A new study offers evidence to support the theory that beaked whales get the bends when they surface rapidly, possibly after being startled by naval sonar.
The report could help scientists understand why beaked whales appear to be more vulnerable to the potentially harmful effects of sonar than other marine mammals.
Together with other studies, the results may also help scientists and regulators think of how navies could adjust their sonar use during training to prevent beaked whale strandings and deaths.
"It provides more evidence that beaked whales that are being found dead in association with naval sonar activities are likely to be getting decompression sickness," said Robin Baird, a marine biologist at Cascadia Research Collective and one of the report's authors.
The study, published online this week in the journal Respiratory Physiology and Neurobiology, uses data gathered from three species in the beaked whale family. Two of the species, Cuvier's and Blainville's, were observed in Hawaii waters. The third, northern bottlenose whales, were studied off the coast of Nova Scotia.
Military ships use mid-frequency active sonar by firing bursts of sound through the water and listening for an echo off a vessel's hull. The technology has become increasingly important to the U.S. navy as other countries, including China, have added quieter, harder-to-detect diesel-powered submarines to their fleets.
In 2000, several beaked whales washed ashore with bleeding around their brains and ears during exercises in the Bahamas. Scientists believe the bleeding may have been caused by bubbles that formed in the whales' bloodstreams when they surfaced more quickly than normal.
U.S. navy funds research, but resists restrictions
The navy has since agreed to adopt some measures to protect whales, such as having ships turn off their sonar when sailors spot marine mammals nearby. But it has strongly resisted many more stringent restrictions, saying there isn't enough scientific evidence to require them.
The navy is also pushing for more research in the area, budgeting $26 million US per year over the next five years to understand how marine mammals hear and how sound affects them.
The new beaked whale study was also funded in part by the Office of Naval Research.
The study concludes the animals are at higher risk of suffering the bends because they live with extremely high levels of nitrogen in their blood and body tissues.
Nitrogen builds in mammals, including humans, when they dive. Beaked whales likely accumulate such high levels of nitrogen because they repeatedly dive to great depths — sometimes almost 1,500 metres below the surface — for long periods of over an hour.
When mammals ascend slowly, the nitrogen in their blood stays dissolved. But when they surface too quickly, the nitrogen comes out in bubbles. This gives them a form of decompression sickness, or the bends, a condition also known to scuba divers.
Since 1960, there have been 41 cases of mass strandings of Cuvier's beaked whales around the world, according to a 2006 report in the Journal of Cetacean Research and Management.
Some of the strandings happened at the same time as naval sonar exercises, including in Greece in 1996, the Bahamas in 2000 and the Canary Islands in 2002.