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The wet world's symphony

Despite Jacque Cousteau referring to the underwater world as the silent world, it is a cacophony. As you dive you hear your breathing quite loudly but if you stop and focus on the ambient noise you start to make out an entire orchestra: crackling, banging, popping and scratching. They are not always audible, sometimes they are very quiet other times impossible to ignore depending on location, depth, time, season and so on. Underwater creatures, contrary to common belief, are very loud. There is an Italian expression silent as a fish but it couldn’t be more wrong. In fact, fish are rather chatty, the entire reef is breaming with sounds.

It is not only the well known clicking of the whales and dolphins but also sea urchins, fish, barnacles and loudest of all the snapping shrimp.

More than 1,000 species of fish make and use sound for defence, courtship, hunting and navigation. Like in big cities, coral reefs are loud: crustaceans emit sounds to defend themselves, fish tell off their neighbors when they trespass, sea urchins click their spines as they move.

And like that one annoying colleague everyone has, they are rather loud eaters.

However, the loudest noise on a reef is made by the snapping (or pistol) shrimp. whose deadly claws stuns the prey. The sound is produced by popping of micro - yet deadly - bubbles it shoots. The actual sound is generated by the formation and subsequent popping of a bubble or bubbles rather than the clapping sound of the two parts of the claws. The bubble forms when the shrimp closes the claw very fast, in a process know as cavitation (like the bubbles formed by a ship’s propeller underwater).

Sound underwater isn’t only used to kill it is also used in courtship. Cod for example “talk” through their swim bladder to attract mates. Fish from different regions even speak different languages! Recordings of American cod are very different to those from their European cousins.

Prof Steve Simpson, from the University of Exeter, and his research team believe that cod populations have different accents also within smaller geographical areas like the Cornish cod vs the ones from Liverpool. These vocal fish have very traditional spawning grounds making them quite isolated. However, with climate change as sea temperatures rises cod are migrating north, different populations are coming more and more frequently in contact with each other for the first time. However, due to their language barriers males cannot chat up the ladies. This leads, like for us humans, to a struggle to integrate, share territory and breed.

Another loud little chap is the water boatman (Corixidae) who wins the contest of noisiest if we measure loudness to size. By rubbing his penis across his abdomen this 2mm long water insect produces 99dB (that is about the noise level of a hand held drill). You can walk along the river bank and here his penis singing at the bottom of the river!

But perhaps the loudest sound produces by underwater animals is that of the Gulf corvina's (Cynoscion othonopterus) spring orgy. All adults, millions of fish, gather in one specific area of the Colorado River Delta for their spawrgy. When they mate, the males start to emit short, loud cracking noises, pulses of sound similar to machine-gun fire. The volume of a single fish would be equivalent on air to the human ear to about 114 decibel (some sources say it is even louder) that is as loud as an emergency vehicle siren… and there are millions of fish in the same place at the same time!

Credit: Amanda Montañez; Source: “A Sound Worth Saving: Acoustic Characteristics of a Massive Fish Spawning Aggregation,” by Brad E. Erisman and Timothy J. Rowell, in Biology Letters, Vol. 13, No. 12; December 2017

Noisy sex never makes the neighbour happy. It is postulated that the sound levels are loud enough to cause at least temporary if not permanent hearing loss in marine mammals that were observed nearby or preying on the fish.

Unfortunately though, fishing trawlers can hear all too easily where all the grunting is coming from and join the party to fish them. Indeed, due to overfishing Corvina’s are now listed as vulnerable to extinction.

Overfishing isn’t the only way we disturb and disrupt the marine environment. Acoustic pollution might have a greater impact on the ecosystem that we previously thought.

Boats are the source of a lot of noise underwater, and more worryingly, the frequency emitted by engines is about the same as that of the fish sounds just much louder, as shown in the graph below. The similarity in noises might be an important impediment for fish that don’t produce very loud sounds and don’t hear all that well. So we are drowning their chatter with our roaring engines. Imagine trying to have a conversation right next to a runway in Heathrow, verbal communication would most likely fail.

BPLs from 62 to 2000 Hz calculated for 10-day subsamples between 7:30 pm and 11:30 pm for July and August (the period in which we recorded the maximum fish vocalization activity). Median with 40% of the data (box) for fish choruses without vessel passage noise (green plot), with vessel passage noise (black plot) (46% of the recordings represented this condition), and for background noise (files with no fish chorus or vessel passage in the spectrogram) (median; Whisker: 30th–70th percentile). From: Busciano et al. Scientific Reports volume 6, Article number: 34230 (2016) doi:10.1038/srep34230

But as we discover more about the acoustics of the oceans we learn that the healthiest reefs are also the noisiest and perhaps we could use this knowledge to save suffering areas. The noisy reefs may act as magnets, attracting more fish and invertebrate larvae than less-diverse reefs nearby. So by playing recordings of bustling reefs on bleached ones, it could attract larvae to repopulate these barren lands.

The underwater soundscape isn’t solely that made by living creatures, you also have the sounds of waves, tide and currents especially in the shallows. And in geological active regions you can hear the earth rumbling.

In 1997 listening stations over 5000 km apart picked up on hydrophones across the Pacific a loud, ultra-low frequency sound. It was names 'the Bloop' and the origin of the sound was an unsolved mystery. In 2002 it was thought that it could’ve even been produced by a large marine animal or by icequakes in large icebergs, or large icebergs scraping the ocean floor. Only in 2012 did NAOO conclude that it was probably due to an icequake.

Sound waves travel 5 times faster in water than through air meaning that for the human ear it is impossible to determine the direction where the noise is coming from. As a diver you are wrapped by the underwater symphony: the grumbling of the earth, the chatting of fish, the munching of the sea urchins and the punches of the shrimp. If you are lucky, a dolphin will come and sing as well. In the meantime you can click below:


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