Engineers tracked Ganges river dolphins in muddy water by listening for their ultrasonic echolocation clicks with passive hydrophone arrays. The detectors did not send sound into the river: they recorded clicks and used differences in their arrival time at multiple sensors to estimate where dolphins were, including their depth. That can reveal animals and high-use areas that visual surveys miss, giving conservationists better information for monitoring and protection.
Why Ganges river dolphins are difficult to count
The Ganges river dolphin (Platanista gangetica) lives in opaque freshwater and is nearly blind. It relies on echolocation to navigate and hunt, producing ultrasonic clicks that can be detected underwater even when the animal is invisible from a boat. A visual survey may miss a dolphin that does not surface during the observer’s brief window; listening provides another way to detect it.
The National Mission for Clean Ganga identifies the species as endangered. It is protected under Schedule I of India’s Wildlife Protection Act and listed in Appendix I of CITES. Dams and barrages fragment its habitat, while pollution and fishing nets pose additional threats; nets can prevent dolphins from surfacing to breathe.
Population figures need their dates and methods attached. IEEE Spectrum’s 2016 field report said fewer than 2,000 dolphins were thought to exist at that time. A 2026 Government of India Press Information Bureau release reported a nationwide population status of 6,324. These are figures from different survey contexts and dates, and without directly comparable methods they should not be read as a measured increase from one number to the other.
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How hydrophones track dolphins without sonar
What a hydrophone does
A hydrophone is an underwater microphone: it listens for sound in water and turns it into a signal researchers can record and analyze. In the Kanpur mission described by IEEE Spectrum in 2016, the stationary detector was passive. It received dolphin clicks rather than transmitting pulses, so it was not active sonar.
How the fixed array estimated location
The team arranged six hydrophones on a metal X, with a sixth sensor below the center. A click reaches sensors at slightly different times because each is a different distance from the animal. Comparing those arrival times lets researchers estimate direction and distance; the lower sensor adds information about depth, supporting a three-dimensional location estimate. A compass, GPS and inclinometer helped correct for the array’s orientation and tilt.
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The technical scale described in the 2016 report was small and fast: a click lasted about 40 microseconds, with an average frequency of about 65 kilohertz. The report used roughly 1,500 metres per second for the speed of sound in water in its triangulation explanation. The fixed array was prepared to remain submerged for about six months; that was the system’s planned deployment duration, not a claim that every station operated continuously for that long.
How the towed detector surveyed a stretch of river
For mobile surveying, researchers towed a detector with four hydrophones behind a boat. A signal processor and laptop displayed detected clicks as the team moved along the river. In one run reported in 2016, the researchers surveyed nearly 7 kilometres and located 14 resident dolphins. The display showed streaks of detections despite the animals being hard to see at the surface.
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Acoustic surveys and visual counts answer different questions
| Survey approach | What it can do | Important limitation |
|---|---|---|
| Visual observation | Record animals seen at the surface and their visible behavior. | In turbid water, a count depends on animals surfacing where and when observers can see them; submerged dolphins can be missed. |
| Hydrophone survey | Detect echolocation clicks underwater and, with multiple sensors, estimate direction and location without relying on a sighting. | Click detections must be interpreted and localized; a detection is not automatically a unique animal, so survey design and analysis matter when counting. |
| Long-term passive station | In principle, collect repeated acoustic observations at a fixed location, helping identify movement or recurring use of a stretch. | The Kanpur report described a system prepared for a roughly six-month submerged deployment and a future network vision, not a completed river-wide autonomous network. |
Acoustics complements rather than replaces observation. A hydrophone can reveal activity where visibility is poor, while a visual survey can provide direct sightings. The Kanpur account establishes that researchers detected and triangulated clicks; it does not give a general accuracy rate for acoustic counts or prove that every click can be assigned to an individual dolphin.
What the Kanpur mission found—and what it did not establish
The 2016 field report’s mobile run produced a concrete result: nearly 7 kilometres surveyed and 14 resident dolphins located. It also showed why acoustic information can matter locally. Researchers found fishermen using nylon nets near an area where dolphins were concentrated, which conservationist Viveksheel Sagar planned to address through education work with local fishers around the “dolphin hot spot.” The observation points to a possible focus for conservation outreach; it is not evidence that the acoustic system itself removed the net threat.
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The Kanpur trial built on earlier field work. In Odisha, researchers had used hydrophones to triangulate a solitary dolphin’s position in three dimensions. The team also spent six dry seasons refining equipment in a comparatively clean 12-kilometre stretch between dams before taking it to the more difficult Ganges trial. This history matters: the Kanpur work was a field experiment with a developing instrument, not a finished, nationwide census system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How this work fits with newer monitoring and rescue efforts
In Bangladesh’s Jamuna River, a 2026 Chinese Academy of Sciences report described a separate acoustic-monitoring effort using Chinese technology. The team refined recognition thresholds to reduce false alarms, and the report noted that systematic surveys in Bangladesh remain scarce. That work is distinct from the earlier Kanpur experiment: it concerns a different river and monitoring context, and its threshold-refinement detail highlights that acoustic detections need careful filtering. The reported information does not establish a directly comparable count or validation result between the two efforts.
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India’s conservation program also includes actions beyond acoustic monitoring. A 2026 Government of India release reported a nationwide population status of 6,324, a Dolphin Rescue Ambulance inaugurated on 13 January 2026, and eight dolphins rescued and released. A separate government release said the National Mission for Clean Ganga approved the “Advancing Rescue System for the Protection of Stranded Ganges River Dolphins” project with an estimated budget of ₹1 crore. Rescue capacity addresses stranded animals; it is a different tool from monitoring where dolphins travel and gather.
The Project Dolphin action plan calls for systematic research and monitoring and identifies technology as part of conservation implementation. Harumi Sugimatsu, the University of Tokyo engineer who led the Kanpur team with Rajendar Bahl of IIT Delhi, Indian conservationist Sagar, SGK System Giken and KDDI, described the longer-term goal as understanding migration patterns. The team’s vision was a network of autonomous stations sending data from multiple stretches of the Ganges, potentially giving conservationists a more continuous picture than occasional boat surveys. That network was a goal, not a system the 2016 report said was already operating.
What technology can—and cannot—do for dolphin conservation
Reliable acoustic records could help identify stretches used for travel, feeding or raising calves, and direct attention toward places where net-risk education, habitat protection or further monitoring may matter most. Those benefits depend on sound data being interpreted in context and translated into local action. Detection alone does not remove pollution, reconnect fragmented habitat or keep fishing nets from endangering dolphins.
For readers interested in the equipment, the relevant category is an underwater hydrophone, but the Kanpur detectors were custom research systems: a six-sensor fixed array and a four-sensor towed unit, with signal processing and location analysis. A consumer hydrophone or waterproof recorder may be useful for listening or recording, but it is not a plug-and-play substitute for a calibrated multi-sensor array, synchronized timing, orientation data and specialist analysis.
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