The dolphin census going on in the Sundarbans area is indicating that the number of the mammals in the mangrove forest rivers is dropping and the experts are blaming climate change and withdrawal of fresh water from upstream.

Experts think rise of salinity in the coastal rivers, poor flow of fresh water in the Ganges river system and growing commercial activities in and around the Sundarbans are threatening the iconic animal. 

“We have not completed the census yet, but still we can say that the number of dolphins is decreasing, comparing it with the number of the last census in 2006,” said Monirul H Khan, eminent wildlife expert who is leading the census team.

His two-member team has completed their work in most areas of the Sundarbans.

The department of forest is carrying out the census with support from the United Nations Development Programme (UNDP). 

The first dolphin survey in the Sundarbans was conducted by Wildlife Conservation Society (WCS) in 2006. It found 225 gangetic dolphins, locally known as Sushuk, and 451 irrawaddy dolphins.

Of the 40 species of dolphins in the world, four are found in the Sundarbans area. Finless porpoise and the pink Indo-pacific humpback dolphins are occasionally seen here.

“This time we have seen one family of four pink dolphins and two finless porpoise. It is hard to determine their status as these two species are not found in the Sundarbans on a regular basis,” said Monirul.

This is the first time a survey team has seen the pink dolphins in the Sundarbans area. They are one of the largest dolphins and found in the east coast of India, and through the Indo-Malay archipelago and east towards Australia where the water is very saline.

The pink dolphin was first sighted in the country in 2002.

Asked if the pink dolphins were living in the Sundarbans as the water salinity increased, Monirul said it could not be said without research.

“What we can say is that we have seen a family of four dolphins, may be for the first time a full family has been sighted in the Sundarbans,” he said.

Dolphins are among the world's most endangered mammals as per the International Union for Conservation of Nature (IUCN) red data book. Dolphin experts have been pushing the government to announce parts of the Sundarbans rivers as dolphin sanctuaries as the mangrove forest is the world's largest single habitat of freshwater cetaceans.

In 2012, the government declared, 10.7 square km area of the Pashur and Andharmanik rivers and their channels in Dhangmari, Chandpai and Dudhmukhi areas as “dolphin sanctuaries”.

During dolphin census last week near the Dhangmari sanctuary, 15 to 20 dolphins, adults and juveniles, were spotted.

Modinul Ahsan, divisional forest officer Wild life Sundarbans and also the census project director, said it was necessary to impose more restrictions to conserve the dolphins in the Sundarbans.

Even the sanctuary areas are not protected. In some areas, one side of a river is marked as protected while the other half is not. Fishermen fish in one side of the river there. This is not realistic, said Modinul.

Although dolphins are not usually captured with intent, every year dozens of dolphins get entangled in fishing nets and die. They are also threatened by pollution of water transports operating in the Sundarbans.

Poor flow of fresh water from upstream, blamed on the upper riparian country, is increasing salinity of river water, resulting in dolphins disappearing, experts said.

They said just a few years ago, dolphins could be sighted in all major rivers of the country. But now they can only be seen in some large rivers, like the Padma and the Karnaphuli.

Three decades ago, there were dolphins in the Buriganga near Dhaka, said Khasru Chowdhury, eminent Sundarbans expert who went to see the census being carried out. 

Experts said over-fishing was a major cause for the decline in dolphin numbers. They also reported that dolphins become entangled in bank-to-bank fishing nets and are hunted in some areas for their alleged medicinal properties.

Khurshid Alam, assistant country director to the UNDP, Bangladesh, said, they were supporting the dolphin survey of Bangladesh as part of supporting Bangladesh to achieve the Sustainable Development Goal (14), which is protecting underwater biodiversity.

Under this project, they are also supporting local fishermen community with alternative livelihood so that they do not disturb the dolphin habitat.

They might extent their support for expanding sanctuaries in future, Khurshid said. 

Rezaul Karim Chowdhury, manager of the project titled, “Expanding the Protected Area System to Incorporate Aquatic Systems Project”, said they were trying to create awareness among locals so that they do not kill dolphins.

The other member of the two-member Dolphin census team is Dr Aziz of the zoology department at Jahangirnagar University.