Correcting lipid extraction effects on nitrogen isotopic values (δ15N) in cetacean skin

Stable isotopes of carbon (δ13C) and nitrogen (δ15N) are widely used to study the feeding ecology of cetaceans, as they provide critical insights into diet and migratory behaviors. Lipids in tissues may bias the interpretation of δ13C. Because of this, lipids need to be extracted before measuring stable isotope ratios, but their removal may alter δ15N values. Using nearly 900 skin samples of five cetacean species, this study examined the impact of lipid extraction (Folch method) on skin δ15N values and developed mathematical corrections for these effects. The mathematical corrections were then validated with three species sampled in the same or other environments. Lipid extraction increased δ15N values in all species, albeit by a variable amount. This effect was accurately predicted by both species-specific and multi-species linear relationships. The proposed mathematical corrections across species and environments offer a cost-effective alternative to analysis of separate aliquots for δ13C and δ15N determination, and a valuable tool for retrospectively correcting existing lipid-extracted samples for which raw material may no longer exist.