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The fatty acid (FA) composition was determined in the blubber of 5 adult Baikal seals Phoca sibirica caught in July 2001, nine 1-mo old pups caught in April 2002, and 25 adults and fetuses from 3 of these, caught in October 2002, all along the SE shores of Lake Baikal, eastern Siberia. For comparison, FA in the blubber of ringed seals P. hispida, the closest marine relative of the Baikal seal, caught along the ice edge around Svalbard during 1999 and 2000 were also analyzed. The FA of the main prey of the Baikal and ringed seals, the endemic golomyanka Comephorus baikalensis and polar cod Boreogadus saida, respectively, were also studied. The same suite of FA was found in all samples. Saturated FA (SFA) with branched chains, all n6 polyunsaturated FA (PUFA) and 18:3n3 and 20:3n3 were more abundant in the blubber of the Lake Baikal seal and in the golomyanka than in the blubber of the marine ringed seal and marine fishes. In contrast, the relative levels of monounsaturated FAs (MUFA) with 20 and 22 carbons and polyunsaturated 20:5n3 were lower in the Baikal species than in the Svalbard species. The FA composition of the inner blubber was significantly different from the FA composition of the outer blubber in both species. Despite large differences in FA composition between Baikal and ringed seal blubbers, vertical differences were manifested in the same FA: MUFA with 14, 16 and 18 carbons were present in higher relative abundance in the outer than in the inner layer. In both species all the other quantitatively important FA had lower or equal relative values in the outer layer. The FA composition of the seals' blubber differed from that of their main prey. The difference was, however, smaller between prey and inner blubber than between prey and outer blubber. The FA composition of the inner blubber of adults showed temporal differences. Since the change from prey to blubber in relative concentration occurred in the same direction for all but 5 FA, the selection of FA during deposition in the inner blubber was similar for the 2 species, even though there was a large difference between the general FA composition of the freshwater and marine lipids. The FA composition in the blubber of adults, pups and fetuses differed.

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