Pacific walrus diet across 4000 years of changing sea ice conditions

Abstract

Declining sea ice is expected to change the Arctic's physical and biological systems in ways that are difficult to predict. This study used stable isotope compositions (δ13C and δ15N) of archaeological, historic, and modern Pacific walrus (Odobenus rosmarus divergens) bone collagen to investigate the impacts of changing sea ice conditions on walrus diet during the last ~4000 yr. An index of past sea ice conditions was generated using dinocyst-based reconstructions from three locations in the northeastern Chukchi Sea. Archaeological walrus samples were assigned to intervals of high and low sea ice, and δ13C and δ15N were compared across ice states. Mean δ13C and δ15N values were similar for archaeological walruses from intervals of high and low sea ice; however, variability among walruses was greater during low-ice intervals, possibly indicating decreased availability of preferred prey. Overall, sea ice conditions were not a primary driver of changes in walrus diet. The diet of modern walruses was not consistent with archaeological low sea ice intervals. Rather, the low average trophic position of modern walruses (primarily driven by males), with little variability among individuals, suggests that trophic changes to this Arctic ecosystem are still underway or are unprecedented in the last ~4000 yr.

Using claws to compare reproduction, stress and diet of female bearded and ringed seals in the Bering and Chukchi seas, Alaska, between 1953–1968 and 1998–2014

“…Typically, instrument precision is < 0.2 ‰. Carbon stable isotopes were Suess corrected to the Bering Sea and the earliest year represented by these data (1953) to account for increased fractionation of carbon due to increased use of fossil fuels from 1850 to present day ( Clark et al 2019 ). Methods for Suess correction were the same as in Clark et al (2019) .…”

Determining sex of adult Pacific walruses from mandible measurements

“…Female and male walruses have different physiological needs and display strong sexual dimorphism ( Fay 1982 ; Noren et al 2012 , 2014 , 2015 ). Determining sex from mandibles also may allow past research on Pacific walruses to be revisited to increase sample size and improve the interpretations of analyses (e.g., stable isotope, trace element, and hormone analyses) carried out on bones of unknown-sex walruses (e.g., Charapata et al 2018 ; Clark et al 2019 ).…”

Stable Isotope Studies of North American Arctic Populations: A Review

“…These differences in baseline values are passed on to upper trophic level consumers: for example, caribou are greater consumers of lichens than other herbivorous species and as a result, tend to have higher δ 13 C values than other local taxa (Britton, 2010;Drucker, Hobson, Ouellet, & Courtois, 2010;Guiry, Noël, Tourigny, & Grimes, 2012;Harris et al, 2019). Marine organisms have considerably higher bone collagen δ 13 C values ranging from -17‰ for some seabirds to between -12‰ and -15‰ for marine mammals (Britton et al, 2013;Coltrain, Hayes, & O'Rourke, 2004;Clarke, Horstmann, de Vernal, Jensen, & Misarti, 2019;Guiry et al, 2012;Harris et al, 2019;McManus-Fry, Knecht, Dobney, Richards, & Britton, 2018;Nelson, Lynnerup, & Arneborg, 2012a;Szpak, Buckley, Darwent, & Richards, 2017;Szpak et al, 2019). Additional variation in δ 13 C values is also observed between nearshore/benthic areas and the pelagic zone (Sherwood & Rose, 2005), but the degree of benthicpelagic coupling varies across the Arctic Ocean and neighbouring seas (Feder, Iken, Blanchard, Jewett, & Schonberg, 2011;Grebmeier, Cooper, Feder, & Sirenko, 2006).…”