Marine Bioacoustics: Exploring the depths of a hidden world

Beneath the ocean surface lies a realm of mystery. We like to think we know plenty about the environment that we do not reside in, but the truth is, that we know very little. Vastly different to the environment we experience on land, marine life thrives in a realm where sight takes a back seat, and sound reigns supreme. In order to study the sound-driven environment, researchers use a field known as marine bioacoustics. This is a rapidly developing area which aims to use underwater sound and its characterizations to see relations in acoustic habitat and acoustic ecology to discover the role of sound among marine life. In this blog post, we’ll dive into the world of marine bioacoustics, briefly exploring the intricate language of marine animals, the technology used for these studies and a continuing need for conservation efforts.

An underwater orchestra

From the serene songs of humpback whales to the rhythmic clicks and whistles of dolphins as well as the well-known sounds of snapping shrimp, many marine animals have evolved to be able to communicate in their water dominant world. As previously mentioned, visibility below the surface is limited, however, in this underwater environment, conditions allow for the effective transmission of acoustic signals, needed for acoustic communication between very vocal cetacean species. As previously mentioned, visibility below the surface is limited, however, in this underwater environment, conditions allow for the effective transmission of acoustic signals, especially needed for acoustic communication between very vocal cetacean species.

Cetaceans are a diverse group of marine mammals that includes whales, dolphins and porpoises and are well studied within the field of bioacoustics. With a lack of light in the ocean, sound is much needed for a range of purposes for these animals from mating calls, foraging behaviours, social activities, navigation and establishing territory. Within the order Cetacea, we have the baleen whales as well as toothed whales. The baleen whales sub-order is call Mysticeti, are Odontoceti for the toothed whales, both producing sound in different ways. With similarities existing between the groups such as live birth and blowholes, they feed differently. Mysticetes use baleen, which are larges plates of keratin, in which they use to filter feed. However, odontocetes will swallow prey whole by grabbing their prey. One of the greatest similarities is that they both produce sound, however, they produce completely different sounds and with different methods.

Within the group of mysticetes, one most common sound we hear about are the detailed and complex songs of humpback whales. Produced only by the male humpback whales, these ‘songs’, travel huge distances below the surface and are mostly heard in breeding grounds. Often described as haunting, these melodies often have a clear structure, and occur in a specific sequence. The structure is as follows: individual sounds, known as ‘units’ are arranged into a phrases. These phrases are then repeated to created themes, which then create the song. Once thought that these songs are only used in breeding grounds, it has been revealed they are also being sung in feeding grounds. A now hot topic between scientists is the reason why this is occurring with a number of hypotheses suggesting it’s a way to intimidate other males, continuing to attempt to impress the females or they are just simply practicing before their big day.

Another popular sound of the sea is the whistles and clicks of dolphins. The whistles they produce are used for social communication. Consisting of many different pitches and durations, they are highly variable and easily studied, but little is known about the complexity of call changes in relation to other sources of noise. The sounds that cetaceans produce are complex but, among odontocetes, they will they vocalise to explore their environment, known as echolocation, or to communicate via both whistles and burst calls. For odontocetes, vocalisations are grouped into three types: short-pulsed sounds, less distinct burst pulse calls and narrowband tonal whistles. Whistles, described as narrowband tonal calls, can last up to a few seconds, with a frequency range between 5 and 20 kHz.

Whilst sound, is popularly studied among cetaceans, many other marine animals vocalise and even the snapping shrimp, a prominent sound you will hear within the water column, contributes its staccato rhythm to this underwater orchestra of sound. If you’ve ever been snorkelling or diving, and hear a crackling in the water, these are the snapping shrimp just doing their thing.

Sound exploration: Marine Bioacoustics

The rapidly developing area of marine bioacoustics aims to use the recordings of underwater sound and its characterisations to see relation in acoustic habitation and acoustic ecology to discover the role of acoustics among marine life. Marine bioacoustics is emerging as a vital tool for researchers seeking to better understand the behaviours, migratory patterns and general ecology of marine life. Passive acoustic monitoring (PAM) involves the deployment of underwater hydrophones, serving as a way for scientists to eavesdrop into the secrets being ‘spoken’ beneath the surface. Through this method, there have been massive breakthroughs for the scientific world, and they can finally begin to understand how different marine animals are communicating.

We’ve come a long way in the face of technological advancement and the same goes for marine bioacoustics. The advancement of this technology has revolutionised the field of marine bioacoustics. We now have autonomous underwater vehicles equipped with hydrophones which can navigate the depths for extended periods, capturing sound in remote and challenging environments, extending the reach into exploration of the ocean realms. Moreover, there is now sophisticated algorithms and machine learning techniques in which vast amounts of acoustic data can be processed and analysed to identify species specific vocalizations and begin to unravel the intricate communications of the underwater world.

So we have underwater recordings, and we can hear the chatter of our ocean animals but how do we study it? Most researchers in the bioacoustics field, will use spectrograms to be able to view sound. Here is an example of some spectrograms from the software Raven Pro, a popular software used in bioacoustics, both terrestrial and marine. In these screengrabs in particular you can see some whistles of common dolphins off the coast off South-West UK.

Recordings are often viewed in time-frequency domains, to be able to hear but also view the sounds being produced. As you can see from the image above, the y-axis shows the frequency of the sounds, and the x-axis is showing the minute and seconds of a recording. The whistles that dolphins produce are the most widely studied as they are easily recognizable in these spectrograms. Whistles are often seen with contours, typically grouped into categories of whistles with a constant frequency, upsweeps, down sweeps, concave, convex or sinusoidal whistles as seen below. This is taken from a paper by Bazua, Carmen & Au, Whitlow, (2003), showing examples of the whistles contours we may see on these spectrograms. (To read: Whistles of Hawaiian spinner dolphins. The Journal of the Acoustical Society of America. 112. 3064-72. 10.1121/1.1508785)

With this basis, whistle repertoires remain overly complex and there are wavering types between these categories, with whistles often having breaks in their contours. Moreover, often with overlapping whistles, there may be a combination of these several types, or repeats of the same whistle, known as a signature whistle. This concept was first introduced by Caldwell and Caldwell (1965), described as whistles of the same contour but sometimes with slight variations in intensity or duration. These signature whistles are only studied within bottlenose dolphin populations and have been described as a way in which dolphins ‘name’ each other with their own unique, distinctive signature whistles!

A noisy ocean

Bioacoustics has aided conservation efforts by identifying crucial habitats and migration routes, both essential parts in being able to create protection for marine species. While we are able to tune into the noise of the ocean, marine bioacoustics also highlights the growing concern of anthropogenic noise pollution. Shipping, underwater construction, naval sonar and oil exploration all create bands of noise that can disrupt the vocal communications of marine life, creating a whole host of issues for individual species. These consequences can include the disruption of feeding and mating behaviours. With this being recognised, efforts can be made to establish ways of combatting this by introducing effective regulation of human noise-inducing activities within marine habitats.

Conclusion

With a plethora of remarkable discoveries, marine bioacoustics is a captivating field that offers a route into the hidden underwater world. By being able to decipher the sounds produced by marine animals, we are slowly uncovering secrets about their behaviour, ecology and relationships. This field, however, also highlights the need for mitigations of anthropogenic noise produced by humans due to their significant impact on the acoustic environment. As we continue to explore this unknown world and learn its secrets, its important to listen carefully and act humbly in the face of our own impact on the delicate balance of life that is known as the marine world.

Written by Emily Irving