Tracking trends in foraging sea turtle aggregations at Reunion Island using aerial and photo-ID surveys (2008–2023)

Abstract

The sea turtle populations of Reunion Island, a volcanic island in the western Indian Ocean between Madagascar and Mauritius, were significantly impacted by human colonization in the 17th century. However, after 40 yr of local and regional conservation efforts, these populations are showing signs of recovery. This study examines spatiotemporal trends in the abundance and distribution of green turtles Chelonia mydas and hawksbill turtles Eretmochelys imbricata along the west coast of Reunion Island from 2008 to 2023. Data were collected through microlight aerial surveys and photo-ID. The results reveal a consistent increase in turtle abundance, particularly juveniles, with juvenile green turtles comprising 78%, juvenile hawksbill turtles 15%, and the remaining observations based on photo-ID being adult green turtles. Size-class estimates from aerial surveys suggest that annual fluctuations in abundance are derived largely from variations in the number of smaller turtles, underscoring Reunion Island’s role as a critical developmental habitat. The highest concentrations of turtles were observed in fringing reef areas characterized by shallow, gently sloping bathymetric zones. Photo-ID also revealed strong site fidelity for both species. Combining aerial surveys and photo-ID—an innovative approach—this study provides life-stagespecific abundance trends across 9% (5.5 km2) of the surveyed area (60 km2) and links distribution patterns to food availability. Notably, in one area, a decline in green turtles correlates with the loss of seagrass habitat. These findings advance understanding of the spatial ecology of in-water turtles and offer valuable insights for local and regional conservation planning. They further emphasize the role of sea turtles as indicators of coastal ecosystem health.