Using unoccupied aerial vehicles to estimate availability and group size error for aerial surveys of coastal dolphins

Aerial surveys are frequently used to estimate the abundance of marine
mammals, but their accuracy is dependent upon obtaining a measure of the
availability of animals for visual detection. Existing methods for
characterizing availability have limitations and do not necessarily
reflect true availability. Here, we present a method of using small,
vessel‐launched, multi‐rotor Unoccupied Aerial Vehicles (UAVs or drones)
to collect video of dolphins to characterize availability and investigate
errors surrounding group size estimates. We collected over 20 h of aerial
video of dive‐surfacing behaviour across 32 encounters with the Australian
humpback dolphin Sousa sahulensis off north‐western Australia.
Mean surfacing and dive periods were 7.85 sec
(se = 0.26) and 39.27 sec (se = 1.31)
respectively. Dolphin encounters were split into 56 focal follows of
consistent group composition to which example approaches to estimating
availability were applied. Non‐instantaneous availability estimates,
assuming a 7-sec observation window, ranged between 0.22 and 0.88, with a
mean availability of 0.46 (CV = 0.34). Availability tended to increase
with increasing group size. We found a downward bias in group size
estimation, with true group size typically one individual more than would
have been estimated by a human observer during a standard aerial survey.
The variability of availability estimates between focal follows highlights
the importance of sampling across a variety of group sizes, compositions,
and environmental conditions. Through data re‐sampling exercises, we
explored the influence of sample size on availability estimates and their
precision, with results providing an indication of target sample sizes to
minimize bias in future research. We show that UAVs can provide an
effective and relatively inexpensive method of characterizing dolphin
availability with several advantages over existing approaches. The example
estimates obtained for humpback dolphins are within the range of values
obtained for other shallow‐water, small cetaceans, and will directly
inform a government‐run program of aerial surveys in the region.