Gharial nesting in a reservoir is limited by reduced river flow and by increased bank vegetation

Abstract

The gharial (Gavialis gangeticus Gmelin) is a fish-eating specialist crocodylian, endemic to south Asia, and critically endangered in its few remaining wild localities. A secondary gharial population resides in riverine-reservoir habitat adjacent to the Nepal border, within the Katerniaghat Wildlife Sanctuary (KWS), and nests along a 10 km riverbank of the Girwa River. A natural channel shift in the mainstream Karnali River (upstream in Nepal) has reduced seasonal flow in the Girwa stretch where gharials nest, coincident with a gradual loss of nest sites, which in turn was related to an overall shift to woody vegetation at these sites. To understand how these changes in riparian vegetation on riverbanks were related to gharial nesting, we sampled vegetation at these sites from 2017 to 2019, and derived an Enhanced Vegetation Index (EVI) from LANDSAT 8 satellite data to quantify riverside vegetation from 1988 through 2019. We found that sampled sites transitioned to woody cover, the number of nesting sites declined, and the number of nests were reduced by > 40%. At these sites, after the channel shift, woody vegetation replaced open sites that predominated prior to the channel shift. Our findings indicate that the lack of open riverbanks and the increase in woody vegetation at potential nesting sites threatens the reproductive success of the KWS gharial population. This population persists today in a regulated river ecosystem, and nests in an altered riparian habitat which appears to be increasingly unsuitable for the continued successful recruitment of breeding adults. This second-ranking, critically endangered remnant population may have incurred an "extinction debt" by living in a reservoir that will lead to its eventual extirpation.

Sand addition promotes gharial nesting in a regulated river‐reservoir habitat

“…The eventual vegetation succession in the above areas reduced the nesting site and nest numbers (Vashistha et al, 2021).…”

“…However, when the river enters the flood plains below Chisapani, it is subject to water extraction, diversion and manipulation of channel flow. Furthermore, a flood event in 2010 shifted active flow of the main Karnali at a natural bifurcation below Chisapani, from the eastern Geruwa to the western Karnali channel (Khanal et al., 2016; Vashistha et al., 2021). The immediate effects were reduced water flow and water depth in eastern Geruwa channel.…”

“…A small resident breeding population of gharial inhabits the KWS. Exhaustive surveys in the Karnali River basin in Nepal and India indicate that today, gharial only occur along a 10–20 km stretch of the Girwa River, inside the KWS and within the reservoir (Bashyal et al., 2021; Vashistha et al., 2021). In response to the mainstream channel shift from east to west in the Karnali, a small resident freshwater dolphin population moved into the adjoining Kaudiyala channel where the flow was greater and the water deeper than in the Girwa (Khanal et al., 2016; Khatiwada et al., 2019; Paudel et al., 2015; Shah et al., 2020).…”

“…Open river banks and sand bars in Girwa have converted to woody vegetation following the channel shift of 2010 (Figure 1(a)). Prior to 2010, there were 22–27 nests, this number increased to 30+ till 2017 but fell back to 25 in 2018 (Vashistha et al., 2021). Before the channel shift of 2010, regular monsoon floods were restructuring the riparian habitat.…”

“…During the years 2016–2019, the number of gharial nesting sites reduced from eight to two and the number of nests reduced by more than 46%. The eventual vegetation succession in the above areas reduced the nesting site and nest numbers (Vashistha et al., 2021).…”