Photographic identification of sea turtles: method description and validation, with an estimation of tag loss

Abstract

Recognition of individual sea turtles is mostly achieved by checking artificial tags previously attached to them, a method which is made difficult by the considerable tag loss rate and which requires repeated manipulation of the marked individuals. We describe an individual recognition method for sea turtles of the family Cheloniidae based on a mark-recapture study that relied on both artificial tagging (Inconel tags, style 681) and natural marks (facial profile photographs). Juvenile green Chelonia mydas and hawksbill Eretmochelys imbricata turtles were manually caught at Arvoredo Island, southern Brazil, and through visual comparison of facial profile photographs we were able to identify recaptured individuals with 2, 1, or no artificial tags. Additionally, Bayesian inference based on tag loss information indicated that the way a tag is attached (position and distance from the flipper edge) affects significantly the probability of its loss. We encourage the use of photographic identification (facial profile) as a reliable method for individual recognition in studies of cheloniid turtles.