Counting crocodiles from the sky: monitoring the critically endangered gharial (Gavialis gangeticus) population with an unmanned aerial vehicle (UAV)

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 high-priority areas with lower operating cost and acquisition of high-resolution data.