Fatty acid metabolism in marine carnivores: Implications for quantitative estimation of predator diets

A number of metabolic processes can potentially affect the relationship between dietary and adipose tissue fatty acid (FA) compositions and will influence the use of FA as trophic tracers in predators. The objectives of my thesis were to use juvenile grey seals (Halichoerus grypus) and mink (Mustela vison) to investigate de novo FA synthesis by the liver, the deposition and modification of individual radio-labelled dietary FA, the relationship between dietary and circulating chylomicron FA signatures, and the relationship between dietary and blubber FA signatures in seals fed completely homogenous diets. In vitro studies with 3H-labelled acetyl-CoA showed that the primary product of de novo FA synthesis in the liver of a marine carnivore was 16:0. In vivo studies using labelled FA showed that while some dietary FA are deposited directly, others may be extensively modified. The employment of a dual-label radioisotope tracer technique demonstrated reduced deposition of 3H-22:1n-11, relative to 14C-18:1n-9, in both seals and mink, coupled with the appearance of 3H-labelled shorter monounsaturated products, likely due to peroxisomal chain-shortening in liver. A further study demonstrated that individual dietary FA likely experience differential metabolism in the early assimilation stages leading up to chylomicron formation, thus also affecting incorporation patterns into adipose tissue. Although absolute differences existed, chylomicron FA signatures most resembled diet signatures at peak chylomicron formation and with greater fat intake, likely reflecting the greatest ratio of dietary-triacylglycerol to phospholipid FA. When metabolic aspects were accounted for, chylomicron FA signatures accurately predicted diet composition using a quantitative model. Finally, a series of captive feeding studies with juvenile grey seals demonstrated that even over a relatively brief period of time, FA signatures of a new diet are reflected in predator stores. When differential metabolism of individual FA within the predator was accounted for, blubber FA provided accurate estimations of diet composition using a quantitative model. In addition, while whole blubber provided a longer-term integration of the dietary history of an individual, the inner-most half of the blubber layer provided a view of more recent diet, consistent with expected turnover rates. These studies provide greater insight into the metabolic processes underlying the use of FA to estimate predator diets.