Using minimally invasive techniques to determine green sea turtle Chelonia mydas life-history parameters

• Photo-ID is a valid research tool for mark-recapture studies on sea turtles.
• Laser photogrammetry is used to accurately measure individuals for the first time.
• Immature individuals have prolonged residency patterns at the study site.
• Developmental habitats can be identified with these minimally invasive techniques.

Determining the life history and population parameters of sea turtles typically involves the live capture of animals. Photo-identification (photo-ID) of free-ranging individuals is a cost-effective alternative to conventional tagging for mark-recapture studies that also minimises the risk of injury or stress to the animals. Paired laser-photogrammetry (photogrammetry) is a minimally invasive technique to size animals and has been proven effective in a range of different taxa. Photo-ID and photogrammetry were combined as minimally invasive techniques to determine life-history parameters of free-ranging green sea turtles Chelonia mydas at a site in Oslob, Cebu, Philippines. The pattern-recognition software I3S Pattern was used to validate results and confirm visual matching. A total of 38 individuals were identified at the site through photo-ID. Facial scutes remained unchanged for a min. of 3.6 years. Photogrammetry was employed to measure straight carapace lengths (SCLs) of a sample of 17 individuals, with a mean straight carapace length of 52.5 ± SD 1.1 cm. Modified maximum likelihood methods were used to model mean residency of 873 d at the site, with c. 12 individuals present at any one time. Based on photogrammetry measurements, individuals at the site were found to be of immature age class. This, along with the extended residency times, indicates that the site could be an important developmental habitat. Photo-ID is a valid minimally invasive technique for identifying individual C. mydas for mark-recapture analyses. The use of paired laser photogrammetry for categorising the life-history stage of an aggregation of sea turtles is demonstrated for the first time. Identification of such developmental habitats through the employment of these minimally invasive techniques is useful for conservation of the species.