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Ecology & Environment
Mahesh

05/03/25 09:20 AM IST

First-ever comprehensive survey of India’s river dolphins

In News
  • Prime Minister Narendra Modi released the results of the first-ever comprehensive population estimation of riverine dolphins – Gangetic and Indus dolphins – done in India.
Key Findings
  • The estimation survey was done by the Wildlife Institute of India under the Union Environment Ministry between 2021 and 2023.
  • It covered the main channels and tributaries of the Ganga and Brahmaputra rivers, as well as the Beas river in Punjab. 28 rivers were surveyed by boat, and 30 were mapped by road.
  • The survey spanned 7,109 kilometres of the Ganga and its tributaries in Uttar Pradesh, Bihar, Jharkhand, West Bengal, Madhya Pradesh, and Rajasthan.
  • A total of 1,297 kilometres of the Brahmaputra were surveyed, including its tributaries Subansiri, Kulsi, Beki, Kopili, and Barak. 101 kilometres of the Beas river were surveyed as well.
  • It thus estimated an average of 3,275 dolphins on Ganga’s main stem, 2,414 in its tributaries, 584 in Brahmaputra’s main stem and 51 in its tributaries.
  • In Beas, it only found 3 Indus River Dolphins, considered a separate species from the Gangetic dolphins.
  • The highest number of Gangetic dolphins, 2,397, was found in Uttar Pradesh.
  • This was followed by 2,220 in Bihar, 815 in West Bengal, 6235 in West Bengal, 162 in Jharkhand, 95 in Rajasthan and Madhya Pradesh, and three in Punjab.
Challenges
  • River dolphins live in opaque, turbid waters and briefly appear on the surface, making any estimates of their population tricky.
  • According to the population estimation report, dolphins surface only for 1.26 seconds and dive for 107 seconds.
  • This generates the possibility of observer error, which prevents observers from sighting all surfacing dolphins, and availability error, meaning not all dolphins surface during the counting time.
  • A combination of visual and acoustic surveys is used.
  • The acoustic survey uses multiple underwater microphones or hydrophones to capture ‘dolphin clicks.’
  • Dolphins, being functionally blind, navigate by making clicking sounds that travel through water and bounce back after hitting objects.
  • This process is called echolocation.
  • Hydrophones record underwater dolphin clicks to counter observer error and reliably triangulate dolphin occurrence.
  • Since multiple hydrophones are used, they can pick up clicking sounds made by different individuals.
  • Different kinds of visual surveys are used, depending on the vessel available, water depth and channel width.
  • For deep and wide channels, the double observer method is used, where two teams positioned on different decks cover different angles around the vessel on its left and right flank.
  • These teams scan for dolphins surfacing around them as the boat travels at 8-10 km/hour.
  • This allows for efficient detection and helps avoid double counting individual dolphins, given that the boat travels faster than the dolphins.
  • A tandem method is used for channels less than 600 metres wide and 3 metres deep, while a single boat method is used for channels with a width smaller than 300 metres and a depth lower than 2 metres.
Coldspots & Hotspots
  • While a major stretch of the Ganga’s main stem flows through Uttar Pradesh, dolphins are sparse or absent in certain stretches of the river called ‘coldspots’.
  • In the 366-kilometre stretch from Narora to Kanpur, dolphins are almost non-existent with an encounter rate of 0.1 per km, according to the population estimation report.
  • Similarly, the Farukkhabad-Kannauj stretch between the Narora and Kanpur barrages is considered a coldspot.
  • “Similar coldspots were found in the Yamuna River, from Kaushambi-Chitrakoot, in the Sharda River, in Pilibhit, and in Rapti River, from Balrampur – Sidhharth Nagar,”
  • The  estimation exercise found several hotspots or stretches where dolphins were abundant. 
  • As against an encounter rate of 0.62 dolphins/km in Uttar Pradesh, the encounter rate in Bihar was 1.62/km. 
  • The estimation survey attributed high dolphin occurrence in Bihar to a higher river depth, particularly at the confluence of tributaries such as Ghaghara, Gandak, Kosi and Son. 
  • The survey found the Chausa-Manihar stretch, spanning 590 kilometres, to be most densely populated, with an encounter rate of 2.20 dolphins/km. 
  • Similarly dense was the population between Manihari in Bihar and Rajmahal in Jharkhand, with an encounter rate of 2.75 dolphins/km.
  • In Assam, despite a healthy water depth on the main stem of Brahmaputra, its tributaries had low average depth. The Barak river was identified as a coldspot in Assam.
  • Compared to past estimates, the dolphin population in the Subansiri and Kulsi rivers was declining, according to the estimation report.
Source- Indian Express

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