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River dolphin distribution in regulated river systems: implications for dry-season flow regimes in the Gangetic basin

Authors

  • Sunil Choudhary,

    1. Vikramshila Biodiversity Research and Education Centre, T.M. Bhagalpur University, Bhagalpur, India
    2. P.G. Department of Botany, T.M. Bhagalpur University, Bhagalpur –, India
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  • Sushant Dey,

    1. Vikramshila Biodiversity Research and Education Centre, T.M. Bhagalpur University, Bhagalpur, India
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  • Subhashish Dey,

    1. Vikramshila Biodiversity Research and Education Centre, T.M. Bhagalpur University, Bhagalpur, India
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  • Viveksheel Sagar,

    1. Freshwater & Wetlands Division, WWF-India, Lodi Estate, New Delhi-, India
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  • Tarun Nair,

    1. Post-graduate Program in Wildlife Biology and Conservation, Wildlife Conservation Society-India Program, Centre for Wildlife Sciences, National Centre for Biological Sciences – TIFR, Bangalore
    2. Gharial Conservation Alliance, Centre for Herpetology, Madras Crocodile Bank Trust, Chennai, Tamil Nadu, India
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  • Nachiket Kelkar

    Corresponding author
    1. Vikramshila Biodiversity Research and Education Centre, T.M. Bhagalpur University, Bhagalpur, India
    • Nature Conservation Foundation, 3076/5, IV Cross, Gokulam Park, Mysore, India
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Nachiket Kelkar, Nature Conservation Foundation, 3076/5, IV Cross, Gokulam Park, Mysore 570002, India. Email: rainmaker.nsk@gmail.com.

ABSTRACT

  1. River flow regulation and fragmentation is a global threat to freshwater biodiversity, ecosystem processes, and associated human activities. Large dams in the Ganges river basin of the Indian subcontinent have severely altered natural flow regimes, particularly in the low-flow dry season. Altered flows could have negative impacts on endangered species such as the Ganges river dolphin Platanista gangetica.
  2. Habitat use by river dolphins was investigated in relation to river channel depth and morphology, over 332 km of the flow-regulated Gandak River in India. Dolphin distribution patterns were compared across multiple spatial scales in the Gandak, Kosi, Chambal, Sone Rivers and the upper and lower sections of the Ganges main stem.
  3. Dolphin presence was recorded in 40% of segments in the Gandak river, with a best count of 257 (range 250–267) and average individual encounter rates at 0.75 dolphins km-1 (SD 0.89). Bayesian zero-inflated spatial models showed that river dolphin abundance was positively influenced by river depth, presence of meanders and corresponded closely with gillnet fishing. Minimum mid-channel depth requirements were estimated at 5.2 m for dolphin adults and between 2.2 and 2.4 m for mother–calf pairs.
  4. Adult dolphins showed highly similar habitat preferences across regulated or unregulated rivers, for depths >5 m, and meandering channels. Dry-season habitat availability was reduced as the degree of flow regulation increased across rivers, mainly owing to loss of lateral and longitudinal channel connectivity.
  5. Overall encounter rates were reduced from >3 km-1 in less regulated stretches, to <0.3 km-1 in regulated rivers. Clustering of dolphins in deep pools increased along the gradient of river flow reduction, with dolphins almost absent from intervening segments because of low flow rates. These results indicate the importance of maintaining adequate dry-season flows to ensure river habitat availability and connectivity for dolphins.

Copyright © 2012 John Wiley & Sons, Ltd.

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