• Aldabra Atoll
• capture-mark-recapture
• fidelity
• foraging ecology
• foraging grounds
• sea turtle
• stable isotope analysis
• UAV

Knowing a population’s spatio-temporal distribution and movements is critical for understanding the ecological roles of marine megafauna and how they influence ecosystem-level processes. The overall aim of my research is to investigate the ecological roles of marine megafauna (green turtles, hawksbill turtles, dugongs, and elasmobranchs) in a relatively undisturbed, long-protected and iconic marine ecosystem: the UNESCO World Heritage Site Aldabra Atoll, in the southern Seychelles. My research focuses on a multi-method approach to quantify population parameters of the foraging aggregations, describe the foraging areas of megafauna to distant foraging grounds, linking wider Western Indian Ocean movements, and quantify abundance and density of megafauna within the lagoon.

First, with a 40-yr long-term capture-mark-recapture data set I estimated the key population parameters of growth rate and age of sexual maturity of immature sea turtles (green turtles Chelonia mydas and hawksbill turtles Eretmochelys imbricata) in the Aldabra lagoon. It showed turtles were using the lagoon for extended periods (ca. 8 and 18 years).

Second, with the same dataset I found that both turtle species show a degree of small-scale fidelity. The fidelity analysis and a network analysis indicted that although both species overlap in sites, they utilize them differently.

Third, I used nitrogen and carbon bulk stable isotopes to investigate foraging ecology, diet selection and fidelity to subareas of juvenile green turtles and hawksbill turtles in the lagoon. Through diet, I showed that these sympatric species have niche partitioning. They also have different foraging strategies – green turtles had ontogenetic shifts in diet, while hawksbills did not. Isotopically, there were differences among sites for both turtle species and among prey items, supporting fidelity to capture sites, as indicated in the capture-mark-recapture analysis. The small-scale differences in prey items (particularly the vegetation), was seen as small-scale baseline differences, which was reflected in turtle tissues.

Fourth, shifting focus to adult green turtles, I linked their reproductive activities in Aldabra with distant foraging locations using stable isotopes and satellite telemetry. With Aldabra standing as a pivotal nesting hub for green turtles in the Western Indian Ocean, this identified broad foraging zones of northern Madagascar and the east African coast (spanning Somalia to Tanzania).

Fifth, I investigated the distribution and abundance of megafauna in part of the Aldabra lagoon through aerial surveys with unmanned aerial vehicles (UAVs) finding high turtle density, followed by sharks, rays and dugongs, and found environmental predictors of abundance varied by taxon. A framework on methods to evaluate how marine megafauna are distributed in the west lagoon through UAVs provides a start for understanding megafauna distribution and abundance at the atoll.

Altogether, my research showed how different approaches can be successfully combined to investigate cryptic biological aspects of marine megafauna, and it also provides baseline information on movements, foraging ecology and resource use, that can be used for comparison as habitats change. By using Aldabra as the focal point, this research illuminates natural aggregation behaviors, providing insights into the prospective impacts of recovering populations and possible climate change-induced behavioral shifts in future.