The ocean. A vast and mysterious world.
A place we like to picture as calm and silent.
But actually, it isn't.
Instead, imagine spending your life on a construction site.
Without ear protectors.
Or standing next to a pneumatic drill.
Because according to researchers monitoring underwater sound pollution
that's what noise levels are like beneath the waves.
For the last 40 years, man-made ocean noise has been doubling every decade.
And it's only getting louder.
Jacques Cousteau famously described the ocean as a silent world,
but in fact, it's full of noise.
Stormy weather, underwater earthquakes and even icebergs breaking up
can cause loud bursts of sound.
The ocean's inhabitants create plenty of noise too.
Many sea creatures use sound to communicate.
Seals grunt or bark.
Dolphins emit high clicks and whistles.
And most famously of all, whales call or sing to one another.
Whales are some of the loudest animals in the world.
With blue whale calls measured at 188 decibels,
equivalent in the air to around 125 decibels.
And the sperm whales clicks at around 230 decibels,
equivalent to 174 decibels above water.
By comparison, a jet engine is only 140 decibels.
And because sound travels 4.5 times faster in water than in air,
not only are these noises loud, they can also be heard far away,
sometimes even thousands of kilometers from their source.
The result is a highly complex underwater soundscape.
But over the last 60 years,
human activity has been gradually altering life in the ocean.
Those natural sounds are being overwhelmed.
By cruise ships and leirsure boats.
Constructing offshore wind turbines.
Commercial shipping.
Sonar.
Military exercises.
And seismic exploration for oil and gas reserves.
But if whales can be louder than a jet engine
and the ocean is a naturally noisy place anyway,
is there really such a problem?
The answer is yes.
Most marine life is dependent on sound to survive
and this works perfectly well in a naturally noisy ocean.
But add in sound like container ship propellers or airgun explosions
and suddenly normal reliance on sound becomes impossible.
Laboratory research and underwater case studies have shown
how continual exposure to manmade sounds can have devastating effects.
Octopus and squid may suffer trauma to sensitive hair cells,
leaving them unable to balance properly in the water.
Fish experience high levels of stress,
causing a reduction in growth and reproduction.
And even crabs can become so stressed,
they're unable to camouflage themselves from predators.
Large marine mammals can be badly affected too.
Some researchers say whales and dolphins can suffer acute hearing trauma
from sudden explosions like air guns.
They may also avoid their usual migratory patterns due to noise stress.
In 2008, it's thought that seismic testing delayed the southward migration
of narwhals from Canada's Baffin Bay for so long
that more than 1000 of them became trapped in the sea ice and died.
But perhaps the biggest danger is the fog of sound,
created by continuous noise like ship engines and propellers.
Shipping noise tends to be the same low frequency as whale song.
When both overlap, the whale's communication with one another can be blocked.
It can also mask sources of dangerm
leading to fatal collisions between whales and ships.
And crucially, the noise fog impairs whales' ability to navigate,
meaning they might not find their seasonal breeding and feeding grounds.
You might think that whales and dolphins are super smart
and could easily adapt to this change in the ocean.
Well, yes and no.
Whales are extremely intelligent and quick learners.
They pick up new skills like hunting techniques
and pass them on down the generations.
But their reliance on sound is an intrinsic part of their biological makeup,
not something that can just be learned.
It's the result of 40 to 50 million years of evolution.
By contrast, humans have changed the underwater soundscape in less than a century.
It would take a lot longer than that for whales' acute sense of hearing
to adapt to their new noisier environment
And humans are only continuing to change the ocean.
Around the globe, 20 to 40 seismic surveys
are usually being carried out simultaneously,
firing air guns every 10 seconds for months at a time.
The world's total offshore wind capacity
will likely increase by almost 10 times between 2018 and 2030.
And global shipping is expected to double between 2015 and 2030.
But is there any way to ensure
these developments don't result in even greater underwater sound pollution?
The International Maritime Organization has established guidelines for reducing noise
from container ships, including new propeller designs,
rerouting and simply slowing down.
Although this is one of the most effective measures for reducing sound pollution,
the guidelines aren't mandatory
and many companies choose not to follow them.
Pilot projects are also being conducted into other sources of noise,
such as pile driving wind turbines into the seabed.
In the Baltic Sea, "bubble curtains" are now being used for this activity,
reducing sound levels by up to four times.
Projects like this are still in the earliest stages of development
and a lot more research is needed before a good solution can be found.
However, the message is clear.
All marine life needs sound to survive.
It just has to be natural and in balance.
Because a healthy ocean isn't a silent world.
But one in which there's room for everyone to be heard.
Want to learn more about the threats facing our oceans?
Watch our ocean conservation playlist.
Just click on the link beside the video.