Listening to Giants
How Towed Arrays Reveal the Secret World of Marine Mammals
The hidden lives of whales and dolphins are being revealed not by sight, but by sound, through innovative technology that listens in on their underwater world.
Beneath the shimmering surface of the Western South Atlantic Ocean exists a world of constant acoustic conversation. Here, where light fades quickly, sound becomes the primary medium for navigation, hunting, and social interaction for marine mammals. For decades, studying these elusive creatures in their vast aquatic habitat posed significant challenges—until scientists harnessed the power of towed hydrophone arrays. This technology has revolutionized our understanding of marine mammal bioacoustics, revealing a complex soundscape of species-specific vocalizations and critical habitats in one of the world's most biodiverse marine regions.
The Ocean's Hidden Symphony
Marine mammals inhabit an environment where vision has limited utility. As light dissipates with depth, sound becomes their most vital sense. Sound travels nearly five times faster in seawater than in air, making it the perfect medium for communication and environmental sensing 4 .
Marine Mammal Vocalizations
Whistles
Tonal, frequency-modulated sounds primarily used for communication
Clicks
Short, high-frequency sounds mainly employed for echolocation
Burst Pulses
Sequences of broadband pulses often used during socializing 4
Each species has developed its own distinctive acoustic signature, much like human languages or regional dialects. Recognizing these signatures allows researchers to identify species presence, distribution, and sometimes even individual animals, without ever laying eyes on them.
Sound travels approximately 5 times faster in water than in air, making it ideal for marine communication.
The Scientist's Ears: Towed Array Technology
So how do researchers listen in on these underwater conversations? The answer lies in towed hydrophone array systems—essentially, underwater microphones strung along a cable that can be kilometers long, pulled behind a research vessel .
Research vessel deploying a towed hydrophone array system.
Why Towed Arrays Work
Towed arrays offer distinct advantages over stationary hydrophones:
- Distance from noise: Trailing hydrophones far behind the vessel keeps them away from the ship's own noise sources (engine, propeller, water flow), dramatically improving the signal-to-noise ratio
- Superior detection: This setup offers superior resolution and range compared to hull-mounted sonars
- Adaptable depth: The array can be adjusted to different depths to sample various thermal layers, crucial because water density and temperature differences can reflect sound, creating "blind spots" for detection
This technology, initially developed for naval purposes, has become an invaluable tool for marine biologists studying acoustically active species in their natural environment.
Cracking the Dolphin Code: A Groundbreaking Experiment
In the waters off southern and southeastern Brazil, from Chuí to Rio de Janeiro, an ambitious research project set out to accomplish what was once thought impossible: acoustically distinguishing eight different delphinid species using their vocalizations 8 .
The Research Mission
Between 2013 and 2015, scientists embarked on multiple surveys during austral spring and autumn. They followed pre-planned zig-zag transects along the outer continental shelf and slope, from approximately 150 to 1500 meters depth. When a group of marine mammals was spotted, the research vessel would approach for simultaneous visual identification and acoustic recording 8 .
The recording system consisted of a 3-element omnidirectional hydrophone array coupled to a digital recorder, capable of capturing sounds at sampling rates of 96 kHz/24 bits—sufficient to capture the full frequency range of delphinid vocalizations 8 .
2013-2015
Multiple surveys conducted during austral spring and autumn
Survey Area
Southern and southeastern Brazil, from Chuí to Rio de Janeiro
Depth Range
150 to 1500 meters along the outer continental shelf and slope
Recording System
3-element omnidirectional hydrophone array with 96 kHz/24 bit digital recorder
The Analysis: Separating Whistles from Clicks
Back in the lab, researchers faced the complex task of analyzing thousands of vocalizations. They took an integrative approach, examining both whistles and echolocation clicks:
- Maximum, minimum, beginning, ending, peak, and center frequencies
- Duration measurements
- Frequency modulation patterns
- Inter-click interval measurements
- 3 dB bandwidth analysis
- 10 dB bandwidth analysis 8
Statistical analyses, including discriminant function analysis and classification tree models, were employed to determine which acoustic parameters could best distinguish between species.
| Species | Common Name | Recording Locations |
|---|---|---|
| Stenella longirostris | Spinner dolphin | Southern Brazilian coast |
| Stenella frontalis | Atlantic spotted dolphin | Southern Brazilian coast |
| Steno bredanensis | Rough-toothed dolphin | Southern Brazilian coast |
| Grampus griseus | Risso's dolphin | Southern Brazilian coast |
| Tursiops truncatus | Bottlenose dolphin | Southern Brazilian coast |
| Delphinus delphis | Short-beaked common dolphin | Southern Brazilian coast |
| Orcinus orca | Killer whale | Southern Brazilian coast |
| Globicephala melas | Long-finned pilot whale | Southern Brazilian coast |
The Revealing Results: Nature's Acoustic Signatures
The research yielded fascinating insights into the acoustic world of delphinids. Each species demonstrated distinctive vocal characteristics that could be used for identification—a crucial development for monitoring and conservation.
Whistles vs. Clicks: Which Works Better?
Interestingly, the study found that echolocation clicks provided more reliable species identification than whistles. When researchers used both types of vocalizations together in what they called "joint classification," the results were particularly impressive:
- Discriminant function analysis 94.2% accuracy
- Classification tree model 81.2% accuracy
- Echolocation clicks provided more reliable species identification than whistles 8
| Vocalization Type | Measured Parameters | Statistical Effectiveness |
|---|---|---|
| Whistles | Maximum, minimum, delta, peak, center, beginning, and ending frequencies; duration | Higher misclassification rates when used alone |
| Clicks | Inter-click interval, 3 dB bandwidth, 10 dB bandwidth | More reliable for species identification |
| Combined Approach | All parameters from both whistle and click analyses | Highest classification accuracy (94.2% in DFA) 8 |
The Researcher's Toolkit: Essentials for Bioacoustic Studies
Conducting marine bioacoustics research requires specialized equipment and methodologies. Here are the key components of the marine bioacoustics toolkit:
Towed Hydrophone Array
System of hydrophones towed behind a research vessel to capture underwater sounds
Digital Recorder
High-sampling rate device to capture full frequency range of vocalizations
ROV
Depth-adjustable device used to position the array at specific depths
Spectrogram Software
Computer programs for visualizing and analyzing sound characteristics
Beyond Identification: The Wider Applications
The ability to acoustically identify marine mammals has profound implications for conservation and ecosystem management, particularly in the rapidly changing marine environment.
Monitoring in a Changing Climate
In Arctic regions like the Greenland and Barents Seas, passive acoustic monitoring has proven especially valuable for tracking species distribution in ice-covered waters during winter months when visual surveys are impossible. These areas are experiencing rapid environmental changes, making baseline acoustic data crucial for assessing future ecosystem conditions 7 .
- Fin whale 20 Hz downsweeps present from October to April
- Bowhead whale songs recorded from mid-October to mid-April
- Sperm whale clicks present nearly year-round at some locations 7
Conservation and Management Tools
The growing understanding of marine mammal bioacoustics has led to practical conservation tools. The Marine Mammal Management Toolkit, for instance, helps Marine Protected Area managers incorporate marine mammals into management plans through factsheets, self-assessment tools, and best practices 5 9 .
Management Toolkit Components
Factsheets
Self-assessment Tools
Best Practices
These resources are particularly important given increasing threats from human activities such as shipping, fishing, and other sources of anthropogenic noise that can disrupt marine mammal communication and behavior 4 .
The Future of Marine Bioacoustics
As technology advances, so does our ability to interpret the complex acoustic world of marine mammals. The success of species identification through towed array systems marks just the beginning. Future research may focus on:
- Understanding how anthropogenic noise affects communication and behavior
- Mapping critical habitats through acoustic presence
- Monitoring population trends through passive acoustic monitoring
- Developing more sophisticated automated detection and classification systems
Western South Atlantic Laboratory
The Western South Atlantic Ocean, with its remarkable diversity of marine mammals, continues to be an important natural laboratory for these discoveries.
Conclusion: Listening for Conservation
Towed array systems have transformed marine mammal research, allowing scientists to decode nature's underwater symphony. What began as a tool for detecting and classifying species has evolved into a vital conservation instrument, helping protect these magnificent creatures and their rapidly changing habitats.
As we continue to listen, we not only satisfy scientific curiosity but also gather essential knowledge to preserve the delicate acoustic ecology of our oceans—ensuring that the hidden symphony of the seas continues for generations to come.