WASHINGTON — The mysterious stranding of about 100 melon-headed whales in a shallow Madagascar lagoon in 2008 set off a rapid international response — a few of the eight- to 10-foot marine mammals were rescued, necropsies conducted, a review panel formed.
Did they follow prey into the lagoon? Were they sick? Was it the weather, or chemical toxins?
The panel recently gave its best answer, and it is causing ripples of concern. For the first time, a rigorous scientific investigation has associated a mass whale stranding with a kind of sonar that is widely used to map the ocean floor, a finding that has set off alarms among energy companies and others who say the technology is critical to safe navigation of the planet’s waters.
The independent review panel appointed by the International Whaling Commission concluded Sept. 25 that a high-powered, ‘‘multi-beam echosounder system’’ (MBES) was ‘‘the most plausible and likely behavioral trigger’’ for the stranding. About 75 of the animals, which normally inhabit deep ocean waters, died.
A contractor for Exxon Mobil was using the sonar system — which sends ‘‘ping’’ sounds from a vessel toward the ocean floor — in a channel between Mozambique and Madagascar to determine where an oil and gas exploration rig might be safely constructed. Computers use the returning echo from the pulses of sound to map the ocean floor.
The panel of five scientists ‘‘systematically excluded or deemed highly unlikely’’ nearly every other possibility before settling on the use of the
MBES, which previously was considered relatively benign, the group’s report said.
‘‘The evidence seems clear to us that [the MBES] was pretty likely’’ the cause, said Brandon Southall, the panel’s chairman and a marine biologist at the University of California at Santa Cruz.
‘From my reading of that report, it’s not clear how they could have come to that conclusion.’
Southall said he hopes the report will cause governments, regulatory agencies, and private companies to ‘‘realize that some of the types of mapping sonars have the potential to cause reactions in marine mammals that can be detrimental.’’
Exxon Mobil, which helped select the panel and partly funded the rescue of some of the whales in 2008, rejects the conclusion, contending that the evidence is too flimsy for a determination that could have a far-reaching impact.
‘‘While Exxon Mobil is not accepting responsibility for the stranding in light of the uncertainties in the report, we did cooperate and provide funding for the response effort in 2008 and the review panel because we are working in Madagascar,’’ spokesman Patrick McGinn said.
Another skeptic is Larry Mayer, a professor at the University of New Hampshire’s Center for Coastal and Ocean Mapping. ‘‘From my reading of that report, it’s not clear how they could have come to that conclusion,’’ Mayer said. ‘‘Any of the other possible conclusions are just as likely.’’
The report could have significant consequences for US government agencies and others around the world that use the MBES to map ocean floors. ‘‘If it endangers the ability to use these sort of systems . . . it could lead to all kinds of dangerous downstream consequences.’’ Mayer said.
And Joseph Geraci, an adjunct professor of comparative medicine at the University of Maryland who has studied cetacean strandings for 40 years, said he was troubled by the strength of the language in the panel report.
‘‘I’m not sure on the basis of a single event where there are two activities that the words ‘most plausible cause’ are the right ones,’’ he said. ‘‘It’s only those three words that made me pay attention.’’
But Howard Rosenbaum, director of the Ocean Giants program of the New York-based Wildlife Conservation Society, hailed the panel for pushing the envelope on possible factors in the strandings and deaths of marine mammals.
Navy sonar has been implicated in harm to whales and dolphins, environmental groups contend.
A federal judge last month ordered federal biologists to reconsider permits that could allow the Navy to kill or disrupt marine mammals during antisubmarine warfare exercises off the coast of the Pacific Northwest.
But in 2008, the Supreme Court allowed similar drills off Southern California to be held without protections for marine mammals.
Other environmental groups are skirmishing with energy companies over the use of ‘‘seismic air guns,’’ devices that send much louder blasts of compressed air toward the ocean floor to help find oil and gas trapped below.
The noise from an MBES is better compared to an industrial-size version of the fish-finders widely used by recreational anglers, Southall said. That is part of the reason his panel’s finding is so controversial: the pinging sound is used so widely around the globe, in so many forms, that most involved have considered it relatively harmless.
But it may be time to adjust that thinking, Southall said. He acknowledged that no study of whale strandings will achieve the kind of certainty that Exxon Mobil and others would like, but said this one provided a rare opportunity to consider a wide range of possibilities and disprove them.