670 R. Pal et al.


PLATE 1 The study was conducted in the trans-Himalayan part (Nilang valley) of Gangotri National Park characterized by dry alpine scrub vegetation, broken terrain, deep gorges, high gradient slopes, and narrow valleys (a), and in the subalpine portion of the Park and Uttarkashi Forest Division (b) within Uttarakhand State, India.


Here, we tested the efficacy of the extension of the


distance sampling method to accommodate camera-trap data for estimating the density of the group-living bharal in the trans-Himalayan region (3,500–5,500 m) and the solitary Himalayan musk deer in the subalpine region (2,500–3,500 m) of the Upper Bhagirathi basin. We exam- ined the field applicability and possible limitations of this method for estimating the density of these two ungulates in mountainous terrain.


Study area


We carried out this study in the trans-Himalayan part (Nilang valley) of Gangotri National Park, in the subalpine portion of the Park and Uttarkashi Forest Division within Uttarakhand State, India (Fig 1, Plate 1). Nilang valley is characterized by broken terrain, deep gorges, steep slopes (.45°) and narrow valleys (Bhardwaj et al., 2010). The study area does not have permanent human settlements, but the alpine and subalpine zones are seasonal grazing ground for livestock from lower parts of the Bhagirathi basin. Tourists also use the area in summer (June–September). Nilang valley forms the international boundary with Tibet and is controlled by military personnel. There is a network of snow-fed tributaries of the Jadh Ganga, which drains the area to meet the Bhagirathi River. We surveyed areas of dry alpine scrub vegetation at 3,000–5,200mfor the bharal, and subalpine habitats dominated by Betula utilis, Pinuswallichi- ana, Quercus semecarpifolia and Cedrus deodara,at 2,500– 3,000 m, for the Himalayan musk deer.


Methods Data collection


Wedivided the study area into grid cells of 3 × 3 km. In each grid cell, we generated 20 random points using a sampling tool in ArcGIS 10.4 (Esri, Redlands, USA), and selected accessible points for the placement of camera traps. Some randomly generated locations were inaccessible because of precipitous terrain or the presence of seasonal pastoral nomad camps.We deployed camera traps (Cuddeback blue


series, Cuddeback, De Pere, USA) to capture the bharal at 21 locations during summer (May–September 2017; 2,205 trap nights) and 25 locations during winter (October 2017– January 2018; 1,786 trap nights) in the trans-Himalayan re- gion of Gangotri National Park (Fig. 1). For the Himalayan musk deer, we set up camera traps in subalpine habitat, at 30 locations during summer (May–September 2018; 2,246 trap nights) and 28 locations during winter (October 2018– January 2019; 964 trap nights). Camera traps were mounted 30–35 cm above the ground, and programmed to trigger immediately and record an image followed by a 30-s video when movement was detected. Distance analysis requires calculating distance of the tar-


get species from the observer, in our case from the camera. To estimate the distance of photo-captured individuals from the camera trap, we calibrated image measurements against actual measurements during camera installation. For this, we took measurements using a calibration pole of known height at known distances from the camera, in the centre and along both sides of the camera’s field of view. This cali- brationwas done for a total of 30 camera traps, andwe consid- ered the measurements taken at these locations representative for others with similar topography and field of view.


Data analysis


Test for sampling bias To test whether we had sampled all elevations and topographic features according to their avail- ability in the landscape, we compared elevation, ruggedness, slope and aspect of camera-trap locations and 100 randomly generated points using a non-parametric Mann–Whitney U test for scale variables (elevation, slope, ruggedness) and Bonferroni confidence intervals for the categorical variable (aspect). We resampled elevation data from the Shuttle Radar Topography Mission, at 1 km resolution (Jarvis et al., 2008). We calculated slope, ruggedness and aspect informa- tion using spatial analyst from the Digital Elevation Model in ArcGIS.


Availability for detection With camera traps, we can only estimate the density of populations that are available for


Oryx, 2021, 55(5), 668–676 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S003060532000071X


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