Written by Carla Leone, Managing Editor at SeaVoice
Soundscapes
Submerged in the sea your body feels weightless, salt and water molecules stick to every inch of your skin, your nose is sealed, and your vision is tinted blue. All your senses have been conquered by the ocean. Water seeps into your ears and muffles the noise of the terrestrial world you came from. The silence underwater has the power to calm your racing mind, bringing your busy days to a restful end, and you are the ocean’s captive.
Engulfed, all your senses interact with the foreign saline water- a substance in which you feel ethereal as you float into an unknown world. Although the silence is initially deafening, you realise this world is in fact a 24/7 orchestra, assembled by the backing rhythm of crashing waves, the squeaks, crackles and snaps of crustaceans, eerie melodies sung by cetaceans, the low grunts belted by fish, and the loud cymbal clapping by seals. The concert you have unknowingly attended is titled “the soundscape”, and just as every concert hall has unique acoustic properties, musicians, instruments, and audiences, you can recognise each ecosystem by its distinctive soundscape.
How sound is used in the ocean
In the ocean, organisms live under high pressures, in limited light, and often spend time completing distant migrations across ocean basins. In water, sound travels four times faster than in air, making communication across these vast spaces possible. The structural properties of marine habitats also influence how sound propagates underwater. Seagrass meadows for example, release oxygen, sometimes creating a soundproof curtain which could potentially protect fish from predators by containing their sounds [1]. Complex coral structures, boulders, long stretches of sand, and kelp forests will alter the movement and distance of signals while also containing nooks and crannies for animals to live.
The sea contains an immense diversity of life, which is reflected in the sounds that travel through its waters.
Many of us have been told of charismatic whale songs travelling thousands of kilometers or of the powerful echolocation pulses used by dolphins to find their prey, but in truth, this is only the tip of the iceberg. Many fish species will use their swim bladder to produce grunts, growls, and low drumming sounds during mating, spawning, and competitive behaviors. Shrimp, most notably snapping shrimp, can create loud popping sounds as they open and close their claws creating and collapsing cavitating bubbles.
The sounds from these small yet mighty crustaceans are now being used to monitor the health of their coral reef homes [2]. Some other crustaceans, such as crabs, can squeak as they rub together their appendages [3]. Somehow, amidst the raucous noise, organisms can ensure their sounds don’t go unheard. This happens because the frequencies (or pitch) of sounds emitted vary for different organisms and their different behaviours. Not only can they communicate through these different frequency bands, but they also alter the times of year and day during which they are most vocally active. Seasonal, diurnal, and moon-phase patterns are observed in most marine ecosystems. Changes in activity and habitat structure manifest in ecological soundscapes, and this is valuable information for animals deciding where they want to travel, rest, mate, spawn, settle and hunt. Some studies have shown that reef fish can use sound as a cue for settlement and will choose reefs that sound healthier (4). A healthier reef is saturated with different sounds because it implies an abundance of different organisms are emitting and receiving auditory information.
Monitoring with sound
To understand how environmental changes affect ecosystem and animal functioning, we need to experience the world as they do. Now that we know how attentively sea creatures listen to their surroundings, protecting and monitoring these soundscapes is our next step. We need insight into the cues they rely on and how disturbances interfere with their natural behaviours. When we listen as they do, we might decipher some meaning from their racket and chatter.
This was one task for my undergraduate research project, where I analyzed recordings of coastal habitats along the west coast of the UK collected by the organisation Sail Britain, with the guidance of Dr. Laurence de Clippele (University of Glasgow) and Tom Grove (Whale Wise). Having just learned about the development of machine learning techniques to monitor reef health with eco-acoustics [5], I was eager to put some headphones on.
I spent countless hours watching spectrograms (visual representations of sound) and listening to recordings while I attempted to identify and categorise acoustic signals. I became acquainted with the unique sound patterns that would appear at different sites and spent hours with my supervisor investigating who might be responsible for them.
Without any accompanying video, we had to be alert to detect even the quietest fish grunt, and it was not possible to know exactly what animals were making these sounds. While our sound analysis could detect cryptic and distant animals, and even allow us to monitor in the dark, the lack of visual data was an impairment.
It was a reminder that we still (at least for baseline data) need to use our senses together to get a holistic picture of the key musicians in each habitat’s orchestra.
Despite this limitation, we delved into some sound archives to match some distinct sounds to the gadoid fish family which contain commercially important species like Haddock and Atlantic Cod. Each habitat had a unique sound community, and if we could listen to each habitat over many days, seasons, and years we might be able to track the progress or decline of these habitats through the changes in their sound.
Protecting soundscapes
The natural world is already noisy, but humans have intruded further and exposed the seas to constant sound disturbances. Even in the distant open ocean massive cargo ships, cruises with giant propellers, seismic survey explosions, and military sonar are a few of the noise pollution culprits. As you approach the shore, noise levels continue to increase. Boats of all sizes dart around, offshore oil rigs rumble in the distance, offshore construction bangs metal on metal - our loud world has seeped into the natural soundscapes of our ocean.
The noise we release into the ocean varies in frequencies and can mask other biological and ecological sounds, for example boat motors emit low frequency rumbles where fish are communicating with their grunts.
We are depriving marine organisms of the sensory cues they rely on for survival, and we are simultaneously causing their homes to become sick and potentially uninhabitable.
The research in marine soundscape ecology is rapidly developing. Engineering feats have also developed methods of dampening noise from boats and offshore wind farms. New and cheaper hydrophones are easier to deploy and can be left in the water over longer periods of time. Machine learning models are being developed to rapidly process and analyse countless hours of recordings and decode important acoustic properties.
While automated analyses are well underway and a sign of rapid progress, people are the key to protecting the sounds of the sea. Whether its participating in citizen science, going to a museum exhibit, educational workshop, or going for a curious dip in the sea just to experience for yourself what underwater songs are being sung in your local area, we can all take a moment to listen amongst the ocean creatures. Then, we can continue local and international work to protect what lies beneath the waves, if not for the life, culture, and peace it provides us, then for its everlasting melodies.
Acknowledgements:
Thank you to Dr. Laurence de Clippele and Dr. Seb Hennige for supervising the initial project and Issy Key for providing her data and knowledge on seagrass habitats. Also Tom Grove the founder of Whale Wise and Oliver Beardon, the founder of Sail Britain, who were responsible for collecting the recordings for the research. You can learn more about the deployment of the hydrophones here.
If you are curious to listen to some ocean singers, you can have a look at these websites:
References
1. Wilson CJ, Wilson PS, Greene CA, Dunton KH. Seagrass meadows provide an acoustic refuge for estuarine fish. Marine Ecology Progress Series. 2013;472:117-127. doi:10.3354/meps10045
2. Song Z, Ou W, Su Y, et al. Sounds of snapping shrimp (Alpheidae) as important input to the soundscape in the southeast China coastal sea. Front Mar Sci. 2023;10. doi:10.3389/fmars.2023.1029003
3. Coquereau L, Grall J, Clavier J, Jolivet A, Chauvaud L. Acoustic behaviours of large crustaceans in NE Atlantic coastal habitats. Aquat Biol. 2016;25:151-163. doi:10.3354/ab00665
4. Simpson SD, Meekan MG, McCauley RD, Jeffs A. Attraction of settlement-stage coral reef fishes to reef noise. Marine Ecology Progress Series. 2004;276:263-268.
5. Lamont TAC, Williams B, Chapuis L, et al. The sound of recovery: Coral reef restoration success is detectable in the soundscape. Journal of Applied Ecology. 2022;59(3):742-756. doi:10.1111/1365-2664.14089
6. Williams B, Balvanera SM, Sethi SS, et al. Unlocking the soundscape of coral reefs with artificial intelligence: pretrained networks and unsupervised learning win out. bioRxiv. Published online January 1, 2024:2024.02.02.578582. doi:10.1101/2024.02.02.578582
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