Assessing the effects of lipid extraction and lipid correction on stable isotope values (δ13C and δ15N) of blubber and skin from southern hemisphere humpback whales

Rationale The coupled analysis of delta C-13 and delta N-15 stable isotope values of blubber and skin biopsy samples is widely used to study the diet of free-ranging cetaceans. Differences in the lipid content of these tissues can affect isotopic variability because lipids are depleted in C-13, reducing the bulk tissue C-13/C-12. This variability in carbon isotope values can be accounted for either by chemically extracting lipids from the tissue or by using mathematical lipid normalisation models. Methods This study examines (a) the effects of chemical lipid extraction on delta C-13 and delta N-15 values in blubber and skin of southern hemisphere humpback whales, (b) whether chemical lipid extraction is more favourable than mathematical lipid correction and (c) which of the two tissues is more appropriate for dietary studies. Strategic comparisons were made between chemical lipid extraction and mathematical lipid correction and between blubber and skin tissue delta C-13 and delta N-15 values, as well as C:N ratios. Six existing mathematical normalisation models were tested for their efficacy in estimating lipid-free delta C-13 for skin. Results Both delta C-13 and delta N-15 values of lipid-extracted skin (delta C-13: -25.57 parts per thousand, delta N-15: 6.83 parts per thousand) were significantly higher than those of bulk skin (delta C-13: -26.97 parts per thousand, delta N-15: 6.15 parts per thousand). Five of the six tested lipid normalisation models had small error terms for predicting lipid-free delta C-13 values. The average C:N ratio of lipid-extracted skin was within the lipid-free range reported in other studies, whereas the average C:N ratio of blubber was higher than previously reported. Conclusions These results highlight the need to account for lipids when analysing delta C-13 and delta N-15 values from the same sample. For optimised dietary assessments using parallel isotope analysis from a single sample, we recommend the use of unextracted skin tissue. delta N-15 values should be obtained from unextracted skin, whereas delta C-13 values may be adequately lipid corrected by a mathematical correction.

Automated summary

The study found that chemical lipid extraction significantly affected the delta C-13 and delta N-15 values of skin in humpback whales, showing differences from the values of non-extracted tissues. Chemical lipid extraction had small error terms for predicting lipid-free delta C-13 values, suggesting the use of unextracted skin tissue for optimized dietary assessments.