Fatty acids (FAs) are used to make inferences about the foraging behavior and diets of free-ranging marine mammals. However, several methods are currently available for determining the FA composition of blubber and these methods may produce different results. We compared in situ direct transesterification methods, where a small amount of tissue is sampled, with more traditional methods involving prior lipid extraction of the entire sample of interest. Using gray seal (Halichoerus grypus) and beluga whale (Delphinapterus leucas) blubber, we found that when the direct in situ method was used on a 2-mg sample of blubber, the resulting FA profile differed significantly from that produced when traditional full-extraction methods were employed. Regardless of where the small spot sample was taken within the blubber depth, it was not representative of the entire blubber FA composition, as blubber is non-homogeneous throughout its depth. We also modified the in situ direct method to allow analysis of the entire blubber layer. Results of this full-layer direct method compared quite favorably with traditional extraction methods and may provide a reasonable alternative for analyses. Although application of our full-layer direct method will require further verification in certain marine mammal blubber samples, we conclude that the large differences obtained when using the direct method are not a consequence of the chemical method itself. Rather, they arise from non-representative sampling of the blubber FA composition.
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