How does marine noise pollution affect whales and dolphins?

Water is a much denser medium than air, presenting a range of challenges that terrestrial organisms don't have to face. This has forced underwater life to adapt their senses to take advantage of the different properties of the marine environment.

Compared to life on land, sight is less important as a sense because light is quickly absorbed by water meaning that below 100 metres 99% of all light entering the water has been absorbed. This has led to marine organisms developing their other senses, of which sound is one of the most important.

As water molecules are clustered more tightly together than the gases in air, sound can travel more rapidly and over greater distances. Whales and dolphins in particular make use of this feature for echolocation and vocal communication.

Some species, such as the humpback whale, can produce calls that can be heard hundreds or even thousands of kilometres from the whale which made them. While these calls can reach volumes of over 180 decibels, louder than a jet taking off, they are directed away from the body and are only temporary.

This is unlike anthropogenic noise pollution. Chronic sounds, such as the noise of a super tanker cargo vessel as it moves through the ocean, can last for over an hour and reach volumes of up to 200 decibels.

'Noise pollution is everywhere, but it's something of a hidden polluter as it's not necessarily that obvious to us,' Richard explains. 'While we're used to living with background noise, it's not that simple for cetaceans and many other marine organisms.'

'Sound is absolutely critical for many of these species, and so it presents a massive problem for them. It affects almost everything they do, because the use of sound has become such an important sense for these species that has been developed over millions of years.'

Anthropogenic noise can change a whale's behaviour, such as causing the marine mammals to feed less or to produce fewer calls. Shipping noise also cause whales to become stressed, with the build-up of stress related chemicals linked to growth suppression, lower fertility and poor immune system function.

Acute noise pollution can prove even more damaging. Military sonar can reach volumes of more than 200 decibels, while seismic air guns can reach up to 250 decibels - louder than the largest rocket ever built taking off.

At this level, the force of the vibrations is enough to kill zooplankton, with the number of dead plankton tripling within the surrounding kilometre of ocean. While larger animals are not killed directly by the sound waves, anti-submarine sonar has been linked to whale deaths caused by strandings and decompression sickness, which is caused by the animals surfacing too quickly.

The recent study reveals how cetaceans are affected in the moments immediately after a high-intensity sound burst. 

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