Abstract
There is growing interest from research and conservation groups in the potential for using small unoccupied aerial vehicles (UAVs; <2 kg) to conduct wildlife surveys because they are affordable, easy to use, readily available and reliable. However, limitations such as short flight endurance, and in many situations, aviation regulations, have constrained the use of small UAVs in survey applications. Thus, there is a need to refine survey methods adapted to small UAVs that conform to standard operations within aviation law. We developed a novel survey approach based on a grid sampling design using two multirotor UAVs (Phantom 4 Pros) flying simultaneously, within visual line of sight, from our vessel base-station. We used this approach to assess the fine-scale distribution and abundance of dugongs (Dugong dugon) in the remote waters of the Pilbara, Western Australia during three field seasons across 2 years. We surveyed 64 non-overlapping survey cells in random order one or more times and obtained complete image coverage of each surveyed cell of our 31 km2 survey area. Our sampling design maximizes sampling effort while limiting survey time by surveying four cells, two at a time, from one location. Overall, we conducted 240 flights with up to 17 flights per day (mean = 14 flights per day) and could obtain complete coverage of up to 11.36 km2per day. A total of 149 dugongs were sighted within the 50,482 images which we manually reviewed. Spatially-explicit models of dugong density distribution (corrected for availability and perception bias) were produced using general additive models to identify areas more or less used by dugongs (range of corrected dugong densities across all field season = 0.002–1.79 dugongs per 0.04 km2). Dugong abundance estimates ranged from 47 individuals in June 2019 (CV = 0.17) to 103 individuals in May 2018 (CV = 0.36). Our method, which proved convincing in a real-word application by its feasibility, ease of implementation, and achievable surface coverage has the potential to be used in a wide range of applications from community-based local-scale surveys, to long-term repeated/intensive surveys, and impact assessments and environmental monitoring studies.
Highlights
A new promising and powerful approach to conducting wildlife surveys is to use unoccupied aerial vehicles (UAVs) and associated aerial imaging technology to collect highly accurate, detailed and verifiable data about the animals of interest and surveyed area (Gonzalez et al, 2016)
The present study focused on the dugong (Dugong dugon), a coastal seagrass specialist listed as Vulnerable to extinction on the IUCN Red List at a global scale (Marsh and Sobtzick, 2019)
The methodology we describe shows that small, affordable UAVs with seemingly limited capabilities can be successfully applied to wildlife surveys
Summary
A new promising and powerful approach to conducting wildlife surveys is to use unoccupied aerial vehicles (UAVs) and associated aerial imaging technology to collect highly accurate, detailed and verifiable data about the animals of interest and surveyed area (Gonzalez et al, 2016). Using small UAVs and aerial imagery requires us to develop aerial survey designs and analytical methods that are adapted to this new technology and to aviation law. In many places around the world the legal horizontal range of UAV operations is restricted to within visual line of sight, which severely limits the area that can be surveyed. These aviation restrictions on flight range reduce the benefits of UAVs that have extended flight endurance, leaving small systems a preferred option for wildlife aerial surveys (Sweeney et al, 2015; Kiszka et al, 2016). To date there has been little focus on developing an aerial survey methodology that augments the capabilities of small UAVs in the context of strict aviation regulations
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