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

Abstract

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. Markrecapture 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.

The impact of injury on apparent survival of whale sharks (Rhincodon typus) in South Ari Atoll Marine Protected Area, Maldives

“…The impact of this heterogeneity on LIR modelling has been noted in other taxa (e.g. marine turtles 70 ). Modelling of days spent outside of the study site is difficult given the lack of absence data and should therefore be cautiously interpreted.…”

Validating the use of stereo‐video cameras to conduct remote measurements of sea turtles

“…Species with stationary and rigid points of measurements, such as the carapace of sea turtles, might provide more reliable and accurate reference points for SVCS measurements. While SVCS measurements of sea turtles have been compared to visual estimates and laser photogrammetry (Araujo et al., 2019), they have not yet been compared to direct hand measurements.…”

Whale sharks Rhincodon typus get cleaned by the blue‐streak cleaner wrasse Labroides dimidiatus and the moon wrasse Thalassoma lunare in the Philippines

“…obs.). An adjacent seagrass area also hosts a resident aggregation of green turtles Chelonia mydas Linneaus 1758 (Araujo et al ., 2019). Understanding the relationship between reef health, the added food as part of the whale shark provisioning, seagrass, L. dimidiatus , T. lunare and the whale shark might help elucidate this rare cleaning behaviour.…”

“…obs.). An adjacent seagrass area also hosts a resident aggregation of green turtles Chelonia mydas Linneaus 1758 (Araujo et al, 2019).…”