Discrepancies in the number of lines of arrested growth (LAG) in the tissues of the humerus and phalanx of sea turtles

Information on the age of vertebrate species such as sea turtles is crucial for planning management and conservation actions. The age of sea turtles has been estimated by skeletochronological analysis using skeletal growth marks in different bones. This study focused on the consistency of the number of visible lines of arrested growth (LAG) observed from the humerus and phalanx bone used for age estimation in Chelonia mydas and Caretta caretta. We collected 67 humeri and phalanges of C. mydas (n = 47) and C. caretta (n = 20) from Samandağ beach, eastern Mediterranean in 2020–2022. LAG in the humerus and phalanx of the same individual were counted by two readers, and their consistency with each other was determined by percent agreement (PA), average percent error (APE), and coefficient of variation (CV). The significance of the difference between them was determined by the McNemar test. The mean number of visible LAG counted from the humerus is greater than the mean number of visible LAG counted from the phalanx, i.e., the humerus contains more growth marks than the phalanx. However, in individuals up to 15 LAG in C. mydas and 10 LAG in C. caretta, the mean number of visible LAG observed in both bone tissues is compatible. This was supported by the differences in the resorption rates calculated in both bones, indicating that the number of LAG lost due to resorption may also differ between these two bone types. It is recommended that the back calculation and/or correction factor applied for the humerus be avoided for the phalanx.