Listening Beneath the Waves

Discover how underwater sound is revealing the hidden lives of harbour porpoises and helping us understand the changing marine environment of the Clyde.

Most of the lives of whales, dolphins and porpoises take place beneath the surface, largely hidden from human observation.

Even in calm conditions, harbour porpoises can be remarkably difficult to see. Their small size, brief surfacing behaviour and tendency to avoid vessels make conventional visual surveys challenging.

Yet beneath the water, these elusive animals are far from silent.

Harbour porpoises produce extraordinary high-frequency sounds that allow them to navigate, locate prey and investigate their surroundings.

By listening to these signals using specialised underwater microphones, Clyde Porpoise can investigate a world that would otherwise remain largely inaccessible.

This technique is known as passive acoustic monitoring, and it forms the foundation of much of our scientific research.

How Porpoises See with Sound

Harbour porpoises rely on a highly developed biological sonar system known as echolocation.

They produce short, high-frequency clicks that travel through the water and reflect from objects, including fish, seabed features and other animals.

By interpreting these returning echoes, a porpoise can determine the location and characteristics of objects around it.

This allows porpoises to hunt effectively even in darkness or in the relatively poor underwater visibility often encountered in coastal environments.

Their echolocation clicks are concentrated at frequencies around 130–140 kilohertz (kHz), far above the upper limit of normal human hearing.

Unlike many dolphins, which produce a wider variety of whistles and broadband clicks, harbour porpoises primarily use narrow-band, high-frequency acoustic signals.

These distinctive clicks make them particularly suitable for passive acoustic monitoring.

What Can a Click Tell Us?

Individual echolocation clicks contain information about the animal’s acoustic behaviour.

But it is often the pattern of clicks over time that provides the most interesting clues.

When searching for prey, porpoises typically produce a succession of clicks separated by short intervals.

As an animal approaches a potential target, the intervals between clicks may become progressively shorter.

During the final stages of a feeding attempt, the click rate can increase dramatically, producing a rapid sequence known as a feeding buzz.

These changes allow scientists to investigate possible feeding activity and other aspects of porpoise behaviour without needing to see the animal.

However, acoustic behaviour must be interpreted carefully. A burst of rapid clicking can suggest prey pursuit, but it does not necessarily establish that a fish was successfully captured.

What Is Passive Acoustic Monitoring?

Passive acoustic monitoring, often abbreviated to PAM, involves recording sounds already present in the marine environment.

Unlike active sonar, which transmits sound into the water and listens for returning echoes, passive monitoring simply listens.

The equipment does not need to produce sound or deliberately attract animals.

Instead, sensitive underwater microphones known as hydrophones detect acoustic signals generated by marine life and other sources.

Hydrophones can record the echolocation clicks of harbour porpoises, dolphin vocalisations, vessel noise and many other components of the underwater soundscape.

These recordings provide a valuable opportunity to investigate animals that may be difficult or impossible to observe visually.

Passive acoustic monitoring can also operate in darkness, when conventional visual surveys are ineffective.

Surveying the Clyde aboard Saorsa

Since 2016, Clyde Porpoise has undertaken vessel-based acoustic surveys using our research vessel, Saorsa.

During these surveys, a specialised hydrophone array is towed behind the vessel, recording underwater sounds as we travel along survey routes.

The acoustic recordings are collected alongside GPS information, allowing detected signals to be related to the position and movement of the survey vessel.

Our recording systems have operated at high sample rates, including 500,000 samples per second, enabling us to capture the ultrasonic echolocation signals produced by harbour porpoises.

Why Use More Than One Hydrophone?

Our porpoise survey equipment uses a pair of closely spaced hydrophones.

In the historical survey configuration, the acoustic elements were separated by approximately 24 centimetres and deployed well behind the vessel on a towed cable.

Although the two hydrophones are close together, a sound arriving from one direction will generally reach one hydrophone a fraction of a second before reaching the other.

By measuring this tiny difference in arrival time, acoustic analysis software can estimate the direction from which the sound originated.

As the vessel moves, successive detections can provide information about how the apparent direction of an animal changes over time.

This is the foundation of acoustic bearing estimation and target motion analysis.

It is important to recognise that a simple two-hydrophone linear array cannot always distinguish whether an animal is on the port or starboard side of the vessel. Additional information and careful interpretation are needed to resolve this ambiguity.

PAMGuard — Turning Sound into Information

A central part of Clyde Porpoise’s research involves a specialised software system called PAMGuard.

PAMGuard was developed for passive acoustic monitoring of marine mammals and other underwater sounds.

It enables researchers to record, detect, classify and analyse acoustic signals.

Rather than requiring someone to listen manually to thousands of hours of recordings, PAMGuard can identify candidate acoustic events and display them for further investigation.

For harbour porpoise research, the software examines very short, high-frequency signals that may correspond to echolocation clicks.

These detections can be displayed as waveforms, frequency spectra and patterns of activity over time.

The Bearing-Time Display

One particularly useful PAMGuard feature is the bearing-time display.

This shows detected sounds according to their estimated direction relative to the hydrophone array and the time at which they were recorded.

When a porpoise produces a succession of clicks, the detections can form a recognisable pattern as the animal and survey vessel move relative to one another.

These patterns are known as click trains.

An experienced analyst can examine their timing, acoustic characteristics and changes in bearing to help distinguish genuine porpoise activity from other sounds.

Where multiple porpoises are present, overlapping click trains can make interpretation particularly challenging.

A central part of our research therefore involves carefully examining acoustic detections and identifying which can be reliably attributed to harbour porpoises.

Finding Where a Porpoise Was Clicking

Detecting a porpoise is only the beginning.

One of our objectives is to understand where animals were located in relation to the survey vessel.

PAMGuard includes analytical techniques known as target motion analysis, or TMA.

As the vessel moves, the direction of the recorded clicks may change. By combining successive acoustic bearings with information about vessel movement, TMA can estimate a possible location for the sound source.

The resulting estimates may provide information about the distance between the porpoise and the vessel’s survey track.

However, these calculations involve assumptions.

Porpoises are moving animals, and their movements can make acoustic localisation more difficult. The accuracy of an estimate also depends on recording quality, hydrophone geometry, vessel movement and the characteristics of the click train.

Consequently, not every detected porpoise can be reliably localised.

Clyde Porpoise is undertaking detailed examination and validation of historical acoustic detections to identify which observations are suitable for scientific analysis.

From Clicks to Population Research

One of the most important questions we hope to address is how harbour porpoises are distributed throughout the Clyde.

Simply counting the number of detected clicks does not tell us how many porpoises are present.

A single animal can produce thousands of clicks, while another may pass through an area without being detected.

Detection also becomes less likely as the distance from the hydrophone increases.

Other influences include the direction in which the animal is facing, its behaviour, background noise and the sensitivity of the recording equipment.

To account for these complications, researchers can use methods such as distance sampling.

By combining suitable acoustic localisations with the distance travelled during valid survey effort, it may be possible to estimate porpoise density.

Such analyses require careful treatment of detection probability, survey coverage and other sources of uncertainty.

For Clyde Porpoise, developing reliable and consistent methods for analysing historical surveys remains an important research objective.

Why the Historical Recordings Matter

Over successive survey years, Clyde Porpoise has accumulated a substantial archive of underwater acoustic recordings.

These recordings represent much more than a collection of individual porpoise detections.

They are historical records of the underwater acoustic environment at particular places and times.

They contain information about porpoise activity, vessel movements and other sounds encountered during our surveys.

As acoustic analysis techniques improve, original recordings can be revisited to investigate questions that were not necessarily considered when they were collected.

For example, it may be possible to re-examine older recordings using improved signal classification, click-train identification and acoustic localisation methods.

This is an important part of our current work.

We are preserving original recordings and historical analyses while developing a more consistent approach to processing data collected across different survey years.

Standardisation is particularly important because differences in recording configurations, equipment performance and analytical methods can influence the number and characteristics of detected signals.

Careful comparisons are essential before interpreting differences between years as genuine environmental or population changes.

The value of a long-term acoustic archive lies not only in what it has already revealed, but in the scientific questions it may help answer in the future.

Investigating the Underwater Soundscape

Although our principal interest is harbour porpoises, the hydrophones also record sounds generated by other activities in the marine environment.

The Clyde is a busy and acoustically complex sea.

Shipping, ferries, fishing vessels, aquaculture, sonar and other marine activities contribute to its underwater soundscape.

Different sound sources produce different combinations of frequencies, intensities and temporal patterns.

Some may overlap with frequencies used or heard by marine mammals, while others may influence behaviour through disturbance or other mechanisms.

Clyde Porpoise has undertaken investigations into underwater acoustic sources, including sonar and acoustic deterrent devices associated with marine aquaculture.

These studies aim to improve understanding of the acoustic characteristics of particular signals and their potential implications for marine mammal exposure.

Such investigations require careful scientific analysis.

Detecting a sound does not automatically demonstrate that it has harmed an animal. Assessing potential effects involves considering sound levels, frequencies, duration, distance, animal movement and relevant scientific evidence.

Our broader interest is understanding how underwater sound forms part of the environmental conditions experienced by cetaceans in the Clyde.

Scientific Collaboration and New Methods

Clyde Porpoise’s recordings and practical field experience have also contributed to scientific research undertaken with academic collaborators.

This includes work supporting Dr Mel Cosentino’s doctoral research at the University of Strathclyde and the development of the Porpoise Click Classifier, known as PorCC.

Published in 2019, PorCC was developed to improve the identification of harbour porpoise echolocation clicks within acoustic recordings.

David Nairn of Clyde Porpoise was among the co-authors of the scientific publication.

This research illustrates how community-based fieldwork and collaboration with academic institutions can contribute to advances in marine mammal monitoring.

Our acoustic recordings also contributed to the research into Kylie, the solitary common dolphin known to associate with harbour porpoises in the Clyde.

These studies demonstrate that recordings originally collected to investigate one species may contain information relevant to a much wider range of scientific questions.

What Are the Limitations?

Passive acoustic monitoring is a powerful research method, but it is not a perfect window into the underwater world.

A porpoise must produce a detectable signal, within range of the hydrophones, for the system to record it.

Animals may be missed because they are too distant, their clicks are directed away from the equipment, background noise interferes with detection or they are not clicking at the time.

Automated detection systems can also mistake certain non-biological sounds for porpoise clicks.

Even where genuine clicks are detected, identifying individual animals and estimating their positions can be difficult.

For these reasons, we place considerable emphasis on quality control, careful review and distinguishing confirmed observations from uncertain detections.

We also recognise the importance of combining acoustic research with visual observations, environmental information and other scientific methods.

Understanding these limitations is fundamental to producing meaningful and defensible scientific results.

Listening Towards the Future

For Clyde Porpoise, listening beneath the waves is about more than detecting marine mammals.

It is about gaining a better understanding of how harbour porpoises use the Clyde, the habitats on which they depend and the environmental conditions they experience.

Our long-term acoustic recordings provide opportunities to investigate porpoise distribution, feeding behaviour, habitat use and exposure to underwater sound.

They also contribute to a wider understanding of the Clyde as a changing marine ecosystem.

Much of this work has been undertaken independently and with limited financial resources.

Through continued scientific investigation and collaboration, we hope to make better use of the recordings already collected and develop increasingly robust methods for monitoring the Clyde’s harbour porpoises.

By listening carefully to the underwater world, we can begin to reveal the lives of animals we rarely see — and gather evidence that may help protect them and their habitats for generations to come.