Back-calculating length from skeletal growth marks in loggerhead sea turtles Caretta caretta

Growth rates within and among sea turtles are highly variable, and gaining an understanding of this variability is difficult using traditional means, such as mark–recapture. Skeletochronology is becoming a standard technique for the assessment of individual growth rates in sea turtles. Here we present an analysis of the relationship between humerus diameter and somatic growth in loggerhead sea turtles Caretta caretta, demonstrating that this relationship is allometric, with a steeper slope for small pelagic turtles and a gentler slope for larger neritic turtles. We compare this relationship to models fit to neritic turtle data only and validate the ability of this relationship to accurately back-calculate carapace lengths from diameters of skeletal growth marks using 12 neritic, juvenile loggerheads that were captured, tagged, released, and subsequently recovered as dead strandings. We estimated the length at capture by back-calculation, using the diameter of the skeletal growth mark most representative of the time of capture as a predictor. The mean difference between the measured carapace length at capture and the estimated carapace length obtained through back-calculation was 0.6 cm ± 0.2 SE. For corresponding estimates of annual growth rate, the mean error was 0.2 cm yr–1 ± 0.05 SE. Although we were unable to validate the back-calculation equation for pelagic turtles, we provide indirect evidence that this equation will allow for backcalculation of sizes through this stage. We suggest that, with proper application, back-calculation in combination with skeletochronology can be a powerful tool in studying the growth dynamics of individual sea turtles.