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

Abstract

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.KEY WORDS: Skeletochronology · Ontogenetic habitat shift · Growth rates · Allometry · Caretta caretta Resale or republication not permitted without written consent of the publisher

Impact of exceptional growth rates on estimations of life-stage duration in Hawaiian green sea turtles

“…In order to back-calculate SCL from skeleto chro nology growth marks, the relationship between humerus diameter and SCL must be established (Francis 1990, Snover et al 2007. To do this, we used 168 humeri represented by hatchlings (N = 10) and juveniles and adults (N = 158) that span the full ontogenetic range for Hawaiian green sea turtles (Table 1).…”

“…Hatchling measurements, SCL (cm) and humerus diameter (mm), were taken with a digimatic plastic caliper (Mitutoyo™). Humerus diameters were measured at the sectioning site and compared to recorded SCLs in order to confirm a predictable and proportional relationship between the humerus and carapace (Snover et al 2007). …”

“…We used the back-calculation methods presented by Snover et al (2007) to estimate SCL at interior growth marks (see Goshe et al 2010). We fit the following equation to the 158 paired measurements of the humerus diameter, at the sectioning site used for skeletochronology, and calculated SCL using leastsquares non-linear regression:…”

“…We then used the fitted parameter estimates for b and c from Eq. (1) to estimate SCL at each interior LAG using the following relationship (Snover et al 2007): …”

“…Skeletochronology is the technique of using growth marks in skeletal structures to obtain information on age and growth rates (Snover et al 2007, Avens & Snover 2013. It has been used successfully to estimate age and growth in numerous species of reptiles and amphibians (Castanet 1994, Smirina 1994, in cluding sea turtles (see Avens & Snover 2013 for re view).…”