New research explains how naval sonar can cause whales to die from decompression sickness.

Although scientists have known for a while that naval sonar can cause beached whales until recently they were unable to explain why. According to research published in The Royal Society’s Proceedings B journal, beaked whales, in particular, are so distressed by the sonar that they will beach themselves or dive to extreme depths resulting in decompression sickness.

According to the paper, mass stranding events were extremely rare before the 1960s. Although naval mid-frequency active sonar was developed and first used in the 1950s mass stranding events were not observed until naval sonar switched to lower frequencies starting in the 1960s.

Between 1960 and 2004, 121 beached whale mass stranding events were recorded and at least 37 of the events “were either strongly correlated in time and space with naval activities, or occurred in naval training areas where US Navy and/or NATO fleets were based or operated.”

Autopsies of the dead whales found “microvascular haemorrhages” and gas and fat bubbles in the blood vessels and vital organs, all symptoms consistent with decompression sickness.

Decompression sickness or “the bends” is most well-known for striking human divers when they ascend and undergo depressurization too fast resulting in the same bubbles seen in the whales. It can be fatal in humans, but the findings confounded scientists. How could a marine animal adapted to diving great depths for hours at a time suffer from decompression sickness?

The answer was stress. According to the paper, the sonar is so distressing to the whales that they will do anything to get away from it including beach themselves and dive to extreme depths. The stress of the sonar causes the whales to dive to depths and in speeds beyond their physical capacity, resulting in decompression sickness.

While the exact physical changes occurring in the whales allowing decompression sickness to set in are unknown, researchers suggest that most likely the animals are experiencing a “flight or fight” response to escape the sonar. Then the flight response overrides the dive response and when the physiological changes are severe enough especially in prone individual animals, it can lead to death in the animal.

Beached Whales in the Canary Islands

In one of the most deadly single examples of mass stranding events, 14 whales beached themselves in 2002 over a 36-hour period in the Canary Islands during a NATO naval exercise.

“Within a few hours of the sonar being deployed, the animals started showing up on the beach,” Bernaldo de Quiros, lead study author and a researcher at the Institute of Animal Health at the University of Las Palmas de Gran Canaria, Spain, told AFP.

Even more mystifying at the time, the whales showed no signs of infection, disease, damage or had any wounds. But the autopsies would show signs of decompression sickness with their brains haemorrhaging blood and gas bubble in their veins.

The Canary Islands soon after banned naval exercises around the island in 2004 after another study published in 2003 suggested a possible link between naval exercises and the beached whales.

“Up until then, the Canaries were a hotspot for this kind of ‘atypical’ strandings,” said Bernaldo de Quiros. “Since the moratorium, none have occurred.”

Scientists have urged other countries to follow suit and ban naval sonar in areas where at-risk whales are known to congregate.

Other Causes of Beached Whales

Sonar is not thought to be the only cause of beached whales as researchers also suspect that navigational errors caused by following prey into too shallow waters could be a culprit.

Another cause could be healthy whales following sick whales into shore and then getting stranded themselves as they struggle to remain near the dying whale. Whales and dolphins are known to have strong social bonds.

Seaquakes and other natural causes of loud noises could also cause similar flight or fight responses in whales akin to naval sonar.

New Zealand has one of the highest annual rates of stranded whales and dolphins. Project Jonah, which works to save whales in New Zealand, reports that one of the most common causes of mass strandings is when one or two whales gets stranded then sends out distress signals to their pod. The pod will then attempt to aid the distressed whale and can get caught inshore when the tide recedes.

You can learn more about what to do in case you see a beached whale from Project Jonah here.