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The oceans are a strange world: they not only harbor extremely bizarre creatures, but also very special soundscapes.

The most curious examples include the squirrel crabs: These shrimps, which live on coral reefs, produce gun-like noises when they catch their prey with specially shaped claws.

But there is much more to be heard underwater: The males of the toadfish attract females to the bottom of the Gulf of Mexico with toad-like tones;

Heats of cod emit grunts during the spawning season;

the arctic sea echoes in spring with the drawn-out whistling calls of the bearded seals.

Probably the most famous are the songs of the humpback whales.

And blue whales not only sing to find a partner - but also to coordinate activities such as seasonal migrations.

The soundscapes of some marine areas are so characteristic that larvae and young animals of several species use them to find their habitat.

This is the conclusion of an international team of researchers in the journal “Science”.

"Animals produce sounds for many reasons, for example for orientation, for hunting, as a signal of readiness to fight, for defense of the territory, for partner recruitment and for reproduction," writes the group around Carlos Duarte from King Abdullah University in Thuwal, Saudi Arabia .

Acoustic signals are therefore of paramount importance for sea creatures - also because the largest living space on earth is predominantly in complete darkness.

Compressed air cannons and sound impulses roar under water

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But the human species also makes noises underwater.

And that has serious consequences: This is how the more than two dozen scientists from eleven countries describe in their article how the soundscape changes in the oceans - and thus also the behavior of the animals.

To this end, the researchers evaluated 538 studies.

Around 90 percent of those studies that focused on marine mammals show clear consequences of human noise;

for fish and invertebrates it is more than 80 percent each.

The data situation is poor for seals, reptiles such as sea turtles and sea birds such as penguins. 

Before the Industrial Revolution, the soundscapes of the oceans consisted largely of biological and geological sources;

Natural causes included seaquakes, underwater volcanoes and the cracking of ice.

Since then, however, the oceans have become significantly louder - and hardly any marine region is spared.

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Expeditions use seismic compressed air cannons to search the ocean floor for natural resources.

These airguns continuously emit loud shots over a wide frequency spectrum, the echoes of which provide information about the nature of the ground.

The military uses sound pulses to locate submarines, for example.

And more and more ships are cruising the oceans: "In the last 50 years, the increased shipping traffic has increased the low-frequency noise along the main routes by an estimated 32-fold," writes the team.

But that's not all: when drilling rigs and offshore wind farms are built, anchors are rammed into the seabed and natural resources - such as sand for the construction industry - are extracted from the ocean floor.

And dynamite is used to fish in many coastal regions of Southeast Asia and Africa.

Escape from the noise is not possible for all animals

This noise pollution is very harmful to wildlife.

The sonic cannons that are used in the search for raw materials could sometimes cause permanent hearing damage on marine life, says co-author Ilse van Opzeeland from the Alfred Wegener Institute (AWI).

This has been proven in harbor porpoises and seals, for example.

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Many species flee to calmer waters from the human noise.

For many marine animals, however, this is not possible, emphasize the "Science" authors.

As an example, they cite the Maui dolphin (Cephalorhynchus hectori maui), which is only native to New Zealand and is threatened with extinction. 

In addition, the human noise superimposes the animal sounds.

The AWI director Antje Boetius confirms this.

Your institute maintains a number of acoustic observatories deep under the water surface in both polar regions.

There the experts record noises - including in the Fram Strait between North Greenland and Spitsbergen.

 “On the recordings, you can always hear the sound waves from technologies used to find oil and gas,” she says.

When the noise occasionally falls silent - for example at Christmas - a completely different soundscape emerges: "Only then do you hear nature itself, for example the variety of whales singing."

One of the few quiet spots is in Antarctica

The Weddell Sea in Antarctica is one of the few marine regions in which it is still quiet: “This is one of the last soundscapes without much human influence,” says co-author van Opzeeland.

However, with reservations: once there were significantly more whales - the number of blue whales has fallen by around 98 percent since the early 20th century.

Environmental groups often associate whale stranding with underwater noise;

however, these are not mentioned in the “Science” report.

There is no clear evidence for the killing by sound, says Boetius.

Because often dead marine mammals have stomachs full of garbage.

Cases are known in which whales and bottlenose dolphins perished as a result of military exercises or dynamite fishing.

But there is no evidence that noise directly increases the mortality of marine life.

The research team predicts in "Science" that the noise in the oceans is likely to increase.

This is due to coastal shipping, seismic explorations, military operations, dredging and pile driving, and deep-sea mining.

In international agreements such as the Convention on Biological Diversity (CBD), however, this problem is ignored.

"The topic is not so prominent because it is not as visible to people as a beach full of plastic waste," says Antje Boetius.

The researchers cite the European Union's Marine Strategy Framework Directive (MSFD) as an exception: It expressly recognizes noise as a stress factor and calls on the member states to monitor and reduce noise pollution.

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As a concrete measure against noise, the researchers are calling for regulation of shipping routes and speed limits.

In the future, electric motors or quieter propellers could also reduce ship noise.

The noise during the construction of offshore wind farms can also be reduced with technical measures such as bubble curtains: Rising air bubbles should reflect the sound waves and thus reduce the sound input by up to 90 percent.

In German waters it is now mandatory to dampen the noise when driving the piles into the seabed.

For the harbor porpoises in the Baltic Sea, however, that comes too late.

Because the construction of such offshore plants drove the only native whale species out of their habitat, says the director of the German Oceanographic Museum in Stralsund, Harald Benke: "Some have never come back."