When about 100 whales ended up in the shallow waters of Madagascar's Loza Lagoon system in May and June 2008, scientists suspected that underwater sonar played a role.
Now, an independent panel of scientists has confirmed that there is a link between the sonar, used by Exxon Mobil in late May 2008 to map the ocean floor for oil, and the death of three-quarters of the displaced melon-headed whales.
When marine animals end up in areas where they wouldn't naturally swim, such as shallow waters or on shore, it is called stranding. Stranding refers to any displaced marine animal, whether found dead or alive.
This is not the first study to confirm a link between sonar and whale stranding. When about 150 melon-headed whales became stranded in Hanalei Bay off the Hawaiian island of Kaua'i in 2004, the National Oceanographic and Atmospheric Administration (NOAA) linked the event with U.S. Navy sonar.
When NOAA released its report in 2006, Robert Brownell, a cetacean biologist at NOAA's Southwest Fisheries Science Center in Pacific Grove, Calif., explained that the organization was not entirely sure why sonar causes whales to change their behavior. He hypothesized that the noise from the sonar "forms an acoustic barrier, and they want to escape."
Others argued that the lunar cycle caused the strandings. Bromwell investigated 21 similar mass stranding events, determined their timing within the lunar cycle, and reported that there was no link.
In the Madagascar case, the sonar in question came from a 12 kilohertz multibeam echosounder system (MBES) that ExxonMobil used on May 29. The first stranded whale turned up about 65 kilometers from the area where ExxonMobil was mapping the ocean floor.
It was the most "plausible and likely behavioral trigger for the animals initially entering the lagoon system," the report's five authors wrote.
In 2006, the U.S. Navy refuted Bromwell's paper connecting the Hawaii stranding event with its ships sonar. This week, an ExxonMobil representative denied the link between the company's sonar and the Madagascar stranding event, reports the Agence France-Presse.
"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 told AFP in an email.
Matt Huelsenbeck, of the advocacy group Oceana, says the mounting evidence confirms the danger that sonar poses to marine animals.
"Now we are seeing that (sonar) disturbance is an even more important thing than was previously assumed," he told the AFP.
"You don't have to kill the animal outright. If you are scaring it into a situation where it can die, that is just as serious of an issue."