- Many Indo-Pacific finless porpoises die each year along Goa’s coast, but without a population baseline, their conservation status remains unknown.
- A new study found acoustic surveys detect finless porpoises four times more often than visual surveys, suggesting current population estimates based on visual surveys may be unreliable.
- With no dorsal fin and only fleeting surfacing in shallow coastal waters, the species is easily missed, making acoustic monitoring the best tool to assess its future.
The Arabian Sea was listless as we headed out one morning to spot Indian Ocean humpback dolphins, famously extroverted and known for their acrobatics. A needlefish darted across the surface, pirouetting over the chop as the sun beat down on the deck, turning the railings hot to the touch. When you stare at the ocean long enough, your mind starts to play tricks: every shifting shadow looks like a flipper, every cresting wave, a dorsal fin. Then a shape appeared.
To my untrained eye, it looked like a dolphin. But Imran Samad, a Ph.D. researcher at the Centre for Ecological Sciences at the Indian Institute of Science, whose work focuses on marine megafauna bycatch in India’s multi-gear fisheries, leaned forward over the roar of the outboard engine and said, “It’s a porpoise.”
It was a blink-and-you-miss-it moment. The animal surfaced for barely a second before vanishing back into the brown water. And that, as Samad would explain over the course of the morning, is precisely the problem — not just for researchers trying to spot them, but for the conservation of a species that most of India does not know exists in these waters.
An animal built for invisibility
The Indo-Pacific finless porpoise (Neophocaena phocaenoides) is one of the smallest cetaceans in Indian waters, measuring just 1.5 to 1.9 metres in length and weighing 30 to 45 kilograms. It belongs to the family Phocoenidae — an entirely different lineage from dolphins — and the differences run deeper than the obvious absence of a dorsal fin. It has a blunt, rounded head and small, spade-shaped teeth set inside a mouth that appears perpetually smiling. In place of a dorsal fin is a low ridge of hardened tubercles running along its back.
It is an animal built for the margins — shallow, muddy estuaries, tidal creeks, mangrove channels, inshore shallows.
“In contrast to humpback dolphins, finless porpoises are highly inconspicuous,” Samad said, his eyes still scanning the surface even after we had cut the engine. “They surface for barely a second to breathe before diving right back in. The only way you can spot a porpoise is if you are looking at the right place at the right time.”
The sea that looks like an impenetrable wall of green-brown to a human observer is a detailed soundscape to a finless porpoise. In sediment-rich nearshore waters of the Arabian Sea, the finless porpoise navigates by echolocation, emitting narrow-band, high-frequency pulses, far above the range of human hearing. The number, speed and rhythm of the clicks seem to be rearranged for different forms of communications. “Unlike dolphins, porpoises have evolved to communicate and find food using these high frequency “ultrasonic” clicks. What’s interesting or even cool about this is that dolphins, including humpback dolphins, and even Orcas cannot hear porpoises at such high frequencies. A great defense mechanism for porpoises to survive in a sea full of larger cetaceans,” says Dipani Sutaria, a marine mammal researcher studying the coasts of India since 2001.
A protected coast under pressure
The Sindhudurg-Karwar stretch of India’s west coast was designated an Important Marine Mammal Area (IMMA) by the IUCN in 2019, as the coastline supports resident populations of both the Indian Ocean humpback dolphin and the Indo-Pacific finless porpoise.
The two species share a habitat, but not a harmonious relationship. “Humpbacks and porpoises are found in the same habitat, but they are more combative neighbours than good friends,” Samad said.
The shallow coastal zones that finless porpoises depend on are becoming increasingly hazardous, heavily fished, industrialised, and crowded with boat traffic. Underwater, their private acoustic world is being shattered by high-speed vessels, commercial engines, and the constant din of mechanical noise, making it harder for them to detect the fine-grained echoes that help them navigate the difference between food and danger.
And then there is bycatch. “Bycatch in fisheries is by far the greatest threat to porpoises,” Samad said. His Ph.D. research tracks the patterns and mechanisms of marine megafauna bycatch across India’s multi-gear fisheries. The numbers from Goa alone are stark. “Every year, an estimated 26 porpoises die in the mere 100 km coastline of Goa alone, many of which are likely a result of fisheries interactions,” Samad said.
Why bycatch is so lethal for porpoises comes down to teeth. “Porpoise teeth are paddle or flat shaped, in a mouth without a long beak, that can scoop, lift, loosen, grasp and crush,” Sutaria said. Dolphin teeth are conical and sharp, with a beak, acting like chisels to pierce. “Dolphins can sometimes tear out of nets, but porpoises cannot,” Sutaria noted. “They get stressed easily and fishers say they cry like human children when entangled.”
The danger is relatively recent. “Before commercial fishing, motorised net fisheries and synthetic net material came into being, this overlap was one of cordial coexistence,” Sutaria said. Nylon nets and motorisation turned it into a fatal competition. Finless porpoises are especially vulnerable to nets their echolocation cannot perceive or escape, including gillnets, seines, and trawl nets.
The blindspot in the data
Marine mammal research in India barely existed as a field when Sutaria started. In 2001, funded to survey coastal cetaceans in Goa and the Gulf of Kutch, she got around by bus, interviewing fishers and documenting strandings along the coast. Her equipment was a single borrowed handheld GPS, a borrowed marine binocular, and a film camera. Her research vessel was a small sailboat lent by the Goa Sailing Association.
Two decades later, the tools have changed almost beyond recognition.
A 2025 study published in Nature Scientific Reports, led by Isha Bopardikar of IISER Tirupati and Cornell University’s K. Lisa Yang Center for Conservation Bioacoustics, compared acoustic and visual survey methods for both species across a 376 sq km area off the Sindhudurg coast, north of Goa. For humpback dolphins, the two methods were broadly comparable. For finless porpoises, the gap was dramatic: visual surveys recorded 0.03 groups per kilometre, while acoustic surveys recorded 0.12 groups per kilometre — four times as many.
In other words: every population assessment using conventional visual boat surveys has been working with, at best, a quarter of the picture.
“Reliable visual estimates exist for finless porpoises,” Bopardikar said, “primarily from areas like Hong Kong, where visibility is better. They’ve also implemented double-observer surveys to estimate correction factors for visual detection. In our waters, we’re limited by visibility conditions, and we don’t have that kind of data for correction factors. So relying solely on visual estimates here is a drawback.”
Finless porpoises have no dorsal fin — the most reliable visual marker in cetacean surveys. Their surfacing is brief and irregular. Group sizes are small. They prefer turbid water that shortens sighting distances. And rough sea conditions push visual detection close to zero, while acoustic detection remains unaffected. The study’s conclusion is clear: Acoustics is a more efficient tool for tracking this species in this region.
Bopardikar, however, believes a combination of various factors contribute to the difficulty in population assessment. “There’s a major factor of behaviour which wasn’t really possible to quantify during the study I did, but that should be considered in future work.”
This is not a minor technical footnote. It means the impact of bycatch mortality figures cannot be based on population estimates from visual counts. The conservation assessments, the IUCN evaluations, all of it has been built on incomplete detection.
Read more: Protected on paper, forgotten by people, humpback dolphins struggle for survival
Listening as a last resort
Cetacean research in India remains in its early stages. “Visual surveys are limited for this species, and we don’t have a solid pool of trained observers,” Bopardikar said. “Changing observers frequently, having new folks without experience, can introduce bias during visual surveys.”
A shift in approach could begin generating the long-term population trend data that is currently absent for the species. That baseline of population size and trends does not yet exist for finless porpoises in the Sindhudurg-Karwar IMMA, according to Sutaria.
We restarted the engine. The roar of the outboard shattered the quiet. The late afternoon light turned the turbid water into a sheet of liquid gold. Somewhere beneath that churned-up surface, an animal no bigger than a young child was rolling up to breathe — leaving the briefest, quietest trace before slipping back into the dark.
To the tourists a few kilometres away scanning the horizon for leaping dolphins, it would register as nothing. But once you have seen that smooth, finless back break the water, it becomes impossible to unsee. And harder still is to ignore what the science is now confirming: We have been missing this animal almost entirely, and that by the time the right tools are in place to conserve them, there may be very little left to save.
Banner image: A pod of Indian Ocean humpback dolphins photographed from above off the Sindhudurg-Goa coast. Image by Imran Samad.
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