Relative value of stomach contents, stable isotopes, and fatty acids as diet indicators for a dominant invertebrate predator (Chionoecetes opilio) in the northern Bering Sea

Stable isotopes (δ13C and δ15N) and fatty acid biomarkers have increasingly replaced stomach contents in diet studies. Stable isotopes (SIs) and fatty acids (FAs) can indicate diet over longer periods than stomach contents (SCs), and provide data when stomachs are empty. While SCs yield greater taxonomic resolution, SI methods are less invasive and labor intensive, and SIs and FAs can indicate the relative mass of foods assimilated. For invertebrates, however, data on fractionation of SI and calibration coefficients for FA needed for definitive quantitative estimates are often lacking. To assess differences in inference from the different methods for an omnivorous invertebrate, we compared SC, SI, and FA analyses as diet indicators for snow crabs (Chionoecetes opilio) in the northern Bering Sea. Stomach contents (relative percent frequency of occurrence) consisted mainly of crustaceans, bivalves, and polychaetes, with lesser frequency of gastropods and ophiuroids. SCs varied among regions and correlated strongly with local prey abundance. Diets inferred from individual values or Bayesian mixing models of SIs did not correlate well with local prey abundance or SCs, suggesting a need for a better understanding of the fractionation of δ15N and δ13C from different foods in snow crabs. FAs suggested consumption of nemertean worms which, lacking hard parts, were not identified in stomach contents. Resemblance of FA composition among prey taxa, similar diet diversity among different areas, and unknown assimilation efficiencies for different FAs by snow crabs limited inference from FAs about the magnitude of diet differences among regions. Stomach contents yielded the most definitive diet information for an invertebrate whose prey mostly contained hard, easily identifiable structures.