The present study was conducted between January and June 2007 to assess the abundance and density of Ganges River dolphin (Platanista gangetica gangetica) and their prey in a 28-km stretch of the River Ganges between Narora Barrage and Anupshahar. Two different sampling methods were used to estimate dolphin densities. Estimated density was 2.58 ± 0.40 individuals/km 2 (mean ± 1 SE) using the direct count method and 4.97 ± 0.60 individuals/km 2 using the boat-based, line-transect distance methods, with an encounter rate of 0.52 ± 0.068 individuals/km and detection probability of 0.647. No significant differences between the upstream and downstream counts (t = 1.29, df = 9, p > 0.05) were detected. The adult male to adult female ratio was 0.66: 1.00, whereas the calf to adult female ratio was 0.42: 1.00. We collected 16 fish species of length class varying from 3.5 to 20 cm (range of preferred size of dolphin prey) with a total density of 176.42 fish/km 2 and a total average biomass of 5.36 kg/km 2 . Dolphin density showed a significant positive relationship (R 2 = 0.587) with density of Reba fish (Cirrhinus reba) (β = 0.31, p = 0.00) and Baam fish (Mastacembelus armatus) (β = 0.50, p = 0.04) and also with water depth (β = 0.17, p = 0.03). Presence of dolphins varied across different water depth categories (χ 2 = 106.38, df = 3, p < 0.01) and different parts of the river (χ 2 = 21.68, df = 2, p = 0.00) with more than 50% of dolphin sightings occurring in confluences, indicating their preference for deep water pools.

We monitored the underwater movements of Ganges River dolphins using stationed stereo acoustic data loggers. We estimated these movements using changes in the relative angle of the sound source direction (trajectory). Of the total acoustic recordings (66 h), 26.2% contained trajectories of dolphins, and 78.6% of these trajectories involved single animals, suggesting that dolphins tended to swim alone and were localized near the monitoring station. The observed trajectories were categorized as follows: staying type characterized by small changes in the sound source direction, moving type A (moving in the same direction), and moving type B (moving up and down the stream during recording). The average interpulse intervals of sounds in moving types A and B were significantly shorter than that of the staying type, suggesting that dolphins produce the former types of trajectories to echolocate across shorter distances during movement. The frequency of occurrence of moving type A increased during the night, whereas that of type B increased in the late afternoon and that of the staying type increased during the daytime. These results indicate that dolphins moving at night tended to use short‐range echolocation, whereas during the day, they remained in relatively small areas and used long‐range sonar.

River Dolphins are among the world's most threatened mammals. The rapid growth of human populations, widespread habitat degradation, pollution, flow regulation and water extraction, fisheries overexploitation and weaknesses of protective legislation have caused severe declines in the range and abundance of Ganges River Dolphins in India. Due to rising conservation concerns, dolphin biologists in the country have conducted a great deal of research over the past few decades on the status and trends of the sub-species in its distribution range. However, empirical studies are still lacking on the ecology of River dolphins, both to inform conservation efforts and to help address broader concerns related to biodiversity conservation and the sustainability of human use of river systems. Existing conservation efforts might be insufficient to stem the ongoing decline of freshwater dolphins in the country. In this article we review the status of Ganges River Dolphins, with a focus on threats the sub-species is facing in its current distribution range, and provide recommendations for its conservation.

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