Stable isotope discrimination (delta C-13 and delta N-15) between soft tissues of the green sea turtle Chelonia mydas and its diet

The foraging ecology and movements of vertebrate species have been increasingly studied via stable isotope analyses of small quantities of body tissues. However, the theoretical and experimental basis of this method remains poorly validated for most taxa despite numerous studies using these techniques in natural systems. In this study, we measured stable carbon and stable nitrogen diet-tissue discrimination (Delta(dt) in whole blood, red blood cells, blood plasma, and epidermis of 8 captive green turtles Chelonia mydas maintained on a control diet (41% protein, 12% lipids, 4% fiber) for 619 d. During the course of the study, mean straight carapace length increased from 45.2 1.2 to 53.7 +/- 2.1 cm, whereas mean body mass increased from 11.7 +/- 0.7 to 19.9 +/- 2.2 kg. Both diet and tissue isotope values remained constant throughout the study, indicating that diet-tissue equilibrium had been achieved. Whereas Delta C-13(dt) ranged from -1.11%. (red blood cells) to +0.1.7%o (epidermis), Delta N-15(dt) ranged from +0.22%. (red blood cells) to +2.92%. (blood plasma). These results contrast with the widely accepted discrimination factors of 0 to 1%. for VC and 3 to 5%, for delta N-15. Comprising the initial enrichment factors available for green turtles, these data will be useful in future interpretations of field isotopic values for this species.