Using unmanned aerial vehicles (UAVs) to investigate shark and ray densities in a shallow coral lagoon
Résumé
Unmanned aerial vehicles (UAVs) are being increasingly used in studies of marine
fauna. Here, we tested the use of a UAV (DJI Phantom II®) to assess fine-scale variation in densities
of 2 elasmobranchs (blacktip reef sharks Carcharhinus melanopterus and pink whiprays Himantura
fai) on reef systems off Moorea (French Polynesia). We flew parallel transects designed to sample
reef habitats (fringing, channel and sandflat habitats) across 2 survey blocks. Block 1 included a
shark and ray provisioning site with potentially higher elasmobranch densities, whereas Block 2
most likely had lower densities with no provisioning activities. Across 10 survey days in July 2014,
we flew 3 transects (400 m) within each survey block (n = 60 total transect passes). As expected,
densities (animals ha−1) were significantly higher in Block 1 than in Block 2, particularly where provisioning
activities occur. Differences between habitats surveyed were also found. Our study provides
the first direct estimates of shark and ray densities in coral-reef ecosystems and demonstrates
that UAVs can produce important fishery-independent data for elasmobranchs, particularly in
shallow-water habitats.