DOI: 10.5061/dryad.fttdz08ww
title: Gharial abundance in Rapti river, Chitwan, Nepal

description: 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’.
description: 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.
description: Microsoft Excel can open the data files.