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ARTICLE Counting crocodiles from the sky: monitoring the critically endangered gharial (Gavialis gangeticus) population with an unmanned aerial vehicle (UAV) Gokarna Jung Thapa, Kanchan Thapa, Ramesh Thapa, Shant Raj Jnawali, Serge A. Wich, Laxman Prasad Poudyal, and Suraj Karki Abstract: Technology is rapidly changing the methods used in the field of wildlife monitoring. Unmanned aerial vehicles (UAV) are an example of a new technology that allows biologists to take to the air to monitor wildlife. A fixed-wing UAV was used to monitor the critically endangered gharial population along 46 km of the Babai River in Bardia National Park, Nepal. The UAV was flown at an altitude of 80 m along 12 pre-designed missions and, with a search effort of 2.72 h of flight time, acquired a total of 11 799 images covering an effective surface area of 8.2 km2 of riverbank habitat. The images taken from the UAV could differentiate between gharial and muggers. A total count of 33 gharials and 31 muggers with observed density (per square kilometre) of 4.64 and 4.0 for gharial and mugger, respectively. Comparison of count data between one-time UAV and multiple conventional visual encounter rate surveys’ data showed no significant difference in the mean. Basking season and turbidity were important factors for monitoring crocodiles along the riverbank habitat. Efficacy of monitoring crocodiles by UAV at the given altitude can be replicated in highpriority areas with lower operating cost and acquisition of high-resolution data. Key words: UAV, gharial, mugger, count, monitoring.
Résumé : Les méthodes dans le domaine de la surveillance de la faune évoluent rapidement grâce aux avancées technologiques. Le véhicule aérien sans pilote (UAV) est un exemple d’une nouvelle technologie qui permet aux biologistes de se mettre en mode vol pour surveiller la faune. Un UAV à voilure fixe a été employé afin de surveiller la population de gavials gravement menacés de disparition sur 46 km le long de de la Rivière de Babai dans le Parc national de Bardia. L’UAV a été dirigé à une altitude de 80 m le long de 12 missions prédéterminées pour une recherche de 2,72 heures de temps de vol qui a permis d’obtenir un total de 11 799 images couvrant une superficie efficace de 8,2 km2 d’habitat de berge. Les images prises de l’UAV permettaient de différencier entre les gavials et les crocodiles des marais. Un dénombrement total de 33 gavials et de 31 crocodiles des marais dont la densité observée (par km2) était de 4,64 et 4,0 pour le gavial et le crocodile des marais, respectivement. La comparaison des données de dénombrement entre les données de levé Received 11 November 2017. Accepted 1 February 2018. G.J. Thapa. WWF Nepal, Baluwatar, Kathmandu, Nepal; Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, the Netherlands. K. Thapa and S.R. Jnawali. WWF Nepal, Baluwatar, Kathmandu, Nepal. R. Thapa and L.P. Poudyal. Department of National Parks and Wildlife Conservation, Babarmahal, Kathmandu, Nepal. S.A. Wich. Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, the Netherlands; Liverpool John Moores University, School of Natural Sciences and Psychology, James Parsons Building, Byrom Street, L33AF, Liverpool, UK. S. Karki. Advanced College of Engineering and Management, Kupondole, Lalitpur, Nepal. Corresponding author: Kanchan Thapa (e-mail:
[email protected]). Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. J. Unmanned Veh. Syst. 00: 1–12 (0000) dx.doi.org/10.1139/juvs-2017-0026
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une fois au moyen d’un UAV et les levés classiques d’observations visuelles multiples n’a montré aucune différence significative au niveau de la moyenne. La saison des bains de soleil et la turbidité étaient des facteurs importants au niveau de la surveillance des crocodiles dans leur habitat le long des berges. L’efficacité de la surveillance des crocodiles au moyen d’UAV à l’altitude donnée peut être reproduite dans des zones de haute priorité à moindre coût d’exploitation et à moindre besoin d’acquisition de données à haute résolution. [Traduit par la Rédaction]
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Mots-clés : UAV, gavial, crocodile des marais, dénombrement, surveillance.
Introduction Technology is rapidly changing the methods with which wildlife is being monitored (Pimm et al. 2015). Unmanned aerial vehicles (UAVs) are one such example of new technology that allows biologists to take to the air to monitor wildlife, allowing for more costeffective wildlife monitoring (Watts et al. 2010; Chabot and Bird 2015). UAVs allow for very high-resolution data acquisition in both the spatial and temporal domain (Whitehead and Hugenholtz 2014). UAVs have been used in several civilian disciplines for research and monitoring: agriculture (Hunt et al. 2010), forestry (Wing et al. 2013), and biodiversity monitoring (Getzin et al. 2012) including wildlife (Koh and Wich 2012; Wich et al. 2015; Hahn et al. 2017). The use of UAVs in wildlife studies is relatively recent and has focused more on the possibility of species detection than on determining wildlife density and abundance (Linchant et al. 2015). In this study, we tested whether UAVs can facilitate the detection of the critically endangered gharial (Gavialis gangeticus). The species was selected for two reasons. First, their survival is increasingly threatened as a result of changes in land-use, water flow and river morphology, poaching, and through being caught in fishing nets (Dudgeon 2000; Smith and Reeves 2000; Hussain 2009). Second, as the species occurs along long stretches of rivers there is a need for cost-efficient survey methods as current ground-based methods are too costly and logistically challenging to conduct on a regular basis which is needed for monitoring purposes. The gharial is a critically endangered crocodilian species found only in running freshwater ecosystems (IUCN 2012). Currently, their distribution is limited to Nepal and India with an estimated population size of
