Fatty Acid Variation in Beluga Blubber: Implications for Estimating Diet Using Fatty Acids

Diet is one of the most important aspects of the ecology of any species, yet remains difficult to estimate for many marine mammals. Quantitative fatty acid signature analysis is a recently developed technique to estimate predator diets by comparing the fatty acid composition of predator tissues with that of potential prey items. This type of analysis is well-suited to estimating marine mammal diet, as most possess a large and relatively easily-accessed blubber layer. However, prior to making diet inferences using FA, it is imperative to examine FA variability within the blubber layer and potential impacts of this variability on QFASA diet estimates. Fatty acid composition of beluga (Delphinapterus leucas) blubber from eastern Chukchi Sea belugas was examined. Composition did not vary across the dorsal portion of the thorax, but FA were stratified with depth in the blubber, and the degree of stratification increased with age. Isovaleric acid, a short-chain FA that is not derived from diet, was investigated separately. Isovaleric acid was found in minute amounts in the liver and much larger amounts in the blubber and melon. This supports a hypothesis of local biosynthesis in the blubber and melon. Isovaleric acid was also stratified with depth in the blubber, and the degree of stratification increased with age. Finally, the impact of several sources of FA variation was explored on QFASA diet estimates. Diet estimates were insensitive to laboratory technique used to process fat samples (methyl esters vs. butyl esters) and the sample collection site on beluga thorax. However, blubber sampling depth affected diet estimates in terms of both prey species identified and proportions of the diet they represented. This study illustrates the great promise in using QFASA to estimate diet in belugas and other cetacean species. It can be used to guide sample collection protocols. While the production of accurate diet estimates remains contingent upon satisfying QFASA model requirements, this study clearly shows that collecting cetacean blubber and potential prey samples is already a worthwhile endeavor, and that baseline datasets can now be developed for detection of future changes.