Abstract:
Population surveys of crocodilians using uncrewed aerial vehicles (UAV) or
drones may become accurate and cost-effective alternatives to more
traditional approaches. However, there are currently no quantitative methods
for deriving body condition scores of crocodilians through remote sensing. This
study presents seven UAV-based morphometric measures collected from
rectified aerial imagery of farmed Nile crocodiles. Two hundred and eightyeight
Nile crocodiles, from two commercial crocodile farms in South Africa were
included in this study. One farm housed crocodiles which appeared to have
wider abdominal girths than those on the second farm, allowing comparisons for
a range of sizes and body condition states. An initial disturbance assessment was
conducted, and an appropriate flight altitude selected for image acquisition of
farmed Nile crocodiles. Altitudes between 40m and 60m above ground level
suited the studies requirements and minimized disturbance. A UAV-based body
condition index for Nile crocodiles was then developed, offering a non-invasive
alternative to traditional condition scoring methods. The body condition index
(BCI) was calculated for each crocodile by measuring the relationship between
total length and belly width (with the equation: BCI = BW/TL*10) derived from
photogrammetrically processed orthophotos in GIS. The BCI values were then
normalized to form a body condition score (BCS) with the equation: BCS = (BCI/
1.27)*4 + 1. The BCS ranked crocodile body conditions from 1-5, where a score
of 1 identified a crocodile that was comparatively thin or emaciated, while a score
of 5 identified a crocodile that was relatively fat or obese in contrast to the other
crocodiles assessed. A BCS of 3 was the most frequent across all crocodiles in the
study, with few animals scoring a 1 or 5. The farm housing crocodiles with
narrower abdominal girths had no BCS 5 occurrences, and the farm housing
crocodiles with wider abdominal girths had no BCS 1 occurrences. This UAVbased
body condition score could be applied to large wild or captive populations for a fast-paced health and welfare evaluation.
