Are Whales Beaching Themselves Stressed By Sonar?

Imagine if accidentally hearing a private conversation could threaten your survival. This is what scientists suspect is happening to beaked whales when they strand themselves on land after hearing the underwater sounds of military sonar.

Beaked whales are fantastic divers. Cuvier’s beaked whales can dive almost three kilometres in search of squid to eat. That’s about five-and-a-half CN Towers!  Military sonar can interrupt a whale's dive for food. If this occurs they will sometimes wash ashore with signs of decompression sickness. Human divers suffer from decompression sickness when they surface too quickly. What is the connection between beaked whale strandings and military sonar? And why would beaked whales suffer from decompression sickness if they are such excellent divers? Let’s look into this to understand better.

Sound underwater - a different kind of wave

Sound moves as a wave underwater, but not like waves that crash onto the beach. A vibrating sound source starts a wave of compression and expansion of water molecules. It’s similar to how sound moves through air. Sound waves travel very well underwater. In fact, the speed of sound is almost five times faster in seawater versus air.  Changes in ocean temperature, depth and salinity change how fast and how far sound travels underwater. 

What is Sonar?

Sonar is an acronym for sound navigation and ranging. It is a tool to “see” underwater using sound waves. There are two main types of sonar: passive and active.

Passive sonar devices are just receivers that listen for underwater sounds. They are used to find whales and dolphins that make noises to communicate. The military also uses passive sonar to listen for submarines.

Active sonar devices have a sound source, called a projector, in addition to a receiver. The projector sends sound waves into the water. The sound waves reflect off underwater objects and return to the receiver. Active sonar measures precisely how long it takes the sound to return to the receiver. It also measures changes to the sound quality. These measurements can identify objects such as the ocean floor, shipwrecks, fish and whales. Active sonar also determines how far away the objects are. 

There are many different types of active sonars. A Mid-frequency active sonar (MFAS) uses sound waves with a frequency between 1 kHz and 10 kHz. (kHz is the abbreviation for kiloHertz and one kHz is 1000 wave cycles per second.) Naval forces around the world use MFAS for submarine detection and training exercises. 

How is MFAS connected to beaked whale strandings?

Beaked whales can hear the frequencies used for MFAS. They also spend a lot of time diving to the same ocean depths used for MFAS. In the mid-1980s, scientists studied mass stranding events (MSEs) of Cuvier’s beaked whales. They found that MSEs happened at the same time as naval training exercises around the Canary Islands in Spain. This association was later confirmed in Greece and the Bahamas. MSEs are when two or more animals wash ashore at approximately the same place and time. Naval MFAS is also associated with strandings of individual beaked whales.

Why are beaked whales stranding?

Sometimes, stranded whales survive if humans quickly roll them back into the ocean. When they don’t survive, scientists can dissect them to try to figure out what caused their stranding and death. The beaked whales that died after MFAS exposure had gas bubbles where they did not belong, in their veins and organs. This looked exactly like decompression sickness in humans. 

What is decompression sickness? During a deep dive, the weight of water causes a lot of pressure on the body. This pressure forces extra breathed gas (mostly nitrogen) to dissolve in blood. Human divers must surface slowly to give their bodies time to exhale the extra gas. Surfacing too quickly traps gas bubbles in blood and tissues. Decompression sickness is also called the “bends” because people bend over from the pain.

Why would beaked whales get the bends?

Decompression sickness, also called the bends, is caused when a diving animal or human surfaces too quickly. When underwater, pressure is much higher than at the surface. As a human or animal surfaces, those gases under a smaller pressure want to take up more space. If surfacing slowly, the gas diluted in blood and tissues are evacuated safely. But if those gases are not given enough time to be liberated in those safe ways, they can go into places they can cause problems, such as the articulations, lungs and in some deadly cases, the nervous system and brain.

Decompression sickness in beaked whales was a surprising discovery. Marine animals that dive have adaptations to avoid this life-threatening condition. They adapt their behaviour to swim to the surface slowly, just like human divers. They also adapt some of their body functions (physiology) such as heart rate, blood flow, and gas exchange in the lungs. These adaptations seem to fail in some beaked whales that are exposed to MFAS. Scientists think that MFAS can cause a lot of stress to beaked whales. This stress changes both their diving behaviour and diving physiology. Sonar-stressed whales don’t decompress normally and can strand themselves.

This story is complicated. Sometimes, beaked whales are exposed to MFAS and appear to be just fine. And sometimes, beaked whales strand themselves with no MFAS exposure. Different populations of whales - and even different individual whales in one population - can react differently to MFAS. Marine mammal scientists are working to solve this mystery.

What is being done to protect beaked whales?

To protect beaked whales and other marine mammals, the Canary Islands banned MFAS in 2004. Not a single MSE has been observed in the area since. In 2015, the U.S. Navy agreed to limit MFAS use in parts of the Pacific Ocean that are important habitats for marine mammals. However, the U.S. Navy was allowed to relax some of these restrictions in 2020. 

Marine mammals may be accidental victims of military training. In the wrong place at the wrong time, hearing the wrong conversation.