Scientists have long known that some fin whales strand and die in agony after exposure to naval sonar, and now they know why: gigantic marine mammals suffer from decompression sickness, as do divers.
At first glance, the explanation exposed today by 21 experts in the Journal of the Royal Society Procedures B seems implausible.
Millions of years of evolution have turned whales into perfectly calibrated diving machines that plunge miles below the surface for hours on end, seeking food in the dark depths.
The heart rate decreases, blood flow is restricted, oxygen is conserved. So how could the most complete deep-sea diver in the ocean do away with the nitrogen bubbles poisoning his veins, like a dive novice soaring too fast to the surface?
Short answer: beaked whales – especially a species known as Cuviers – are really scared.
"In the presence of the sonar they are stressed and swim vigorously away from the sound source, changing their diving pattern," said lead author Yara Bernaldo de Quiros, a researcher at the Institute of Animal Health at the University of Las Palmas de Gran Canaria , Spain.
"The stress response, in other words, nullifies the response of the dive, which causes animals to accumulate nitrogen," she added. "It's like a shot of adrenaline."
One kind of sonar, in particular, unbalances these whales.
"ATYPICAL" MASS GRASSES
Developed in the 1950s to detect submarines, active mid-frequency sonar (MFAS) is used today in naval patrols and drills, especially by the United States and its NATO allies.
Beginning around 1960, ships began to emit underwater signals in a range of about 5 kHz (kHz).
That's when the mass of beaked whales, especially in the Mediterranean, began.
Between 1960 and 2004, 121 of these so-called "atypical" mass strandings occurred, with at least 40 closely linked at the time and place with naval activities.
These were not single strandings of old or sick animals, nor were they mass stranded like last November in New Zealand, when more than 200 pilot whales stranded together.
Instead, a handful or more whale whales ashore within a day or two, and no more than a few tens of miles away.
The most deadly episode in 2002 saw 14 people arrested during a 36-hour period in the Canary Islands during a NATO naval exercise.
"A few hours after the sonar was implanted, the animals began to appear on the beach," said Bernaldo de Quiros.
Externally, the whales showed no signs of disease or damage: they had normal body weight and no lesions or skin infections.
Internally, it was another story. Nitrogen gas bubbles filled their veins and their brains were devastated by bleeding.
Autopsies also revealed damage to other organs, as well as to the spinal cord and central nervous system.
CANARY ISLAND MORATORIUM
As with altitude sickness, the reactions – in humans and probably in whales – the blood bubbles of nitrogen vary in type and intensity.
A 2003 study in Nature on the possible link between sonar and whale deaths led Spain to ban such naval exercises in the Canary Islands in 2004.
"Until then, the Canaries were a hotspot for this type of atypical stranding," said Bernaldo de Quiros. "Since the moratorium, none have occurred."
The authors have called for similar prohibitions to be extended to other regions where whales are known to be at risk.
The Cuvier's beaked whale grows to seven meters and dines mainly on squids and deep-sea fish. His upturned mouth gives the impression of a permanent smile.
The whale is listed as "vulnerable" on the IUCN Red List of Endangered Species, and is believed to have a global population of 5,000 to 7,000.
Other threats include ship strikes, ocean pollution and changes in habitats caused by climate change.