Stability of facial scale patterns on green sea turtles Chelonia mydas over time: A validation for the use of a photo-identification method

Abstract

Photographic identification (photo-ID) has been increasingly used as a reliable tool to track individuals over time, which provides essential knowledge on a species' population dynamics. For photo-ID to work, natural markings must be individual-specific and stable over time. In sea turtle research, the use of facial scale patterns has been proposed and tested as a reliable means for individual recognition. Nevertheless, as sea turtles are migratory and long-lived individuals, the stability of those patterns over long periods of time is yet to be confirmed to validate this method. Stability of facial scale patterns was evaluated on green sea turtles, Chelonia mydas, sighted and photographed in coastal waters of Reunion Island (21°06 S, 55°36 E) or reared in captivity. From 53 free-ranging individuals previously identified, 90 head profiles were selected based on the photographic quality and the distinctiveness of facial scale patterns. The time interval between two sightings of a same individual ranged from 2 (738d) to 11years (3954d). Additionally, facial scale patterns of captive green turtles from two different age groups were compared: (1) from adult-sized individuals reared in captivity (n=13) and (2) from hatchlings and then at later developmental stages (until 1800d) to assess the stability of facial scale patterns throughout early juvenile development (n=16). In both the free-ranging and captive-reared groups, there were no significant changes in facial scale patterns over time. Conversely, changes in pigmentation were observed in free-ranging turtles at successive sightings and in captive-reared turtles at different developmental stages. These results on the stability of facial scale patterns over time, combined with previous findings on the uniqueness of patterns between individual green turtles, validate the method of using facial scale patterns as a long-term identification tool for green turtles. Nonetheless, the variability of pigmentation patterns should be kept in mind when using photo-identification on sea turtle species.