Applying a fast, effective and reliable photographic identification system for green turtles in the waters near Luichiu Island, Taiwan

• The size of a green turtle population was inferred from non-invasive photo-id.
• Facial scutes on the left and right sides of the head were used for identification.
• 106 to 142 turtles have been identified based on underwater photos.

Liuchiu Island of Pengtung County, Taiwan, is the only island host both foraging and nesting green turtles. Due to almost complete overlap of in-water study with the major tourism activities—in water observation of sea turtles on the island, an effective, stable and fast, non-invasive photo-ID system has been developed to estimate the size of this population. The characteristics of facial scales, such as size, shape and arrangement are used as the diagnostic tools. One hundred and six to one hundred and forty-two turtles, with minimum 6 and maximum 8 were males have been identified based on underwater photos collected from 2011 to 2013. The right and left facial scutes of most turtles are not identical suggesting that both sides of the head must be photographed in order to determine the population size of the turtles. Based on the frequency of observation, these three to five males and forty to seventy-two females or undetermined sex were assumed to be resident while four males and sixty-six to seventy female or undetermined sex turtles were assumed to be migrants. Repeated photographed suggests that at least portion of the resident turtle conduct seasonal migration around the island. Among all sections of the area surveyed, turtles were most concentrated from Beauty Cave to Vase Stone, and least numerous from Shan-fu Fishing Port to Clam Bay. This is the first photo-ID system developed in Taiwan. It will benefit sea turtle research in the future and enhance sea turtle conservation significantly.