Synergistic use of <scp>UAV</scp> 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 species makes estimating their challenging. Abundance estimates sea turtles, marine birds, seals are usually made when breeding adults ashore, while life stages spent at sea, including as juveniles, often poorly sampled. We used a combination high-resolution satellite tracking (Fastloc-GPS), uncrewed aerial vehicle (UAV) surveys, capture-mark-recapture approaches assess immature hawksbill (Eretmochelys imbricata) green turtles (Chelonia mydas) tidal lagoon Chagos Archipelago (Indian Ocean). captured, marked, released 50 (48 2 turtles) prior UAV surveys data from 27 (25 refine estimated numbers marked available for resighting those likely have emigrated study area. total 339 with density variation different heights between 265 km?2 high water 499 low water. Of these, 91% were hawksbills 9% turtles. These densities highest reported among 17 foraging sites recorded around world reflect successful long-term protection Archipelago.