The Science of Underwater Acoustics

Sound travels roughly five times faster in water than in air, making it the most efficient way for marine animals to communicate, navigate, and detect prey over vast distances. For researchers, this property turns the ocean into a living acoustic landscape—one that can be monitored to uncover the hidden migration patterns of whales, dolphins, fish, and even sea turtles. Unlike visual tracking methods that require clear water, daylight, or animal tagging, sound-based tracking works continuously, day or night, through murky or deep waters. Scientists now rely on acoustic data to map migration corridors, identify breeding and feeding grounds, and assess how noise pollution disrupts these ancient journeys.

Key Tracking Techniques

Passive Acoustic Monitoring

Passive acoustic monitoring (PAM) uses hydrophones—underwater microphones—to listen for natural sounds produced by marine animals. No signals are emitted; the devices simply record the surrounding soundscape. This non-invasive technique is ideal for long-term studies because it requires minimal human presence. For example, researchers deploy arrays of hydrophones along continental shelves or at oceanographic moorings to detect the low-frequency songs of blue and fin whales, which can travel hundreds of miles. By comparing the timing and location of these vocalizations across seasons, scientists can reconstruct migratory highways used by endangered populations. PAM also helps estimate population sizes: the number of distinct whale songs recorded in a region correlates roughly with the number of animals present.

Active Acoustic Techniques

Active acoustics involve transmitting short pulses of sound (pings) into the water and analyzing the returning echoes. This approach, commonly used in fish-finding sonar and echosounders, measures the density, depth, and movement of marine organisms in real time. Researchers use downward-facing echosounders on ships or autonomous gliders to track the daily vertical migrations of krill, squid, and fish—movements that often coincide with predator migration patterns. Active acoustics can also detect the presence of large animals like dolphins or whales by their acoustic backscatter signature. While more intrusive than passive monitoring, active methods provide detailed, high-resolution data on the three-dimensional distribution of marine life along migration routes.

Emerging Technologies: Gliders and Cabled Observatories

Autonomous underwater gliders equipped with both passive and active acoustic sensors are revolutionizing migration studies. These battery-powered vehicles can patrol the water column for weeks or months, uploading data via satellite when they surface. Cabled ocean observatories, such as the Ocean Observatories Initiative (OOI) and Neptune Canada, provide permanent, real-time acoustic monitoring across entire basins. Combining these platforms with machine learning algorithms allows researchers to automatically classify species calls and detect rare migration events that would otherwise be missed. The result is an increasingly detailed acoustic map of marine animal movements across seasons and years.

Case Studies: Tracking Migrations with Sound

Humpback Whale Migrations in the Pacific

Each year, humpback whales travel thousands of miles between cold feeding grounds near Alaska and warm breeding areas off Hawaii and Mexico. By analyzing recordings from hydrophones placed on the seafloor, scientists have discovered that male humpbacks sing complex songs that change gradually during migration. These songs are not only heard near breeding grounds but also along the migration route itself. Acoustically tracking the spread of song themes over weeks reveals the direction and speed of whale movement, complementing satellite tag data. In one study, researchers at the Whale Museum used hydrophone arrays to document that some whales pause their migration to feed in unexpected areas, challenging the long-held assumption of a non-stop journey.

Fish Migrations and Spawning Aggregations

Many fish species produce distinctive sounds during spawning—grouper rumbles, croaker drumming, and snapping shrimp crackles. Passive acoustic monitoring has become a powerful tool for locating and protecting fish spawning aggregations, which are often targeted by fisheries. For instance, scientists with the NOAA Fisheries program deployed hydrophones in the U.S. Virgin Islands to record the nightly choruses of red hind grouper. The data revealed not only the exact location of spawning sites but also the timing of fish arrivals and departures over several weeks. This information helps establish seasonal fishing closures and marine protected areas that align with natural migration rhythms.

Eel Migration: A Mystery Solved by Sound?

European eels have one of the most enigmatic migrations—they travel from European rivers to the Sargasso Sea to spawn and then die. Because eels are cryptic and avoid light, traditional tags rarely work. Recently, researchers have experimented with active acoustic tracking using miniature implanted pingers. Although tiny, these devices produce distinctive signals that can be detected by listening stations along the continental shelf. Preliminary results show that eels migrate in deep currents during the night, moving much faster than previously believed. Pure active acoustic tracking is still challenging due to battery life, but passive detection of sounds made by eels (if any) is an active area of investigation. A better understanding of eel migration could help reverse the species’ drastic decline, as noted by the IUCN Red List.

Comparison with Other Tracking Methods

Sound-based tracking offers unique advantages over visual surveys, satellite tags, and genetic sampling. Visual methods require calm seas, daylight, and observer expertise, and they miss animals below the surface. Satellite tags provide precise location data but are expensive, have limited battery life, and can only be attached to large animals. Genetic sampling from water (eDNA) reveals presence but not behavior or movement timing. Acoustics, by contrast, can monitor multiple species simultaneously, operate 24/7, and cover areas too deep or remote for ships. The main limitation is uncertainty: a sound does not always reveal an animal’s exact position, and background noise can mask signals. Combining acoustics with other methods—such as using hydrophone data to trigger satellite tag deployments—is the most promising path forward.

Challenges and Future Directions

Despite its promise, acoustic tracking faces several hurdles. Ship traffic, seismic surveys, and construction generate noise that can drown out animal sounds and even cause hearing damage. Climate change is altering ocean chemistry and temperature, which affects how sound travels, potentially shifting migration corridors that animals have used for millennia. On the technical side, sifting through terabytes of acoustic data remains a daunting task, though machine learning is speeding up classification. Future advancements include miniaturized acoustic tags for small fish, long-duration hydrophone networks powered by ocean energy, and international data-sharing standards to create a global acoustic migration atlas. The Woods Hole Oceanographic Institution is already piloting a network of autonomous listening buoys in the North Atlantic that will provide real-time updates on whale movements to shipping companies, helping reduce ship strikes.

Conclusion

Sound is not just a way for marine animals to communicate—it is a window into their migratory lives. By deploying hydrophones, analyzing echoes, and leveraging new sensor platforms, scientists are uncovering migration patterns that have remained hidden for centuries. These acoustic insights are critical for designing effective marine protected areas, managing fisheries, and mitigating the impacts of noise and climate change. As technology continues to evolve, listening to the ocean will become an ever more powerful tool for tracking marine animals and protecting the health of our seas.