Supplementary file 1_Recovery of the critically endangered gharial (Gavialis gangeticus) amid escalating anthropogenic pressures in the Ganga River Basin.docx

Conservation initiatives launched in the mid-1970s have contributed to its sustained recovery of the species. Nevertheless, information on the current population status, habitat condition and long-term recovery is lacking. Boat-based surveys were conducted across 22 rivers covering 7,680 km (60.3% of the total river length of 12,741 km) to determine the minimum population size (MPS) of gharial in the Ganga River Basin.

We recorded 3037 gharials in 13 rivers, with the highest number from the Chambal (MPS = 2,097), followed by the Ghaghra (MPS = 463) and Girwa rivers (MPS = 158), while less than 10 individuals were observed in the Sharda, Kosi, Bagmati, and Babai rivers. Across 13 gharial occupied rivers encounter rates ranging from 0.02 ± 0.01 ind./km (Ganga) to 7.90 ± 4.29 ind./km (Girwa), highest in Chambal (5.18 ± 0.50 ind./km) and lowest in Ganga and Kosi (≤0.02 ind./km).

The Chambal, Gandak, and Ghaghra rivers had individuals from all size classes, with a high proportion of larger individuals (>1.8 m) in the population. Habitat suitability analysis revealed that most of the river network (10,716 km; 84.1%) was very low suitable for gharials, whereas only 1,320 km (10.4%) was classified as highly suitable habitat. Population trend analysis based on long-term data from the National Chambal Sanctuary revealed an exponential rate of increase of 7% per annum between 1979 and 2024, indicating substantial population recovery in this key conservation stronghold.

However, this trend is specific to the Chambal population and should not be extrapolated to the wider Ganga Basin, where our river-wise survey data reveal a markedly uneven population status. Growth was most rapid during the initial intensive grow and release phase from 1979 to 1993 (15.6% per annum). The identified river stretches potentially represent priority sites, therefore requiring immediate conservation measures to support gharial recovery in the Basin.</p>