Green turtle foraging patterns revealed by seagrass meadow-centric time-lapse camera and drone surveys

Abstract

The recent repression of seagrass meadows threatens multiple ecosystem services. Seagrass overgrazing by herbivorous animals, such as green turtles (Chelonia mydas), may contribute to seagrass repression. Therefore, the foraging ecology of green turtles is important for understanding their interactions with seagrass meadows and assessing the sustainability of seagrass ecosystems. The diel cycle is a major driving factor of green turtle foraging patterns; however, tidal effects remain less well understood. This study aimed to clarify the influence of diel and tidal cycles on green turtle foraging in seagrass meadows by integrating time‑lapse camera and drone surveys. Specifically, the timing and density of green turtle visits to a seagrass meadow on Ishigakijima Island, Yaeyama Islands, Japan, were investigated during 2023–2024. The presence or absence of green turtles in the time-lapse camera data indicated that the bimodal diel foraging activities peaked at 6:00–08:00 and 16:00–18:00. In addition, the green turtle observation probability in the time-lapse camera data and the density estimated from the drone surveys increased as the tide level increased. The results showed that more green turtles aggregated to forage in the seagrass meadow when access to seagrass was available during high tide. Furthermore, green turtle density tended to increase during these two years. The seagrass meadow-centric approach using time-lapse camera and drone surveys is set to complement individual-based biotelemetry and biologging techniques to reveal green turtle foraging ecology. This approach is also important for monitoring herbivores in seagrass meadows and considering their coexistence and conservation management.