In-water methods reveal population dynamics of a green turtle Chelonia mydas foraging aggregation in the Philippines

The green turtle Chelonia mydas is an Endangered species that forms aggregations at neritic foraging sites where juveniles spend, in some cases, over a decade before moving to adult- or subadult-dominated foraging sites. Here, we used photographic identification, behavioural observations, paired-laser and stereo-photogrammetry to determine the population dynamics of a coastal aggregation of green turtles in Oslob, Philippines. We identified a total of 82 individual turtles between May 2012 and October 2018. We recorded behavioural observations during individual identification and found that turtles changed behaviour in 25% of instances. Some turtles were identified consistently throughout the study period, indicating strong site fidelity and residency. Modified maximum likelihood models suggest turtles spend the majority of their time within the study area (>92%), although some (8%) excursions outside the area do occur. Mark-recapture models suggest this foraging aggregation exhibits a pattern similar to that of a closed population. This is likely due to the extended residency periods of some individuals at the site, and the relatively low recruitment rate over time. We measured 18 turtles using paired-laser photogrammetry, with estimated straight carapace length of 55.3 cm, and mean growth rates of 3.4 cm yr(-1). Stereo-photogrammetry measurements were consistent and more accurate than visual or paired-laser photogrammetry estimates. Our results highlight the use of in-water methods to understand population dynamics of green turtles, and the importance of coastal, juvenile-dominated habitats for green turtles, as well as the need for effective conservation and management strategies to safeguard them.