Abundance and Prey Availability Assessment of Ganges River Dolphin (Platanista gangetica gangetica) in a Stretch of Upper Ganges River, India

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/km2 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) (beta = 0.31, p = 0.00) and Baam fish (Mastacembelus armatus) (beta = 0.50, p = 0.04) and also with water depth (beta = 0.17, p = 0.03). Presence of dolphins varied across different water depth categories (chi(2) = 106.38, df = 3, p < 0.01) and different parts of the river (chi(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.