ESR 3:95-104 (2007)  -  doi:10.3354/esr003095

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

Melissa L. Snover1,2,*, Larisa Avens3, Aleta A. Hohn3

1Duke University Marine Laboratory, 135 Duke Marine Lab Road, Beaufort, North Carolina 28516, USA
2NOAA Fisheries, Southwest Fisheries Science Center, Environmental Research Division, 1352 Lighthouse Avenue, Pacific Grove, California 93950, USA
3NOAA Fisheries, Southeast Fisheries Science Center, Center for Coastal Fisheries and Habitat Research, 101 Pivers Island Road, Beaufort, North Carolina 28516, USA

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 back-calculation 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

Full text in pdf format