For decades scientists have known that naval sonar can have a detrimental impact on marine life. The sound is so intense that marine mammals will swim hundreds of miles, dive incredibly deep, or even beach themselves in an attempt to flee the unbearable din. Beaked whales, in particular, tend to strand themselves in response to sonar. A new review paper, published in the Proceedings of the Royal Society B, tells us why.  

Beaked whale strandings were a rare occurrence before the 1960s, but then a kind of sonar called mid-frequency active sonar (MFAS) started to be used by the Navy to detect submarines. MFAS produces an incredibly loud sound, which, if you're a marine creature, can be very alarming.

So, it’s perhaps not all that surprising that from 1960 to 2004, more and more beaked whales were found washed up on beaches, with a total of 121 mass stranding events reported. It first happened in France, followed by Italy, the US, and the Bahamas. Various species of beaked whale were found stranded, but the majority were Cuvier’s beaked whales. Many of the events happened in areas where US Navy and NATO training exercises were taking place, and were described as ‘atypical’.

^{Rare footage of Cuvier's beaked whales swimming off the coast of Mexico}

Typical mass strandings involve two or more animals beaching themselves while alive at roughly the same time and place. Exactly why these events happen isn’t really known – it could be due to sickness, disorientation, or being chased into shallow water by a predator. We’re still not quite sure. Atypical mass stranding events, on the other hand, involve two or more whales washing up in six days no more than 74 kilometers (46 miles) apart. These kinds of strandings are usually associated with sound.

So how exactly does sonar sound cause the whales to beach themselves? According to the new paper, which summarises what was discussed at a 2017 meeting of beaked whale experts in the Canary Islands, the sonar distresses beaked whales to the extent that they end up with nitrogen bubbles in their blood. This can cause hemorrhaging and damage to their vital organs. If you’re a diver, you’ll know this kind of nitrogen poisoning as decompression sickness, aka the bends – a rare but life-threatening effect of coming up to the surface too quickly.

But how can an animal perfectly adapted to life underwater, with the ability to dive 2,000 meters (6,500 feet), end up with the same problem as a panicky scuba diver?

"In the presence of sonar they are stressed and swim vigorously away from the sound source, changing their diving pattern," lead author Yara Bernaldo de Quiros told AFP.

"The stress response, in other words, overrides the diving response, which makes the animals accumulate nitrogen. It's like an adrenalin shot."

The team points out that the response can vary from whale to whale, but it explains why atypical strandings often occur where naval sonar has been deployed. The conclusions are drawn from autopsies of dead whales, although a handful of animals were killed by other threats inflicted by humans, such as collisions with ships or entanglement in fishing nets, as well as disease.

The authors note that to mitigate the impacts of sonar on beaked whales, we must ban its use in areas where they're found. A moratorium on the use of MFAS around the Canary Islands in 2004 shows just how well this works – no atypical strandings have been seen since. The researchers urge other countries where sonar is deployed, such as the US, Greece, Italy, and Japan, to follow suit.