635 dolphins found in Assam, low encounter rate of dolphins a point of concern

Guwahati, March 3: Assam has 635 estimated dolphins out of 6327 in the country, according to the population status of dolphins in the country released on March 3.

The study, covering major river systems including the Ganga and Brahmaputra, assessed dolphin abundance, habitat quality, and anthropogenic pressures. The report was prepared by Wildlife Institute of India.

Out of 635 dolphins in Assam, 584 are in the Brahmaputra mainstream and 51 in its tributaries. These numbers highlight Assam as a crucial habitat for the endangered Ganges River Dolphin, India’s National Aquatic Animal. The Ganga mainstream recorded 2,422 dolphins, while its tributaries hosted 2,414.

In the Brahmaputra, the mainstream held 584 dolphins, while its tributaries had 51. Bihar, with an encounter rate of 1.62 dolphins per kilometre, emerged as a significant dolphin habitat, followed by Assam and Uttar Pradesh. The study used independent double observer methods and underwater acoustic hydrophones to reduce bias, applying statistical models such as Lincoln-Peterson’s Chapman’s corrected formula and the Huggins method for precise estimates. The encounter rate of dolphins in Assam was only 0.49 per km, which is a point of concern as the habitat in the Brahmaputra River is still good. The encounter rate of dolphins refers to the average number of dolphins observed per kilometer during a survey. Essentially, it’s calculated by dividing the total number of dolphin sightings by the total distance covered in the survey. This metric helps researchers gauge the density of dolphins in a given river section and can also indicate changes in population distribution or habitat use over time.

In 2008, the Ganges River Dolphin was declared the state aquatic animal of Assam. Brahmaputra River holds a good and barrier-free population of the Ganges River dolphin in its undisturbed natural habitat, making it one of the major conservation areas for the survival of the species. Approximately 61 % of the total population of Assam is related to agriculture, and 65% is irrigable land.

“The Brahmaputra River and its tributaries Subansiri, Kulsi, Beki, Kopili and Barak, with a length of 1326 Km, were surveyed during the exercise. The estimated population of dolphins in this entire basin was 635 (606- 664). Considering that the habitat status in this river is comparatively good, the encounter rate is low, with 0.49/Km. Barak River is identified as the cold spot in the entire state of Assam. The dolphin populations in Subansiri and Kulsi are declining. Kaziranga and Orang Tiger Reserves and confluences of tributaries with Brahmaputra are important dolphin areas. The average depth of the mainstream Brahmaputra was 5.2 m. However, the overall water depth of the surveyed tributaries was low. The average depth of the tributaries, Subansiri, Kulsi, Kopili and Beki, was 3.8 m, 1.7 m, 2.7 m and 1.7 m, respectively. Ganges River Dolphins were recorded for the first time in the Kopili River during winters,” the report said.

“The depth, width and nature of the Brahmaputra River creates a diversity of morphological features in the river, which helps support a large variety of aquatic fauna. The River Brahmaputra sustains a significant portion of wetlands that are crucial for migratory waterfowls, as well as resident aquatic birds: the report said.

The important areas identified in Assam within the Brahmaputra River and its tributaries are vital for dolphin conservation and local livelihoods. The estimated dolphin population in Assam is 635 individuals, with critical areas in the Kulsi River (Kamrup district), Subansiri River (Lakhimpur), Brahmaputra River in Majuli, and Dibru-Saikhowa National Park (Tinsukia). Kaziranga National Park (Biswanath district), Guwahati (Darang-Kamrup metro), Goalpara-Bongaigaon, and Dhubri. The Barak River has been identified as a cold spot with no dolphin population. Conservation efforts in these critical areas are necessary to maintain dolphin populations and support other species of conservation concern, including the Otter and Greater Adjutant Stork.

Several smaller tributaries were surveyed to assess their potential as dolphin habitats. A resident dolphin population has been observed in the Kulsi River. In 2012, 17 dolphins were recorded, increasing to 29 in 2020. However, recent years have seen dolphins stranded in deep pools due to changing water dynamics. The first-ever dolphin survey in the Kopili River found a small population of 5 dolphins, suggesting it may be a resident rather than a migratory group. The survey covered 45 km in the Beki River, estimating 4 dolphins with an encounter rate of 0.1 dolphins/km. It remains uncertain whether this is a resident or migratory population.

No dolphins were found in a 124 km stretch in the Barak river confirming fears of their local extinction in this river system.

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