Gharial nesting threatened by woody vegetation on riverbanks

A male gharial basks at a nesting site cleared of vegetation. Photo by Gaurav Vashistha

A male gharial basks at a nesting site cleared of vegetation. Photo by Gaurav Vashistha

  • A flood event in 2010 caused a natural shift in the Karnali River reducing water flow on a stretch of the Girwa River where gharials nest. This likely contributed to a subsequent loss of gharial nesting sites on riverbanks and growth in vegetation at those sites.
  • The number of gharial nest sites and nests declined from 2015 to 2019 in Katerniaghat Wildlife Sanctuary near the Indo-Nepal border.
  • Vegetation restricts access to nesting sites and affects the temperature of incubating eggs; roots of vegetation can damage incubating eggs, the team observed.
  • Cross-border cooperation with Nepal is needed to develop a collaborative conservation plan for gharials.

A new study from Katerniaghat Wildlife Sanctuary near the Indo-Nepal border found that reduced flow in the Girwa River has adversely impacted gharial nesting by promoting the growth of vegetation at riverbank sites where nesting used to occur.

A riverine-reservoir in Katerniaghat Wildlife Sanctuary (KWS) in Uttar Pradesh hosts a small breeding population of the critically endangered fish-eating crocodiles. Although the population is small, it is second in ranking to the much larger Chambal population in Madhya Pradesh. In 2010, a flood-related natural channel shift in the Karnali River, upstream in Nepal, reduced seasonal flow in the Girwa stretch where gharials nest. Subsequently, there was a notable rise in woody vegetation at open sites where gharials previously nested. The number of nests sites declined by 70 percent while the number of nests dropped by 46 percent from 2015 to 2019, the study found.

“Our findings highlight the impact of habitat modification on the population ecology of a freshwater crocodile,” the team of authors from the University of Delhi, Wildlife Institute of India, Madras Crocodile Bank Trust and the Uttarakhand Forest Department told Mongabay-India. “When a free-flowing river (Girwa) is converted to a reservoir, the entire riverine dynamics including riparian vegetation changes and impacts gharial ecology,” they explained.

“The loss of nesting habitat for the gharial residents in the Girwa river, that we have documented in this study, could result in local extinction of this species in the KWS,” warned the authors.

Gharials inhabited free-flowing, freshwater rivers across the Indian subcontinent. Endemic to South Asia, gharials nest on sand substrates such as mid-river sandbars and high sandbanks next to water pools. However, the construction of dams or barrages has resulted in fragmented and reduced habitat size, explained the authors.

After monsoon floods in 2010, the active mainstream of the Karnali River, which originates in Nepal and flows into India, shifted naturally from the east channel (Geruwa), known as Girwa in India, to the west Karnali channel, which is known as the Kaudiyala in India. Consequently, the Karnali had a higher water discharge and flow than Geruwa.

Satellite images before (2009; left panel) and after (2011 and 2018; middle and right panels) a flood event in 2010 in Karnali river channels, Nepal. The flood resulted in a mainstream channel shift from east (right side, = G, Geruwa channel in Nepal) to west (left side, = K, Karnali channel in Nepal). Map from Vashistha et al. 2021.
Satellite images before (2009; left panel) and after (2011 and 2018; middle and right panels) a flood event in 2010 in Karnali river channels, Nepal. The flood resulted in a mainstream channel shift from east (right side, = G, Geruwa channel in Nepal) to west (left side, = K, Karnali channel in Nepal). Map from Vashistha et al. 2021.

The researchers investigated the impact and relation of these riverine changes and rise in woody vegetation on the trends and patterns of gharial nesting in KWS. For the study, the team sampled vegetation at the nesting sites from 2017 to 2019 and derived an Enhanced Vegetation Index from satellite data to quantify changes in riverside vegetation from 1988 to 2019. They also documented changes in gharial nesting site distribution from 2015 to 2019 using information collected during nesting surveys and secondary data.

Declining nests and the threat of vegetation

Within KWS, there were eight nest sites, four on riverbanks and four on mid-river sandbars, along a 9-km stretch of the Girwa River. In 2015, five out of the eight sites had nests, which increased to six in 2016 and seven in 2017. However, the trend reversed in 2018 when three sites were abandoned but a new site popped up on an exposed mid-river sandbar. Only two sites had nests in 2019. These two sites had most of the nests from 2015 to 2019. In one of the two sites, trees and shrubs increased along with a drop in the number of nests from 21 in 2017 to six in 2019.

Vegetation in the gharial nesting sites comprised of three species of grasses (Phragmites karka, Saccharum spontaneum, and Typha sp.), two herbs (Euphorbia hirta and Ageratum conyzoides), four shrubs (Tamarix sp., Lantana camara, Ricinus communis and Calotropis giganticus) and three tree species (Adina cordifolia, Wrightia tinctoria and Bombax ceiba).

Apart from limiting access to potential nesting areas by female gharials, vegetation influences the incubation of gharial eggs. The roots of vegetation could damage eggshells during incubation and contribute to egg mortality, explained the authors. “During our sampling period, we found evidence of physical damage to shells of incubating eggs by vegetation roots that ultimately resulted in egg mortality,” the authors observed. By reducing solar irradiation, vegetation affects the incubation temperature of eggs, which plays a role in determining the sex of the hatchlings. Upon hatching, vegetation could impede access of hatchlings to water, exposing them to high summer temperatures, which may lead to desiccation and predation.

A gharial hatchling. Photo by Gaurav Vashistha.
A gharial hatchling. Photo by Gaurav Vashistha.

B.C. Choudhury, an expert in crocodiles and a former professor at the Wildlife Institute of India, who was not involved in the study, said these ecological successions have an impact, but he argues that such successions are “triggered by anthropogenic actions such as irreversible developmental projects on natural river systems as has been the case with Girwa River in KWS.”

Building artificial nest sites and reviving river flow

Active intervention is required for the survival of the small population of gharials in KWS. Recently, vegetation was removed from nesting banks and sand was added by shifting sand to extend river banks. “We collected sand from a sandbar in Girwa and made several artificial sand platforms, mimicking gharial nesting sites in the wild,” said the authors. This method proved successful, the authors noted, as “the artificial sites were readily adapted for basking and nesting and the number of nests doubled in just one year.”

Gharial hatchlings. Photo by Gaurav Vashistha.
Gharial hatchlings. Photo by Gaurav Vashistha.

The researchers have been working together with the forest department at KWS over the past six years and the idea of restoring nesting habitat was jointly agreed upon and executed. “It became a matter of pride for the department when the first attempt at building artificial sandbanks was successful,” the authors told Mongabay-India.

However, “we can’t keep on providing artificial assistance because it is not resourceful and feasible for long-term,” warned the authors. A larger continuous habitat is needed and this can be achieved by a “cooperative initiative by the two countries in the Karnali basin—India and Nepal—to develop a collaborative conservation agenda for this important riverine landscape while it is still largely intact and free-flowing,” stated the paper.

Gharials have breeding and nesting in the Girwa stretch since only 1975. With a channel shift decreasing nesting area, efforts need to be made to revive river flow in the Girwa. This can be possible if “Nepal takes regional efforts near Chisapani (where Girwa bifurcated from Karnali) and assist in gharial conservation taking place in India,” proposed the authors.


Vashistha, G., Mungi, N. A., Lang, J. W., Ranjan, V., Dhakate, P. M., Khudsar, F. A., & Kothamasi, D. (2021). Gharial nesting in a reservoir is limited by reduced river flow and by increased bank vegetation. Scientific reports, 11(1), 1-12.


Banner image: Photo by Gaurav Vashistha.

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