Mathematical lipid correction of δ13C and effect of lipid extraction on δ15N of European eel (Anguilla anguilla) muscle: Lipid correction of δ13C in European eels.

Rationale: Carbon (δC) and nitrogen (δN) 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 δC values, as lipids are C-depleted compared to proteins and carbohydrates.

Methods: We measured δC and δN values of bulk and lipid-free samples of eel muscle. Lipid-free samples were obtained after the extraction of lipids with cyclohexane. Lipid-corrected δC values, using five different mathematical equations based on bulk δC values, were compared to lipid-free δC values. We also evaluated the effect of lipid extraction on δN 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 δC values were highly consistent with the measured lipid-free δC values (R > 0.90). The application of specific equations for each habitat or capture site only slightly increases these R (1.5% or less). The lipid extraction treatment significantly decreased by 0.2‰ the δN values compared to bulk samples.

Conclusions: Given the excellent prediction of mathematical equations and the small decrease of δN values after lipids extraction, we propose to use mathematical correction to estimate δC 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.