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

Abstract

Rationale

Carbon (δ¹³C) and nitrogen (δ¹⁵N) 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 δ¹³C values, as lipids are ¹³C-depleted compared to proteins and carbohydrates.

Methods

We measured δ¹³C and δ¹⁵N values of bulk and lipid-free samples of eel muscle. Lipid-free samples were obtained after the extraction of lipids with cyclohexane. Lipid-corrected δ¹³C values, using five different mathematical equations based on bulk δ¹³C values, were compared to lipid-free δ¹³C values. We also evaluated the effect of lipid extraction on δ¹⁵N 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 δ¹³C values were highly consistent with the measured lipid-free δ¹³C values (R² > 0.90). The application of specific equations for each habitat or capture site only slightly increases these R² (1.5% or less). The lipid extraction treatment significantly decreased by 0.2‰ the δ¹⁵N values compared to bulk samples.

Conclusions

Given the excellent prediction of mathematical equations and the small decrease of δ¹⁵N values after lipids extraction, we propose to use mathematical correction to estimate δ¹³C 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.