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 Jason M. Kolts a, ⁎, James R. Lovvorn b , Christopher A. North a,c , Jacqueline M. Grebmeier d , Lee W. Cooper d a Department of Zoology and Physiology, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA b Department of Zoology and Center for Ecology, Southern Illinois University, 1125 Lincoln Drive, Carbondale, IL 62901, USA c Program in Ecology, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA d Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, 146 Williams Street, PO Box 38, Solomons, MD 20688, USA abstract article info Article history: Received 30 July 2013 Received in revised form 5 October 2013 Accepted 8 October 2013 Available online 26 October 2013 Keywords: Decapod diets Northern Bering Sea Snow crab δ 13 C δ 15 N Stable isotopes (δ 13 C and δ 15 N) 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 δ 15 N and δ 13 C 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Stable isotope (SI) and fatty acid (FA) analyses have become common approaches in diet studies. These methods are advocated as solving certain shortcomings of stomach content (SC) analyses, namely that SC identify only an organism's last meal, and underestimate the importance of prey items that are easily digested and lack hard parts (Hyslop, 1980; Iverson et al., 2004; Williams and Buck, 2010). SI and FA techniques can indicate diet over a longer period than SC, and can provide dietary data on individuals with empty digestive tracts (Lovvorn et al., in press). On the other hand, SC analyses can reveal variations in prey size important to foraging ecology (Richman and Lovvorn, 2003), and typically yield greater taxonomic resolution; the latter aspect is especially important when the average SI signature of two different prey taxa resembles the signature of another (Hart and Lovvorn, 2005). However, in consumers that grind their prey into fragments identifiable only as present or absent in stomachs (Kolts et al., 2013), SIs and FAs can indicate the relative masses of foods that are actually assimilated. Trophic studies based on stable isotopes typically involve analyses of δ 13 C and δ 15 N. δ 13 C is often used to trace the trophic flow of carbon from a specific source, as carbon sources can vary appreciably in δ 13 C, and δ 13 C in animals often remains within 1‰ of values in their diet (Deniro and Epstein, 1978; Michener and Schell, 1994; Post, 2002). δ 15 N is frequently used to indicate trophic position in food webs, as δ 15 N is assumed to become enriched by a mean of ~3.4‰ with each trophic level (Deniro and Epstein, 1981; Michener and Schell, 1994; Post, 2002). However, δ 15 N fractionation can vary appreciably within and among species, trophic levels, and habitats (Vander Zanden and Rasmussen, 1999, 2001). Mixing models, which calculate probable diets based on isotopic values in both consumers and the complement of potential foods, require information on isotopic fractionation between foods and the consumers' Journal of Experimental Marine Biology and Ecology 449 (2013) 274–283 ⁎ Corresponding author at: Department of Biology, Metropolitan State University of Denver, Denver, CO 80217, USA. Tel.: +1 3073999855. E-mail addresses: jkolts@msudenver.edu (J.M. Kolts), lovvorn@siu.edu (J.R. Lovvorn), cnorth@uwyo.edu (C.A. North), jgrebmei@umces.edu (J.M. Grebmeier), cooper@umces.edu (L.W. Cooper). 0022-0981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jembe.2013.10.005 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe