Buriganga too polluted for most fish, yet endangered dolphins survive

The Buriganga River, which runs alongside Dhaka, is among Bangladesh’s most polluted waterways and ranks among the world’s most contaminated urban rivers.

Yet, despite severe pollution and the near disappearance of native fish, around 20 endangered Ganges river dolphins continue to survive in the river, according to a recent study.

The 29-kilometre river receives toxic industrial effluent, tannery waste, untreated sewage and solid waste from the densely populated capital and surrounding areas. The pollution has left large parts of the river severely degraded and unsuitable for many aquatic species.

A study published in the Journal of Asia-Pacific Biodiversity estimated a population of about 19.79 Ganges river dolphins in the Buriganga. Researchers conducted surveys over 12 months, from September 2023 to August 2024, covering a 27-kilometre stretch of the river along the southern edge of Dhaka.

According to MONGABAY, the researchers recorded dolphins in 10 of the 12 survey months. Sightings peaked in December and remained relatively high during winter, while no dolphins were recorded in October 2023 or in July and August 2024.

Dolphin groups averaged 4.18 individuals, while calves accounted for nearly 16% of sightings, suggesting that the species continues to reproduce in the river despite its severely degraded condition.

Surviving on pollution-tolerant fish

“The Buriganga is not a usual place for dolphins, where native fishes have largely vanished,” researcher Md Aminur Rahman told Mongabay. He said the river is now dominated by invasive suckermouth catfish, locally known as sakar fish.

“When we found [the dolphins], we were amazed at how it was possible. Because the river is not livable for fish,” Rahman said, noting that dolphins were recorded during 10 months of the survey.

The continued presence of dolphins appears to be linked partly to their ability to feed on fish that can survive in highly polluted water.

Mohammad Abdul Aziz, a zoology professor at Jahangirnagar University, said some catfish can breathe both underwater and at the surface and are among the species most tolerant of pollution.

“Right now, the Buriganga is full of suckermouth catfish. We believe the dolphins are hunting these fish,” he told Mongabay.

But the dolphins’ ability to survive does not mean the river is healthy.

“If we want to conserve the dolphins in the Buriganga, the priority must be to stop the pollution,” Aziz said. He also warned that the high number of vessels on the river was disrupting the dolphins’ natural movement.

Heavy metals add to the threat

The Buriganga receives untreated industrial effluent from tanneries, textile and dyeing factories, battery plants, metal-processing units, re-rolling mills, dockyards and pharmaceutical industries, according to a 2025 study on the river’s water pollution.

City corporation sewage, solid waste and urban runoff are also major sources of contamination.

The study identified heavy metals including chromium, lead, cadmium, arsenic, mercury, nickel, copper and zinc in the river water and sediments, with concentrations regularly exceeding WHO and national safety limits.

During the dry season, dissolved oxygen levels can fall close to zero, far below the environmental standard of 5 milligrams per litre. At the same time, biochemical and chemical oxygen demand rise sharply, indicating severe organic pollution.

The study also found widespread heavy-metal contamination in river sediments, with pollution and ecological-risk indicators reaching high or very high levels at many sampling sites.

Researchers have linked the contamination to biodiversity loss and the accumulation of pollutants in fish, vegetables and water, creating potential health risks for people who live beside or depend on the river.

A wider decline in Bangladesh

The Buriganga dolphins are part of a much larger but declining population of Ganges river dolphins in Bangladesh.

A joint survey by the Bangladesh Forest Department and the Wildlife Conservation Society recorded 2,307 Ganges river dolphins across the country’s rivers. The survey covered about 4,900 kilometres across 45 rivers between 2024 and 2025 and identified 25 dolphin hotspots.

Researchers found only 27 dolphins along a 305-kilometre stretch of the Turag, Buriganga, Bongshi, Surma and Kalni rivers, highlighting the pressure faced by dolphins in heavily polluted waterways.

The Ganges river dolphin is found mainly in the Ganges-Brahmaputra-Meghna river systems of Bangladesh and India. Its population in Bangladesh is under pressure from pollution, habitat degradation and harmful fishing practices, particularly illegal gillnets.

Aziz said the decline was evident in rivers around Dhaka.

“Back in 2013, we found dolphins in the Turag River during our study, but they are no longer seen there,” he said.

He added that dolphins from the Buriganga sometimes move downstream through the Dhaleshwari into the Meghna before returning, while most of the Buriganga population remains around the Bosila area.

“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,” Aziz said.

The government established three wildlife sanctuaries in the Sundarbans in 2012 to protect endangered freshwater dolphins, including the Ganges river dolphin and the Irrawaddy dolphin.

The continued presence of nearly 20 Ganges river dolphins in the Buriganga, however, highlights both the species’ resilience and the urgency of restoring the river’s ecological health. Conservationists say reducing pollution, controlling harmful fishing practices and managing vessel traffic will be essential if the dolphins are to survive in one of Dhaka’s most degraded waterways.