Vertical fatty acid profiles in blubber of a freshwater ringed seal - comparison to a marine relative

We investigated the vertical stratification pattern of fatty acids in the blubber of the freshwater Saimaa ringed seal (Phoca hispida saimensis; n=35). Blubber was dissected vertically into 3mm thick sequential subsamples, and the fatty acid composition was analyzed separately for each subsample. The combined vertical fatty acid profiles (expressed as numerical gradients) showed that the blubber of >1year old individuals is stratified into three distinguishable layers: superficial (~1.5cm), middle and deep (~1cm). Thickness of the middle layer varied according to the total blubber thickness. We suggest that the observed layering is related to the differences in the tissue temperatures and metabolic activity in the blubber column. The dissimilarity in the fatty acid composition (measured as Euclidean distance) between the different blubber layers and the 5 most important fish prey species available in Lake Saimaa was largest in the superficial blubber. In fact, the fatty acid composition of superficial blubber resembled more that of marine ringed seal (Phoca hispida hispida) blubber than any of the analyzed fish species. The composition closest to that of the prey was found in the deep blubber of the Saimaa ringed seal. The large vertical differences in the fatty acid composition of blubber lipids, which likely affect the vertical distribution of other endogenous or exogenous lipophilic substances as well, will set conditions for the sampling of blubber for biomonitoring and dietary studies. Thus the knowledge on the potential layers with different composition and the depths they span in different individuals (e.g. young versus old; with thin versus thick blubber) is crucial for improving the validity and reliability of monitoring methods utilizing the blubber tissue.