The Use of Sonar in Detecting and Tracking Marine Mammals to Prevent Collisions

Marine mammals such as whales and dolphins are vital to ocean ecosystems but face growing threats from human activities, particularly ship collisions. Vessel strikes cause severe injuries and mortality, with certain species like the North Atlantic right whale pushed closer to extinction. To address this, scientists and maritime authorities increasingly rely on sonar technology to detect and track marine mammals in real-time, enabling proactive collision avoidance. This article explores how sonar works, its applications in marine monitoring, current challenges, and future innovations that promise safer seas for both wildlife and navigation.

Understanding Sonar Technology

Sonar—an acronym for Sound Navigation and Ranging—uses sound waves to detect, locate, and identify objects underwater. The basic principle involves emitting acoustic pulses (pings) that travel through water, reflect off objects, and return as echoes. By measuring the time delay and characteristics of the returning sound, sonar systems calculate distance, size, shape, and movement of the target.

Sound travels efficiently in water, making sonar far more effective than optical or radar methods for underwater detection. The technology has been used since the early 20th century for submarine detection, but its application to marine biology gained momentum in recent decades. Modern sonar systems now combine advanced signal processing with computer algorithms to distinguish biological targets from geological features or debris.

In the context of marine mammal conservation, sonar serves two primary purposes: detecting the presence of animals in the vicinity of vessels and tracking their movements over time. This information can be fed into decision-support systems that advise captains to slow down, alter course, or stop engines until the animals have cleared the area.

Types of Sonar Used in Marine Monitoring

Active Sonar

Active sonar systems emit their own sound pulses and listen for returning echoes. They provide detailed, real-time information about nearby objects, including distance and bearing. In marine mammal applications, active sonar is often mounted on the hull of ships or deployed from autonomous underwater vehicles (AUVs).

Active sonar can detect objects at ranges from a few meters to several kilometers, depending on frequency, power, and water conditions. Lower frequencies travel farther but provide less detail, while higher frequencies offer finer resolution over shorter distances. Many modern systems use split-beam or multibeam arrays to create three-dimensional images of targets, helping operators identify the distinct echo signatures of whales and dolphins versus fish schools or submerged rocks.

However, active sonar has a significant drawback: the sound pulses themselves can disturb marine mammals. Controversy arose in the early 2000s when military exercises involving mid-frequency active sonar were linked to whale strandings. Consequently, conservation-minded active sonar systems are designed to use lower intensity, narrower beams, and to operate in specific frequency bands less likely to cause harm.

Passive Sonar

Passive sonar does not emit any sounds; instead, it listens for the natural sounds produced by marine mammals. Whales and dolphins are highly vocal, using clicks, whistles, and song for communication, echolocation, and navigation. Passive acoustic monitoring (PAM) arrays can detect these vocalizations at distances of tens of kilometers, depending on background noise and sound propagation conditions.

PAM is non-invasive and poses no risk of acoustic disturbance, making it the preferred method for monitoring in sensitive areas such as marine protected zones. Many modern ships deploy towed hydrophone arrays that trail behind the vessel, continuously listening for cetacean calls. When a vocalization is identified by species-specific acoustic signatures (e.g., the distinctive song of humpback whales or the echolocation clicks of sperm whales), the system alerts the bridge crew.

Passive sonar does have limitations: it requires animals to be vocalizing, which is not always the case (e.g., resting or feeding animals may be quiet). Also, it cannot provide precise range or bearing without multiple hydrophones and triangulation. Nonetheless, combined with active systems, passive sonar offers a powerful complementary tool.

The Collision Problem: Scale and Species at Risk

Vessel strikes are a leading cause of death for many large whale species. The International Whaling Commission (IWC) estimates that hundreds of whales are killed or injured by ships each year worldwide, but the true number is likely much higher due to underreporting. Species most vulnerable include the North Atlantic right whale (fewer than 350 individuals remain), fin whales, humpback whales, blue whales, and gray whales.

Collisions occur most frequently in areas where shipping lanes intersect with feeding grounds, migration routes, or calving areas. High-risk zones include the Gulf of St. Lawrence, the waters off Sri Lanka, the Mediterranean Sea, and the California coast. Smaller marine mammals such as dolphins and porpoises are also struck, though their smaller size means collisions often go unnoticed.

The economic impact is also substantial—vessel strikes can damage propellers, rudders, and hulls, leading to costly repairs and delays. A single strike involving a large whale can cost a shipping company tens of thousands of dollars in downtime and repairs, not to mention potential regulatory penalties and reputational harm.

For example, in 2022, a cargo ship entering the Port of Savannah struck a North Atlantic right whale calf, killing it instantly. The incident prompted NOAA to impose temporary speed restrictions and reroute traffic—demonstrating how a single collision can ripple through the entire maritime ecosystem.

To address this, the International Maritime Organization (IMO has adopted guidelines for ship routing and speed reduction zones, but compliance remains voluntary in many regions. Real-time detection via sonar offers a way to supplement these measures with immediate, localized warnings.

Applications in Collision Prevention

Ship-Mounted Systems

The most direct application is installing sonar systems on commercial vessels. These systems continuously monitor the waters ahead and alongside the ship, providing alerts when marine mammals are detected within a danger zone—typically 100 to 500 meters. The warning gives bridge crew time to reduce speed or change course.

For example, the company Ocean Sonics has developed an integrated passive sonar system used by the Canadian Coast Guard. It consists of hydrophones mounted on the hull and a real-time classifier that can distinguish whale calls from background noise. When a detection occurs, the system sends an alarm to the captain's display and logs the event for post-voyage analysis.

Similarly, the Whale Alert system combines satellite tracking of tagged whales with shipboard sonar and AIS (Automatic Identification System) data. First deployed in the U.S. East Coast shipping lanes, it overlays real-time whale positions on electronic chart displays, enabling captains to see both sonar detections and known aggregations from aerial surveys.

Several ferry operators in British Columbia and Washington State have adopted such systems, reporting significant reductions in collision risk. The technology is especially valuable during nighttime or low-visibility conditions when visual observers cannot spot animals.

Autonomous Underwater Vehicles (AUVs) and Gliders

AUVs and ocean gliders equipped with sonar extend monitoring beyond the reach of ship-mounted sensors. These unmanned platforms can patrol shipping lanes, survey migration corridors, and transmit detection data to shore stations or vessels via satellite links. They are particularly useful for monitoring remote or environmentally sensitive areas where human presence is limited.

For instance, the Monterey Bay Aquarium Research Institute (MBARI) uses long-range AUVs with passive acoustic recorders to track blue whales in the Pacific. The data helps delineate critical habitat and supports dynamic ship-routing decisions. In the Atlantic, the Woods Hole Oceanographic Institution deploys gliders with active sonar to detect right whales in the Gulf of Maine.

One key advantage of AUVs is their ability to operate continuously for weeks, providing near-real-time situational awareness that can be integrated into maritime traffic management systems. This allows port authorities to issue "whale advisories" that recommend speed reductions or route changes for incoming vessels.

Integration with Vessel Traffic Services (VTS)

Ports and coastal states are increasingly integrating sonar data into Vessel Traffic Services (VTS). When sonar sensors—whether on buoys, AUVs, or land-based platforms—detect marine mammals near shipping lanes, the information is relayed to a central coordination center. The VTS operator can then broadcast warnings to all ships in the area via VHF radio or digital alerts.

The St. Lawrence Seaway Management Corporation, in partnership with Fisheries and Oceans Canada, operates such a system in the Gulf of St. Lawrence. A network of fixed passive sonar stations monitors the migration corridor of endangered North Atlantic right whales. When detections occur, the VTS issues a "slow down" zone, complemented by aerial surveillance. Since implementation, evidence suggests reduced collision rates and lower ship speeds during whale presence.

Real-Time Monitoring and Predictive Analytics

Machine Learning for Acoustic Classification

One of the biggest challenges in passive sonar is distinguishing the calls of different marine mammal species from each other and from background noise (ship engines, seismic surveys, snapping shrimp). Manual analysis is time-consuming and operator-dependent. Modern systems employ machine learning algorithms trained on large libraries of known vocalizations to achieve high classification accuracy.

For example, the National Oceanic and Atmospheric Administration (NOAA) Pacific Islands Fisheries Science Center uses a deep learning model called "Whale Detector" that can identify 12 species of dolphins and whales in real time from streaming hydrophone data. The model achieves over 95% precision for some species, with false alarm rates low enough for operational use.

Similarly, the company Greenridge Environmental developed an AI-powered active sonar system that can differentiate a whale's body shape from a submarine or large fish based on high-frequency imaging. These advances make sonar-based collision avoidance increasingly reliable, even in noisy environments.

Integration with AIS and Dynamic Speed Zones

Sonar detection data can be combined with Automatic Identification System (AIS) transponders on ships to create dynamic speed reduction zones. In this system, when a whale detection is confirmed within a defined area, the VTS automatically activates a temporary "right whale towing zone" on electronic charts, visible to all AIS-equipped vessels. Ships entering the zone receive a mandatory speed limit (e.g., 10 knots) until the alert is lifted.

This approach is being piloted in several regions. A notable example is the collaboration between the Port of Savannah, NOAA, and the nonprofit Whale and Dolphin Conservation. A network of bottom-mounted passive sonar arrays in the shipping channel reports detections to a cloud server, which triggers AIS alerts. In the first year, compliance with recommended speeds increased by 40%, and no collisions occurred.

Challenges and Limitations

Distinguishing Marine Mammals from Other Objects

Even with advanced active sonar, differentiating a whale from a large fish, a submarine, or a rock can be difficult. The echo from a whale's body may resemble that of a submerged shipwreck or a dense school of fish. Active sonar returns also vary with the animal's orientation—a whale swimming directly toward the sonar may produce a weaker echo than one crossing broadside.

Machine learning is helping, but training data is limited for many species in different ocean environments. False alarms can arise from bubble curtains, kelp forests, or thermoclines. Conversely, missed detections occur when whales are diving or sleeping near the surface.

Passive sonar has the advantage of specific acoustic signatures, but only if the animals are vocalizing. Silent whales or those producing faint calls (e.g., minke whales with low-amplitude sounds) are invisible to PAM. Combining both active and passive modalities seems the best path forward, but it adds cost and complexity.

Potential Disturbance from Sonar Noise

Active sonar has been shown to cause behavioral responses in marine mammals—including avoidance, temporary hearing threshold shifts, and, in extreme cases, strandings. The issue came to global attention in 2000 when a mid-frequency active sonar exercise by the U.S. Navy in the Bahamas coincided with the stranding of several beaked whales. Subsequent studies linked the injuries to decompression sickness-like effects, likely due to the whales' diving behavior being disrupted by the sound.

While commercial sonar for whale detection typically uses lower power than military systems, the risk cannot be fully eliminated. To minimize disturbance, many systems operate at frequencies outside the sensitive hearing range of the target species (e.g., above 200 kHz for toothed whales) and use directional beams to limit exposure. Nonetheless, environmental impact assessments are required before deploying active sonar in sensitive habitats.

For this reason, many conservation groups advocate for passive sonar as the primary detection tool, reserving active sonar for situations where passive detection is insufficient—such as in very quiet waters where animals may not be vocalizing.

Limited Detection Range in Certain Conditions

Sonar performance is affected by water temperature, salinity, depth, and ambient noise. In shallow or turbid waters, sound waves may be reflected or scattered, reducing range. Heavy ship traffic, storms, and wind also raise background noise levels, masking faint echoes or calls. These factors can lead to detection gaps, especially for small cetaceans like porpoises that produce lower-intensity signals.

Warm-water environments like the Gulf of Mexico can have strong thermoclines that trap sound energy, while cold polar waters often allow long-range propagation but also harbor noise from ice cracking and breaking. Seasonal changes further complicate predictions. Operational sonar systems must be calibrated and maintained regularly to ensure reliable performance across these variable conditions.

Future Developments

Higher-Frequency Imaging Sonar

Advances in transducer technology now enable imaging sonar that operates at frequencies above 1 MHz, offering near-photographic resolution of underwater objects. These systems can produce real-time video-like imagery showing the shape and even the movement of marine mammals. When combined with automated pattern recognition, they promise to dramatically reduce false alarms and improve species identification. Their main drawback is short range (tens of meters), but this is acceptable for bow-mounted systems on ships traveling at slow speeds through high-risk zones.

The company Sound Metrics Corporation has developed the ARIS (Adaptive Resolution Imaging Sonar), used by researchers to observe whale foraging behavior. In the future, such devices could be integrated into hull fairings for continuous wide-angle imaging as ships approach.

AI and Edge Computing

Onboard processing of sonar data is shifting from dedicated hardware to software-defined systems running AI models on edge computers. This allows for sophisticated classification algorithms to run in real time without relying on cloud connectivity, which may be unavailable or delayed at sea. Edge AI also enables continuous learning: the system can refine its detection model based on local acoustic conditions encountered during the voyage, improving performance over time.

Several startups are developing standalone "smart sonar" modules that can be retrofitted onto any vessel, similar to an aftermarket collision avoidance system for cars. These modules include GPS, AIS receiver, hydrophone arrays, and a small computer running open-source detection models. The expectation is that widespread adoption will drive down costs and accelerate regulatory demands for such equipment on large commercial ships.

Multisensor Fusion

Future collision prevention systems will not rely solely on sonar. They will fuse data from multiple sources: radar (for surface detection), infrared cameras (for thermal detection of whales at the surface), visual cameras (for daytime identification), satellite imagery, and even electronic tagging of whales. Sonar will provide the critical underwater layer, but the total system will have redundancy to cover different conditions.

For instance, the "Shipboard Whale Detection System" being developed by the U.S. Department of Transportation combines a forward-looking infrared camera with a low-frequency active sonar and an acoustic listening array. The fusion algorithm weighs each sensor's confidence level and produces a unified risk score, which in turn triggers appropriate alerts. Early trials aboard NOAA research vessels have shown a 90% reduction in close encounters compared to visual observation alone.

Regulatory Drivers and Industry Standards

As the technology matures, regulatory bodies are beginning to mandate sonar-based whale detection on certain ship types. In 2023, Transport Canada proposed amendments requiring all vessels over 20 meters in certain high-risk zones to carry approved PAM systems by 2026. The European Union is considering similar rules for the Mediterranean and Baltic Seas. The International Maritime Organization has published a circular recommending the use of "recommended shipboard detection systems" and encourages ports to provide real-time acoustic data.

Insurance companies are also taking notice. Some marine liability insurers now offer premium discounts for vessels equipped with PAM and active sonar, recognizing the reduced risk of accident-related claims. This financial incentive may accelerate adoption beyond the slow pace of regulation.

Conclusion

Sonar technology has evolved from a tool for military navigation and fisheries science into a vital instrument for marine conservation. By detecting and tracking marine mammals in real time, it enables ship operators to avoid collisions, protecting both endangered species and commercial interests. Active sonar offers precise, real-time imaging of underwater objects, while passive sonar listens for the natural sounds of whales and dolphins without causing harm. Integration with AIS, machine learning, and vessel traffic services creates a layered defense against one of the greatest threats facing whales today.

Challenges remain—distinguishing species, avoiding disturbance, and maintaining performance under variable conditions—but ongoing advances in high-frequency imaging, edge AI, and multi-sensor fusion promise to overcome many of these obstacles. Already, successful implementations in the Gulf of St. Lawrence, the U.S. East Coast, and the Pacific Northwest have demonstrated that sonar-based collision prevention is not just feasible but effective. As regulations tighten and technology becomes more affordable, we can expect sonar systems to become standard equipment on ships navigating whale-rich waters, contributing to a future where human maritime activities coexist safely with the magnificent animals that share our oceans.

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