In-water photo-ID to estimate foraging movements of green turtles Chelonia mydas in Apo Island, Negros Oriental, Philippines

Abstract

Green turtles Chelonia mydas aggregate on the western coastline of Apo Island, Negros, central Philippines foraging mainly on two species of red algae: Hypnea sp. and Gracilaria sp. We identified turtles in the foraging aggregation by photographic identification (photo-ID) of the facial scutes. Studies have shown that the facial scute patterns of sea turtles are individually unique and remain unchanged for at least a decade confirming the validity of Photo-ID as a viable method of identifying marine turtles in the wild. In-water photo-ID surveys from the end of February to the beginning of March 2018–2020 were made by taking photos of the right side of the head of each turtle. We examined 3050 clear photos of the head scute patterns resulting in identification of 129 unique individuals. Fourteen turtles were identified for three consecutive years and eight individuals identified in 2020 were also found in a 2013 poster of a green turtle photo collage taken by a dive instructor residing on the island, suggesting strong site fidelity of green turtles in Apo Island. Out of the 56 turtles identified in 2020, 42 were adults comprising 40 females (95%) and 2 males (5%). This study adds to the existing literature on the effectiveness of photo-ID in identifying turtles to evaluate foraging movements and highlights the importance of Apo Island as a foraging and developmental habitat for green turtles in the Sulu Sea region of the Philippines.