Against the odds, endangered dolphins survive in Bangladesh’s polluted river

• The Buriganga River in Bangladesh is one of the five most polluted rivers in the world, with heavy metal pollution from chromium, lead and cadmium.
• However, a recent survey found that around 20 Ganges river dolphins survive in this toxic river.
• More than 2,000 Ganges river dolphins were recorded across 4,900 kilometers (3,045 miles) across 45 rivers in Bangladesh in another survey.
• For river dolphins to survive in Bangladesh, the country needs to keep a check on the intense pollutions and on harmful fishing methods, conservationists say.

The Buriganga River, which flows by Dhaka city, is one of Bangladesh’s most polluted waterways and ranks among the most contaminated urban rivers in the world. Every day, the 29-kilometre-long (18-mile) Buriganga River absorbs toxic industrial effluent from hundreds of factories, continuous tannery discharges, untreated sewage and solid waste, leaving much of the river virtually lifeless for fish.

However, in the Buriganga, where native fish species have essentially vanished, endangered Ganges river dolphins (Platanista gangetica) continue to persist. A recent study published in the Journal of Asia-Pacific Biodiversity found that around 20 dolphins live in the river.

Researchers conducted the study for 12 months, from September 2023 to August 2024, and systematically surveyed a 27-km (16.7-mi) stretch of this heavily polluted river that skirts the southern edge of Dhaka.

“The Buriganga is not a usual place for dolphins, where native fishes have largely vanished. Leaving the river full of invasive suckermouth catfish [Hypostomus Plecostomus, locally called sakar fish],” one of the researchers, Md. Aminur Rahman, told Mongabay. “When we found [the dolphins], we were amazed at how it was possible. Because the river is not livable for fish. Around the year, we didn’t track any dolphins only during two of the months, but all the other 10 months we tracked them.”

The highest number of dolphins were seen in December, and throughout the winter season, and none were seen in October 2023 or between July and August of 2024. The dolphin groups averaged 4.18 individuals, higher than in earlier reports, and calves accounted for nearly 16% of the sightings, indicating that reproduction continues despite the extreme pollution conditions.

Heavy metal pollution turns the Buriganga toxic

A 2025 study on Buriganga River’s water pollution analysis shows that the river receives untreated industrial effluent from tannery waste, textile and dyeing factories, battery plants, metal-processing units, re-rolling mills, dockyards, and pharmaceutical industries. Other primary contributors to the pollution include city corporation sewage, solid waste and urban runoff. Key heavy metals found in water are chromium, lead, cadmium, arsenic, mercury, nickel, copper and zinc, which regularly exceed WHO and national safety limits in both water and sediments.

During the dry season, dissolved oxygen frequently falls to near zero (well below the environmental standard of 5 milligrams per liter), biochemical oxygen demand and chemical oxygen demand soar, and sediments show severe contamination, with Pollution Load Index (PLI — a tool to measure and compare the degree of heavy metal contamination) values exceeding 1 at most sampling sites and high Potential Ecological Risk Index (PERI — a method to evaluate heavy metal pollution and its potential ecological danger) scores placing many locations in the high-to-very-high risk category, the study reads.

The study also shows that these pollutants have collapsed native fish populations, triggered widespread biodiversity loss, and entered the food chain through bioaccumulation in fish, vegetables and water, posing serious risks of kidney damage, neurological disorders, developmental problems and cancer to the millions of people who live alongside or depend on the river.

A Canada-based awareness nonprofit, Sea Smart, identified Buriganga as the fifth most polluted river in the world.

Mohammad Abdul Aziz, zoology professor at Jahangirnagar University in Dhaka, told Mongabay, “The Buriganga’s condition is not suitable for most fish, yet some [types of] catfish are still present. Some catfish can breathe both in water and by taking air from the surface, and suckermouth catfish are among the most pollution-tolerant species. Right now, the Buriganga is full of suckermouth catfish. We believe the dolphins are hunting these fish.”

He added, “If we want to conserve the dolphins in the Buriganga, the priority must be to stop the pollution. The number of vessels on the river is also excessive now. These vessels severely disrupt the dolphins’ natural movement.”

River dolphins in Bangladesh

A recent survey jointly conducted by Bangladesh’s Forest Department and WCS Bangladesh recorded 2,307 Ganges river dolphins living in the country’s various rivers. The survey covered around 4,900 km (3,045 mi) across 45 rivers between 2024 and 2025.

They identified 25 dolphin hotspots. Along 305 km (190 mi) of the Turag, Buriganga, Bongshi, Surma and Kalni rivers, they found a total of 27 dolphins, despite the rivers’ heavy pollution.

In 2012, the Government of Bangladesh declared three wildlife sanctuaries for endangered freshwater dolphins in the Sundarbans mangroves. The sanctuaries aim to protect two species of freshwater dolphins — the Ganges river dolphin and the Irrawaddy dolphin (Orcaella brevirostris).

The Ganges river dolphin largely inhabits the Ganga and Brahmaputra river systems in India and Bangladesh. However, their presence is declining in Bangladesh for several reasons, including pollution and illegal use of gill nets for fishing.

“Ganges river dolphins in Bangladesh are declining for several reasons,” Aziz said. “Illegal fishing in the rivers has increased, and the widespread use of illegal types of fishing nets trap and kill these dolphins. Back in 2013, we found dolphins in the Turag River during our study, but they are no longer seen there. The Buriganga dolphins sometimes move downstream through the Dhaleshwari River into the Meghna River and then return. Most of them stay around the Buriganga’s Bosila area.”

He added, “To protect the country’s Ganges river dolphins, we must reduce river pollution and at the same time stop harmful fishing practices and other activities.”

This story was published on Mongabay