Habitat occupancy and threat assessment of gharial (Gavialis gangeticus) in the Rapti River, Nepal

Abstract

Gharial (Gavialis gangeticus) is one of the most highly threatened fauna of fresh water ecosystems in the world. Yet, information about the species habitat characteristics influencing its distribution and threats to its existence are still lacking in Nepal. This study identified the most important habitat characteristics associated with gharial presence and existing threats in the Rapti River of Nepal. An opportunistic search method was adopted for field surveys conducted in November and December of 2018. As part of the survey, Global Positioning System (GPS) coordinates were recorded for each sighting location and gharial behavioral activity was captured from photographic documentation. Habitat characteristics were recorded at stations spaced at 500 m intervals along the river in addition to locations where gharial was sighted. Data were collected moving downstream on each side of the river (left and right banks), which included both stations and sighting locations that comprised a total of 200 sampling points. Habitat factors associated with the presence of gharial were analyzed using a combination of MS Excel, ArcGIS and the R Binomial distribution model with logit link function. The dependent variable was the presence or absence of gharial at sampling points and the independent variables included six unique habitat characteristics: riverbank location, river width, mid-river depth, riverbank substrate type, riverbank slope and observed anthropogenic threats. Relative whole-site ranking was used to identify the most prominent threats to conservation. During the survey, 53 gharial individuals were recorded where 62% were found basking while 38% were found swimming or submerged underwater. A majority of individuals were sighted on the south side (left bank) of the Rapti River (31 out of 53 sightings), at river widths of 51–100 m (34 out of 53 sightings) and at mid-river depths of 1–2 m (28 out of 53 sightings). Other important factors were riverbank substrate type where most of the gharials were found basking in the sandy bank (25 out of 53 individuals), flat riverbank topography (42 out of 53 individuals), and at locations with no observed anthropogenic threats (41 out of 53 individuals). Among the six habitat variables examined, no significant differences in the probability of gharial sightings were found for five variables (river side, observed human threats, substrate type, river width and mid river depth) while only one habitat variable, topography, exhibited significant differences in the probability of gharial sightings (χ24,195 = 38.782; p < 0.001). Pollution by industrial toxic wastes, domestic sewage and natural habitat modification due to monsoon flash floods were the major threats to gharial and their habitat. Although the Rapti River currently supports a significant gharial population, conservation efforts need to be increased in the future from authorities responsible for their stewardship in order to maintain and enhance their numbers through both habitat protection and abundance of prey (fish).