Gharial (Gavialis gangeticus) is a Critically Endangered crocodilian species whose abundance in Nepalese rivers are low due to the threat they face. We estimated gharial abundance in the Rapti River, one of the major rivers in Chitwan National Park (CNP) holding the largest numbers of gharials in Nepal. The Rapti River, running across the CNP, was divided into 18 segments, each measuring ~4 kilometers, and gharials were counted directly with three replicates. Gharial count data was analysed using an N-mixture model (negative binomial) and the overall occupancy of gharials was estimated using a single season occupancy model. Covariate effects were also investigated on gharial abundance. Our findings revealed that the Rapti River is home to 150 gharials (119 - 181), with a mean abundance of 8.3 (SD= 3.45) across each segment. The presence of humans and square of Rapti River depth were the significant covariates that had a negative and positive impact on gharial abundance, respectively. Similarly, the number of sandbank presence influenced the detection probability of gharials. Our study shows that gharial population estimation can be improved using the N-Mixture model. The overall gharial occupancy estimated using single season occupancy model was 0.84 (SD= 0.08), with a detection probability of 0.37 (SD =0.02). The management authority should concentrate on segments to minimize human disturbance (e.g., fishing, washing clothes, extraction of riverbed materials). If the gharial population in this river declines, their population in central Nepal will be threatened. Hence, we suggest designating the Rapti River section that passes across the CNP as a ‘no extraction zone’.

Crocodilians are typically aquatic, although they emerge from the water to bask on land during the winter months. Counting basking gharials has been used to determine their population number since it is a reliable and convenient approach. Crocodile surveys are most effective during the winter months, from November to March, when practically all individuals come out for basking and continue basking for extended periods of time, increasing the chance of sightings. Because it is mating season, breeding groups tend to congregate. Furthermore, gharials are less active during the winter months, restricting their frequent migration in our short survey time, implying that the gharial population would remain demographically closed during the course of the surveys, as needed by the occupancy model. As a result, we performed our research during the winter, from November 13 to December 13.

The Rapti River was thoroughly surveyed by dividing it into 18 of 4 kilometer length. Gharial sightings, habitat parameters, and anthropogenic pressure were recorded at every 200 m of each segment, for a total of 18*20= 360 sampling points. With two experienced observers and two boatmen, we used a dugout boat. Each segment was surveyed three times with binoculars to look for gharials, for a total of 1080 (360x3) points. Gharials were approached as closely as possible, and their sizes were determined visually. The Rapti River was thoroughly surveyed by dividing it into 18 of 4 kilometer length. Gharial sightings, habitat parameters, and anthropogenic pressure were recorded at every 200 m of each segment, for a total of 18*20= 360 sampling points. With two experienced observers and two boatmen, we used a dugout boat. Each segment was surveyed three times with binoculars to look for gharials, for a total of 1080 (360x3) points. Gharials were approached as closely as possible, and their sizes were determined visually. Then, we estimated gharial population size using a binomial N-mixture model and a single season occupancy model for gharial occupancy.

Microsoft Excel can open the data files.