(Or: The unsolved riddle of a whale’s song)
The ocean is not a quiet place. In fact, it is full of sounds originating from numerous different sources. Screening over a wide spectrum of frequencies, some of them are inaudible to the human ear.
Rain, wind, and waves constitute the main part of the background vocals in the deep eerie ocean choir.
Earthquakes, plate tectonics, moving glaciers, submarine landslides, and underwater volcanic eruptions form a constant bassline, whereas medium and high-pitched regional sounds of biodiversity produced by the staggering variations of animal communication sing the beautiful underwater melodies.
Some regions in the ocean are louder than others. The abyssal plain, not only said to be the ocean’s desert because of its flat vastness, but also because it can be sparsely inhabited, is hence rather quiet. Every now and then, it is being visited by migrating species, like whales, dolphins or turtles, who temporarily ensonify this otherwise so silent place.
This sound level however drastically increases when approaching a populated seamount. Usually, a high number of the most diverse creatures are romping about, producing an exceptionally vivid and unique sound scenery that stretches over an impressive range of frequencies and amplitudes. Snapping crustaceans really can be the loudest of them all – in fact, they are so loud, that navy submarines use shrimp clouds to acoustically hide in. But also chattering fishes or foraging dolphins, who use clicks to echolocate their prey, are remarkable contributors to this natural ocean soundscape.
Travelling at a speed of 1500 meters per second, sound in water is almost four times faster than in air. It can therefore cover enormous distances. Depending on physical properties like temperature and pressure, especially low frequencies can last for several thousands of kilometres. Indeed, there is a particular layer at 800 – 1000m depth where sound speed is at its minimum, allowing low frequencies to circle almost entirely around the globe. It is known as the SOFAR (Sonar Fixing And Ranging) channel and migrating animals such as whales use it for long range communication with other far-away mates. But they are not the only ones: navy submarines benefit from this way of audio connection to send information around the world within seconds. This triggers a conflict of interest, causing whales to become distracted and straying off, commonly ending up with them beaching in shallow water regions.
But even outside the SOFAR channel, the deep rumble of a feeding fin whale can be heard over 15km away from its position (Watkins 1981). They use a variety of sounds to communicate, some of which are solely used to target food when hunting, others to navigate during their long transits to different places. But the long and low-pitched humming in particular remains an unresolved mystery. Some say whales sing for aesthetics or artistic reasons. A beautiful idea and a very equitable one, too – why should humans be the only species making noise for pure joy?
Of course, anthropogenic noise like ship traffic, drilling offshore installations, or military activities increasingly become the lead voice. We significantly increase the overall volume, not only of the ambient noise, but throughout the entire soundscape spectrum. In fact, the anthropogenic component of the ocean’s ambient noise has doubled in the last 50 years. It is an open secret that natural ocean inhabitants are at the very least distracted by this constant noise shower. Likewise, the fact that animals show different behaviour as a consequence of permanent sonication has meanwhile reached common knowledge as well as the rising number of serious hearing damages or cases of death and beaching due to noise.
Recent studies have revealed that blue whales respond to anthropogenic noise overlapping with their vocalisation (Melcón et al. 2012). When hunting, blue whales amplify and stretch their foraging calls (‘D-calls’, usually less than 100Hz) in the vicinity of mid-frequency sonar and ship sounds. It was also observed that they entirely cease to sing when distracted by ambient, non -‘natural’ sound, such as ship traffic.
The good news is that, in contrast to other pollutants, like chemicals or plastics, noise can be switched off. If we put efforts in reducing the man-made share of sounds in water, we could actually get rid of it. And efforts are being made to reduce submarine noise pollution and monitor sound in the oceans. For instance, permanently installed hydrophones in high-traffic areas or on deep-sea moorings help to understand and observe ocean sounds over the long term. Thus, recent studies have revealed that during the Covid pandemic, the noise level of major shipping routes significantly dropped (e.g. De Clippele & Denise 2021). New developments within engineering technology and smarter engine construction allow for reduction of ship noise.
As this blog is not for once about the visual beauty of the deep sea, there won’t be any pictures this time. But why not instead take a sonic tour about the ocean’s vast soundscape? (Use headphones!)
The Humpback Whale: https://freesound.org/people/MBARI_MARS/sounds/403406/
The Killer Whale: https://freesound.org/people/MBARI_MARS/sounds/458854/
The Fin Whale: https://freesound.org/people/MBARI_MARS/sounds/403841/
Rain at 900m water depth: https://freesound.org/people/MBARI_MARS/sounds/404317/
Live from Monterey bay – Eavesdrop on the deep sea off California: https://www.mbari.org/soundscape-listening-room/
The French band “Doucha – music of the depths” translated deep sounds recorded by hydrophones into lyrics. Lead voice is the captain of the French research vessel RV Pourqoi Pas?: https://youtu.be/QuJ95bBJgIw
And here’s a snippet from our own hydrophone record – can you name it?
References:
Melcón, M. L., Cummins, A. J., Kerosky, S. M., Roche, L. K., Wiggins, S. M., & Hildebrand, J. A. (2012). Blue whales respond to anthropogenic noise. PloS one, 7(2), e32681. https://doi.org/10.1371/journal.pone.0032681
Watkins, W.A. (1981): Activities and underwater sounds of fin whales. Sci. Rep. Whales Res. Inst., No. 33, I981, 83-117.
