Accounting for the effects of lipids in stable isotope (δ13C and δ15N values) analysis of skin and blubber of balaenopterid whales

RATIONALE Stable isotope values (delta C-13 and delta N-15) of darted skin and blubber biopsies can shed light on habitat use and diet of cetaceans, which are otherwise difficult to study. Non-dietary factors affect isotopic variability, chiefly the depletion of C-13 due to the presence of C-12-rich lipids. The efficacy of post hoc lipid-correction models (normalization) must be tested. METHODS For tissues with high natural lipid content (e.g., whale skin and blubber), chemical lipid extraction or normalization is necessary. C:N ratios, delta C-13 values and delta N-15 values were determined for duplicate control and lipid-extracted skin and blubber of fin (Balaenoptera physalus), humpback (Megaptera novaeangliae) and minke whales (B. acutorostrata) by continuous-flow elemental analysis isotope ratio mass spectrometry (CF-EA-IRMS). Six different normalization models were tested to correct delta C-13 values for the presence of lipids. RESULTS Following lipid extraction, significant increases in delta C-13 values were observed for both tissues in the three species. Significant increases were also found for delta N-15 values in minke whale skin and fin whale blubber. In fin whale skin, the delta N-15 values decreased, with no change observed in humpback whale skin. Non-linear models generally out-performed linear models and the suitability of models varied by species and tissue, indicating the need for high model specificity, even among these closely related taxa. CONCLUSIONS Given the poor predictive power of the models to estimate lipid-free delta C-13 values, and the unpredictable changes in delta N-15 values due to lipid-extraction, we recommend against arithmetical normalization in accounting for lipid effects on delta C-13 values for balaenopterid skin or blubber samples. Rather, we recommend that duplicate analysis of lipid-extracted (delta C-13 values) and non-treated tissues (delta N-15 values) be used. Copyright (C) 2012 John Wiley & Sons, Ltd.