The Sunderbans mangroves, which are facing danger from increased salinity in the rivers. (Below) Gangetic dolphins and hilsas are under threat, too.

The Indian Sunderbans will lose several of its signature species, such as hilsa, Gangetic dolphin and Sundari tree, unless its rivers receive additional fresh water at 1,707 cubic metre per second (cumec) in the non-monsoon months, a study has pointed out.

The desired rate, in more familiar terms, is 1.7 lakh buckets (of 10 litre each) per second.

The mangrove forest on this side of the border gets 1,100 cumec fresh water on an average in the dry months.

Around 40 per cent of the 10,000sq km of the Sunderbans is in India and the rest in Bangladesh.

The India-Bangladesh joint study, supported by the International Union for Conservation of Nature (IUCN), says that an increased salinity and a reduction in the depth of the rivers in the delta are threatening the fragile eco-system of the Sunderbans in general and the signature species in particular.

“The pilot study indicates that the present level of the flow (of the rivers feeding the delta) is insufficient to maintain the ecosystem and the ecosystem services in the Sunderbans,” said Sugata Hazra, the director of the oceanographic department at Jadavpur University, who led the study on the Indian side.

The report of the study was discussed at a recent meeting between experts from India and Bangladesh.

The insufficient flow has been blamed for the rise in the salinity of the river water, which the experts said is killing the Sundari trees (after which the forest is believed to have been named) and made the delta an unsafe corridor for hilsa and Gangetic dolphin.

To sustain Sundari trees, the salinity has to be within 14ppt (parts per thousand). The study has pegged the non-monsoon salinity in the Sunderbans rivers at 30. In the monsoon, it varies between 15 and 20ppt.

Salinity of 14ppt means 14 pounds of salt per 1,000 pounds of water.

“To bring down the salinity from 30 to 14ppt, the delta has to get at least an additional 1,200 cumec fresh water from the Hooghly and 507 from the other river systems, especially Ichhamati, during the lean (non-monsoon) months,” Hazra said.

The 1,100 cumec fresh water the Sunderbans now gets in the lean months is entirely from the Hooghly system. “The other systems contribute almost nothing,” Hazra said.

Gangetic dolphins, which were found in abundance even a couple of decades back, can hardly be spotted now.

Hilsa migration from the sea through the Indian Sunderbans is also affected as the fish cannot lay eggs in highly saline water.

“The hilsa yield on our side of the border will improve if the non-monsoon salinity could be brought down to 14ppt,” an expert said. “The monsoon salinity in the Meghna, a principal river in the Bangladesh Sunderbans, is less than 2ppt. That explains why the hilsa yield is much better in Bangladesh.”

An additional 1,707 cumec of water in the lean months will also lead to an increase in the depth of the rivers in the delta. “The increased depth and decreased salinity will go a long way in making the Indian Sunderbans a favoured corridor for hilsa and Gangetic dolphin again,” the expert said.

Hilsa needs around 10 metre of water and Gangetic dolphin at least 6 metre.

The Sunderbans is rich in faunal diversity, too. Experts said the forest is home to 50 species of mammals, 320 species of inland and migratory birds, 50 species of reptiles and around 400 species of fish.

“Our estimate of additional water required to save the Sunderbans ecosystem is based on the species whose details are available. We would like to take up a more comprehensive study,” said Hazra.