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Inter-annual variability in the proportional contribution of higher trophic levels to the diet of Pacific walruses |
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| Authors: | J Seymour L Horstmann-Dehn M J Wooller |
| Institution: | 1. School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, P.O. Box 755220, Fairbanks, AK, 99775-7220, USA 2. Institute of Marine Science, School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, P.O. Box 757220, Fairbanks, AK, 99775-7220, USA 3. Institute of Marine Science, School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, P.O. Box 755860, Fairbanks, AK, 99775-5860, USA 4. Alaska Stable Isotope Facility, Water and Environmental Research Center, Institute of Northern Engineering, University of Alaska Fairbanks, P.O. Box 755860, Fairbanks, AK, 997755-5860, USA 5. Alaska Quaternary Center, University of Alaska Fairbanks, P.O. Box 755940, Fairbanks, AK, 99775-5940, USA |
| Abstract: | Pacific walruses (Odobenus rosmarus divergens) depend on Arctic sea ice as a resting and foraging platform; however, recent years have seen unprecedented seasonal reductions in ice extent. Previous researchers proposed that during unfavorable ice conditions, walruses might prey on other pinnipeds. To examine this hypothesis, we analyzed carbon and nitrogen stable isotope ratios of muscle from walruses (n = 155) sampled from the Bering and Chukchi seas during 2001–2010. We used a Bayesian stable isotope mixing model to examine the proportional contribution of higher trophic level prey (HTLP) (e.g., seals, seabirds) to walrus diets and extrapolated a tissue-specific turnover rate to compare diet of individuals over time. Mode HTLP across years was 19 % ± 8. Results indicate a significant decrease (P < 0.05) in the reliance on HTLP during 2008–2009 (mode HTLP 13 %), one of two sampling periods that experienced great seasonal loss of pan-Arctic sea ice (the other being 2007–2008 with mode HTLP of 23 %). We also reveal intra-annual fluctuations in the contribution of HTLP to the diet of a walrus sampled in 2011 with seal remains in its stomach through high-resolution sectioning along a whisker length. Our findings suggest that walruses forage opportunistically as a result of multiple environmental factors and that sea ice extent alone does not drive consumption of HTLP. |
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