Rise in salinity in the water system that makes the Indian Sunderbans has resulted in a decrease in population of the Ganges River Dolphins in the region.

A recent study covering 100 km of rivers and channels around the Sunderbans has revealed that the national aquatic animal is no longer sighted in the central and eastern parts of the archipelago. Only in the western part of Sunderbans, where the salinity is lower, could researchers find some evidence of the species.

The details of the study have been published in Journal of Threatened Taxa , in an article titled Possible Range Decline of Ganges River Dolphin Platanista Gangetica in Indian Sundarban . The paper, authored by Sangita Mitra and Mahua Roy Chowdhury, states that “sighting records in the present study reveal that distribution of GRD (Ganges River Dolphin) is influenced by the salinity level of the waterways”.

“The study is indicative of how natural changes, including the phenomenon of climate change, and human interventions in the Indian Sunderbans are having an adverse impact on the habitat of the species” Ms Mitra told The Hindu .

Ms. Roy Chowdhury, the other researcher who carried out the study from 2012 to 2016, said that because of its unique body shape, it becomes difficult for the dolphin to remain submerged in waters with high salinity. According to her, freshwater flow to the Sunderbans is crucial for the subsistence of these species. She pointed out that the hyper-saline zone in the central part of the Sunderbans, which includes areas such as Raidighi and Patharpratima, has lost connectivity with the upstream freshwater flow. Though there is some fresh water connectivity and flow in the eastern part, salinity levels were still high and there was therefore no evidence of the Ganges River Dolphin in this region.

The rise in sea level, triggered by climate change, is one of the reasons for the increase in salinity of waters of rivers and channels. “Hydrological modifications like water diversion and commission of large barrages upstream have had a great impact on the salinity profile of the rivers downstream in the Sunderbans,” the publication stated.

In the study, the researchers noted a higher rate of encounter with the species in rivers and stretches that had limited use of motorised boats, less river traffic and more country boats. The encounter rate was higher by almost 55 % in such stretches, the publication stated.

Classified as endangered by the IUCN Red List, the species was once found in tributaries of the Ganga in West Bengal. Researchers and experts said the sighting of the dolphin has dipped over the years in the 534-km stretch from Farakka Barrage to Sunderbans.