Use of an unmanned aerial vehicle (drone) to survey Nile crocodile populations: A case study at Lake Nyamithi, Ndumo game reserve, South Africa

Observer bias and inexperience are challenging aspects of crocodile survey methods for determining population numbers and structure. Aerial surveys with either a helicopter or a fixed winged aircraft are generally preferred methods to ground surveys; however, the high cost of the former is a limiting factor. Recently unmanned aerial vehicles (UAVs) or drones have been proposed for surveys because of their potential of improving over traditional techniques of wildlife monitoring and as they have relatively lower costs. We investigated of the suitability of a UAV to determine numbers and structure of the Nile crocodile, Crocodylus niloticus, population during winter at Lake Nyamithi, Ndumo Game Reserve in South Africa. We used the UAV for eight flights covering ~132 ha. We also conducted a diurnal ground survey of crocodiles for comparison. Using the UAV, 287 crocodiles were identified and body length measured accurately for size class allocation whereas only 211 crocodiles were counted in the diurnal ground survey. Consequently, the UAV aerial survey recorded 26% more crocodiles. The potential of using UAVs to estimate crocodile population size and measure the total length (TL) of individuals accurately and precisely at a relatively low cost should improve management actions, enable monitoring of the crocodile populations annually and importantly avoid observer bias. Implications of this may facilitate improved crocodilian survey techniques.