Mathematical lipid correction of δ13C and effect of lipid extraction on δ15N of European eel (Anguilla anguilla) muscle

Rationale Carbon (delta C-13) and nitrogen (delta N-15) stable isotope analysis is a powerful tool to investigate diverse questions in fish ecology, such as their trophic position or migration strategies. These questions appear particularly important to protect endangered European eel. However, elevated lipid content in eel muscle can bias delta C-13 values, as lipids are C-13-depleted compared to proteins and carbohydrates. Methods We measured delta C-13 and delta N-15 values of bulk and lipid-free samples of eel muscle. Lipid-free samples were obtained after the extraction of lipids with cyclohexane. Lipid-corrected delta C-13 values, using five different mathematical equations based on bulk delta C-13 values, were compared to lipid-free delta C-13 values. We also evaluated the effect of lipid extraction on delta N-15 values. The analyses were based on linear regression performed on 333 individuals captured in nine lagoons and four rivers. Results Independently to the capture site or habitat (river or lagoon), the predicted lipid-corrected delta C-13 values were highly consistent with the measured lipid-free delta C-13 values (R-2 > 0.90). The application of specific equations for each habitat or capture site only slightly increases these R-2 (1.5% or less). The lipid extraction treatment significantly decreased by 0.2 parts per thousand the delta N-15 values compared to bulk samples. Conclusions Given the excellent prediction of mathematical equations and the small decrease of delta N-15 values after lipids extraction, we propose to use mathematical correction to estimate delta C-13 values of eel muscle. As the habitats or sites did not strongly influence the results, the coefficients from our study can be applied to other studies on European eel.