Diet-tissue fractionation of stable carbon and nitrogen isotopes in phocid seals

Diet-tissue isotopic fractionation of carbon (C) and nitrogen (N) isocop in short- and longer-term diet integrators of diet (i.e. blood serum and red cells), that involve non-invasive sampling techniques was examined using three species of phocid seals (harbor seals, gray seals, and harp seals) fed a known diet. Variability in diet-tissue fractionation values within and between species was also scrutinized to determine the legitimacy of using values obtained from one species to explore trophic positions and diets of other related species. All captive seals raised on a constant diet had tissues enriched in C-13 and N-15 relative to their diet. Diet-tissue isotopic fractionation values were generally consistent among conspecifics and among phocid species for a given tissue. Trophic isotopic enrichment in C-13 was significantly higher in red blood cells (+1.5 parts per thousand than in blood serum (+0.8parts per thousand), whereas the reverse was observed fur nitrogen isotopes (+1.7parts per thousand in red cells vs. +3.1parts per thousand in serum). However, C-13-depleted lipids were not extracted from blood tissues in this study. This results in a downward bias in the diet-tissue fractionation factors for carbon for both red cells and blood serum, particularly the latter because of their significantly higher lipid contents ((x) over bar +/- SD = 4.6 +/- 2.3%; n = 20; red blood cells 3.8 +/- 0.9parts per thousand; n = 50, muscle 7.7 +/- 2.0; n = 21) in marine mammals.