Utility of carapace images for long-term photographic identification of nesting green turtles

Abstract

The accurate identification of wildlife is necessary for biological research and population monitoring. In sea turtles, artificial tags have commonly been used for individual identification, but tagging has low long-term reliability because of inherent problems such as tag loss and the stress caused by invasive tagging. Photographic identification (Photo-ID) has been increasingly used as an alternative to tagging. Mostly, facial scute patterns have been used as Photo-ID means for sea turtles; however, for Photo-ID of turtles on nesting beaches, there is a risk of disturbing individuals when attempting to access the head region and capture the entire face. Thus, here, we focused on the posterior part of the carapace (fifth vertebral scute), which can be photographed more easily and less invasively, as a useful natural marker for the identification of nesting green turtles (Chelonia mydas). To establish the utility of the carapace for individual identification, we used photographs (167 images from 77 individuals) collected in a nesting survey conducted over 28 years at Ishigaki Island, Japan, and verified the long-term identifiability of carapace. We initially matched individual turtle images with the HotSpotter program to assess the automated recognition of images, and then conducted a blind test to visually validate the results of automated recognition. High matching accuracy was achieved, especially within the same nesting season (98.1%; 52 out of 53 queries), and logistic regression indicated that the matching accuracy was over 70% when the photographing interval was ≤4 years, which is longer than the mean remigration interval for nesting green turtles on Ishigaki Island. In the blind test of visual identification, 95.4% of the image pairs were correctly judged (as those of the same individual or different individuals). These results show that a query image can be successfully identified in the database even if automated matching to the same individual fails or if the query is taken from a newly recruited individual. This study shows the long-term identifiability of nesting green turtles using carapace photographs and proposes a practical Photo-ID method that is conducive to citizen science.