".

The report said that seismic air guns, long opposed by environmental groups for the potential harm they can caused to marine life, were not to blame for the event.

"They used the multi-beam echo sounder first. That scared the animals into the lagoon and then the air guns were used afterward," marine scientist Matt Huelsenbeck of the advocacy group Oceana said.

"So that is not to say that air guns would not have caused it had they been used first. They are even louder than the multi-beam echo sounder."

A spokesman for ExxonMobil said the company disagreed with the findings.

"ExxonMobil believes the panel's finding about the multi-beam echo sounder is unjustified due to the lack of certainty of information and observations recorded during the response efforts in 2008," spokesman Patrick McGinn said.

He added that observers employed by the Madagascan government and the oil giant "were on board the vessel and did not observe any whales in the area".

Nevertheless, since 2008 the company had developed a "detailed risk assessment process" that took into account the local characteristics and the potential for harm to marine life, McGinn said.

High-frequency echo sounders are often used to map the ocean floor and can be dangerous to smaller whales and dolphins, while the air-gun blasts that follow are lower frequency and may endanger large whales, according to Oceana.

The evidence was compiled by the International Whaling Commission, the US Marine Mammal Commission, the US National Oceanic and Atmospheric Administration, the US Bureau of Ocean Energy Management, ExxonMobil, the International Fund for Animal Welfare, the Wildlife Conservation Society and the government of Madagascar.