Data from: Synergistic use of UAV surveys, satellite tracking data and mark-recapture to estimate abundance of elusive species

Estimating population abundance is central to many ecological studies and
important in conservation planning. Yet the elusive nature of many species
makes estimating their abundance challenging. Abundance estimates of sea
turtles, marine birds and seals are usually made when breeding adults are
ashore, while life-stages spent at sea, including as juveniles, are often
poorly sampled. We used a combination of high-resolution satellite
tracking (Fastloc-GPS), Unmanned Aerial Vehicle (UAV) surveys and
catch-mark-recapture approaches to assess abundance of immature hawksbills
(Eretmochelys imbricata) and green turtles (Chelonia mydas) in a tidal
lagoon of the Chagos Archipelago (Indian Ocean). We captured, marked, and
released 50 turtles (48 hawksbill and 2 green turtles) prior to UAV
surveys and used satellite tracking data from 27 immature turtles (25
hawksbill and 2 green turtles) to refine the estimated numbers of marked
turtles available for resighting and those likely to have emigrated from
the study area. We estimated a total of 339 turtles in the lagoon with a
density between 265 turtles km-2 at high water and 499 turtles km-2 at low
water. Of these 84% were hawksbills and 16% were green turtles. These
hawksbill densities are the highest reported amongst 17 foraging sites
recorded around the world, likely reflecting successful long-term
protection of turtles in the Chagos Archipelago.