A novel method for photo-identification of sea turtles using scale patterns on the front flippers

Abstract

Photo-identification using animals' natural markings is a cheaper and less impactful alternative to the use of more conventional external or internal tags for identifying individual animals. Photo-ID has already been successfully employed to monitor hard-shelled sea turtles, specifically using facial scales. However, photographing facial features might cause stress on photosensitive sea turtles, especially at night. Considering that there are more scales on the flippers than on the face and flipper photography is likely to be less invasive, we proposed an alternative method for photo-identification using the scale patterns on the front flippers. This method might also be suitable for successful photo-identification of hatchlings – which has been ineffectual using facial scales. To test the suitability of using the front flippers for photo-ID sea turtles, we took photos of the right flippers of adult and hatchling green turtles (Chelonia mydas) and olive ridley turtles (Lepidochelys olivacea) from Cabuyal, Costa Rica. The photos were analysed using APHIS, which delimits an area of interest with three reference points and then uses several additional points within this area to mark the intersections between scales. In both species, hatchlings and adults were correctly identified 92.9% and 81.8% of the time respectively but the capability of APHIS to correctly identify individuals was highly dependent on the quality of the photo. We detected similarities in flipper scale pattern between hatchlings of the same species but not between hatchlings from the same nest, indicating that hatchlings have unique flipper markings. The use of flipper scale patterns to identify individual sea turtles is comparable to the use of facial scales, without the risk of disturbing nesting females. Additionally, we were able to reliably identify individual hatchlings, which has not been possible using facial scales or conventional tagging techniques. Flipper scale patterns may potentially be a new methodology for identifying hatchlings both short- and long-term. We discuss the benefits and limitations of using sea turtle flipper scale patterns for identification as well as the benefits and limitation of APHIS.