Rise in salinity in the water system that makes the Indian Sunderbans has resulted in the decrease of population of the Ganges River Dolphins in the region.A recent study covering 100 km of rivers and channels around the Sunderbans have 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- 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 the hyper-saline zone in the central part of the Sunderbans, which includes areas such as Raidighi and Patharpratima, have 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 thus there was 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 freshwater species was also once found in different tributaries of the Ganga in West Bengal. Researchers and experts pointed out that the sighting of the Ganges River Dolphin has decreased over the years in the 534-km stretch from Farakka Barrage to Sunderbans.“An adult species requires a minimum mid-channel depth of 5.2 metres, and for young ones the depth is 2.5 metres. At present the population of the Ganges River Dolphin is confined to some pockets like Nabadweep, Kolaghat, Diamond Harbour and Namkhana,” Ms. Mitra said. She said there was an urgent need for more detailed and comprehensive studies to asses the population and factors resulting in the decline of the species.