In the marine world animals like whales and dolphins rely heavily on sound for communication, foraging and navigation. But increased noise pollution from human activities are impacting these behaviours in a big way.
On the surface it is hard to imagine that the ocean could be very noisy but in reality it is filled with a symphony of sounds that are vitally important for marine life. Below 1000m light fails to illuminate anything and so the deep ocean, the largest habitat on the planet, is also in complete darkness. That is why sound becomes extremely important for animals such as whales and dolphins that rely almost completely on it for communication, foraging and navigation. But these animals are now being forced to share their world with some noisy neighbours that are spoiling their way of life. When you think of humans polluting the oceans the images that probably come to mind are turtles eating plastic bags, seabirds covered in oil or a shark tangled up in discarded fishing gear. But marine noise pollution from increased shipping activity, construction and sonar is also causing major problems for sea life. Whilst it may be invisible to us on the surface the damage its causing below is becoming more and more obvious.
Sound in the sea
The way sound moves through water is considerably different from how it travels through the air. Sound waves are essentially energy vibrations through particles, in a gas the particles are spaced out but in a liquid those particles are densely packed together. As a result sound travels through the ocean at 1500 metres per second which is over 4 times faster than the speed it goes through the air. For the same reason sounds also travels further distances and are considerably louder because the waves carry more energy. However in contrast light struggles to penetrate the oceans. Water is a great absorber of light wavelengths, all colours apart from blue are absorbed quickly, which not only gives the ocean its colour but makes it very dark at depths below 200m. Below 1000m no light can reach at all and the sea becomes a very dark place, however it is still alive with an abundance of sounds.
Due to the properties of light and sound in water marine creatures like whales and dolphins have little need for expensive ocular equipment and often have very rudimentary eyes. Instead they have evolved some of the most complex and effective acoustic anatomy and behaviours in the animal kingdom. Whereas sound hits our ears at angles in cetaceans it hits head on, which would normally make it hard to determine the direction the sound is coming from, so they have specially adapted pockets of air within their ears to compensate. The structure of their ears is also rearranged differently, rather than the ear drum and bones being inside their skulls like humans, they are kept more externally in giant bony bulbous shells. These changes don’t just allow them to accurately locate sounds but also experience a much greater range of pitches. Humans can register frequencies as high as 20,000 Hz and dogs can go as high as 44,000 Hz, but dolphins put us both to shame and can register up to 160,000 Hz. On the other end of the scale blue whales can hear frequencies as low as just 14 HZ.
An oceanic opera
It’s not just cetacean’s ability to register a wide range of frequencies that is impressive, they are also masters of generating complex and varies noises themselves. Baleen, aka non-toothed, whales have some of the most powerful voices in nature capable of reaching 180 decibels which is the equivalent of a jumbo jet. Their songs are legendary and can carry hundreds of kilometres across entire ocean basins. This is partly because their low frequency calls encounter less interference but also because they strategically direct them though deep underwater channels. It is thought that one of the reasons behind their singing is navigation, but primarily it is for communication especially between potential mates. In humpback whales it is only the males who sing and they have been known to vary their highly complex tunes in attempts to outcompete each other. However there is still much we don’t know about their singing and the reasons behind it.
Communicating with clicks
While large baleen whales sing to one another from across the ocean, dolphins and toothed whales use clicks and whistles to communicate with each other over shorter distance. Dolphins in particular are extremely intelligent and social creatures and their ‘language’ of clicking has been compared by scientists to human speech. Not only do bottlenose dolphin individuals have distinct sounds but they communicate through questions and answers and avoid interrupting their peers. A recent study into short-finned pilot whales in Hawaii has even shown that different groups can have their own dialects.
However the clicking is not just social it is also used for echolocation to find prey. For this a special burst of high frequency clicks is fired off using a specialised organ in the head called the melon that concentrates and amplifies the pulses. The sound then bounces off fish, who have air filled swim bladders to control buoyancy, creating an echo which the dolphins then pick up. The time it takes for echoes to be picked up allows the dolphins to determine the distance to their prey. This is the behaviour that inspired the creation of sonar technology.
Unfortunately as human activity has increased in the oceans in the last century so has the noise pollution we create via a number of different factors. Firstly the increase of shipping activity across the seas for transporting goods has been exponential. Boats can be noisy enough for people on board them but underneath the noise of engines for marine life is amplified by the water. We are also building more and more structures in the ocean than ever including wind farms, oil rigs and bridges. Not only does the construction physically damage ecosystems but creates a lot of noise as structures are drilled into the seafloor. As well as this we generate high levels of noise from sonar and radar which is used not just for navigation but also heavily by the military and mining companies to explore the seafloor for valuable minerals or enemy submarines. Finally major ports and harbours across the world have become epicentres for chronic noise due to high levels of activity.
It is only recently that the effects of marine noise pollution have been properly realised and researched. It is an issue that as humans we rarely consider because it is something we are rarely affected by on the surface. But whilst the problem is invisible to us it is one of the most devastating and far reaching forms of pollution. Animals such as cetaceans who experience the world through sound are being constantly bombarded by loud and chronic noises that they are not prepared for. As we have already seen many of their crucial behaviours depend on their sensitivity to acoustic signals and when they can no longer hear these signals or are confused by new ones their behaviours are disrupted. These changes can be so drastic that they can unfortunately often lead to the deaths of these magnificent creatures.
Struggling to speak
Noise pollution interferes heavily with a cetacean’s ability to communicate with other members of its species. Killer whales in the Haro Straight, one of the nosiest shipping areas in the pacific, have been shown to increase the volume of their own calls in response to interference. Whilst this may seem like a good solution to the problem increasing their volume is very energetically expensive and stressful for them. Shipping noise has also been shown to separate calves from their mothers which is often fatal. In response to louder noises bottlenose dolphins have also been shown to reduce the complexity of their dialogue reducing the information that can be exchanged. Helen Baily from the University of Maryland says “It’s kind of like trying to answer a question in a noisy bar and after repeated attempts to be heard, you just give the shortest answer possible”.
It’s not just communication that is effected by interference of signals, as we have seen toothed whales and dolphins rely heavily on echolocation to hunt. If they can’t locate their food then they will struggle to gain enough energy to survive. This problem is made worse by depleted stocks of prey species due to overfishing by humans. The killer whales from the Haro Straight not only struggle to locate the Chinook salmon they love to feed on but populations of the fish have crashed in the area meaning starvation is one of the biggest threats they now face.
Hard of hearing
Chronic exposure to noise or extremely loud explosive sounds, like that from construction work, can also cause temporary or permanent hearing loss. Anyone who has ever been to watch live music loud will understand how hearing can be effected for a short period afterwards. It is the same for whales and dolphins except it can go on for much longer and whilst hearing is compromised so too are those vital behaviours they depend on. Cetaceans can also become permanently deaf if their ears are damaged too severely to recover. A deaf whale is comparable to a blind man that is no longer able to see the world around them. The difference is they have no other real senses to fall back on. Deafness is also one of the biggest causes of collisions between whales and boats as the whales can’t hear them coming.
Marine noise pollution is also linked to mass beach stranding events, such as Hamlin Bay in Australia or Stewart Island in New Zealand both last year, where hundreds of animals can die at once. It is believed the sonar noise of military activity, often training sessions in coastal areas, effects navigation and disorients whales and dolphins into running aground onto beaches. There is also some evidence to suggest that the victims are already dead from noise pollution before they wash up. Postmortems carried out on stranded whales reveal that they can die from the bends, a disease associated with scuba diving in humans, as they are forced to rapidly resurface to escape the noise. This is something that would never happen naturally and as a result the U.S. military have lost major court cases over their use of sonar. However it is hard to regulate and still happens across the world.
Although we have mainly focused on cetaceans they are not the only marine life that are effected by marine noise pollution. Pinnipeds, a group containing seals, sea otters and sea lions, whilst mainly visual hunters have also been shown to use echolocation to forage for food on the seafloor. They can also communicate with each other via sound, although not as well as cetaceans. As well as this they have been known to get stranded like cetaceans due to military sonar, although it is much rarer. Whilst research has focused on marine mammals like cetaceans and pinnipeds it is now also believed that over 800 species of fish also produce sound and can be majorly effected by noise.
Research has shown that with increased marine noise pollution in both volume and frequency fish will swim faster, dive deeper and change direction more quickly all of which are very energetically expensive. They are also less likely to be able to successfully forage for food and are more likely to be predated upon themselves. A ground breaking study has even found that corals can be effected. Their larvae use sound to detect healthy reefs to live on and disorientation by noise pollution can reduce the number that successfully settle on reefs. On top of all this ocean acidification caused by rising atmospheric CO2 increases the distance sound can travel through the sea meaning noise pollution is likely to become even worse in the future.
To sum up
The major increase in marine noise pollution over the last century is creating widespread problems for a majority of marine life, but especially cetaceans who rely heavily on sound to explore the world around them. It is just one of many ways we are damaging our oceans including rising temperatures, ocean acidification, plastic pollution and overfishing. But what is perhaps most damaging about this form of pollution is not how widespread and invasive it is but that we can’t see it happening. How many times have you been confronted with a loud noise and said “I can’t hear myself think”? Just for a minute imagine what it would be like to have to live like that constantly. For whales and dolphins not only can they not think but they can’t talk to each other, they can’t see where they’re going and they can’t find food.
However there is hope, unlike any other form of pollution the effects of marine noise are completely reversible. If we can reduce the amount of noise we put into the ocean then all of the issues highlighted can be avoided. It is also not that hard to achieve. If we can design quitter engines, create more ship free areas, reduce the use of sonar by the military and better plan ocean construction to avoid wildlife then the issue will be majorly alleviated. But we can’t wait too long we must start making changes now if they are to be effective. If we can start sharing the ocean rather than invading it we can prevent thousands of avoidable deaths of these magnificent and intelligent animals.