Pacific Walrus (Odobenus rosmarus divergens) Resource Selection in the Northern Bering Sea

et al. (2014) Pacific Walrus (Odobenus rosmarus divergens) Resource Selection in the Northern Bering Sea. PLoS ONE 9(4): e93035. doi:10.1371/journal.pone.0093035 Pacific Walrus (Odobenus rosmarus divergens ) Resource Selection in the Northern Bering Sea Chadwick V. Jay 0 Jacqueline M. Grebmeier 0 Anthony S. Fischbach 0 Trent L. McDonald 0 Lee W. Cooper 0 Fawn Hornsby 0 Philippe Archambault, Universite du Quebec a` Rimouski, Canada 0 1 Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska, United States of America , 2 Chesapeake Biological Laboratory , University of Maryland Center for Environmental Science , Solomons, Maryland , United States of America, 3 Western EcoSystems Technology, Inc. , Laramie, Wyoming , United States of America The Pacific walrus is a large benthivore with an annual range extending across the continental shelves of the Bering and Chukchi Seas. We used a discrete choice model to estimate site selection by adult radio-tagged walruses relative to the availability of the caloric biomass of benthic infauna and sea ice concentration in a prominent walrus wintering area in the northern Bering Sea (St. Lawrence Island polynya) in 2006, 2008, and 2009. At least 60% of the total caloric biomass of dominant macroinfauna in the study area was composed of members of the bivalve families Nuculidae, Tellinidae, and Nuculanidae. Model estimates indicated walrus site selection was related most strongly to tellinid bivalve caloric biomass distribution and that walruses selected lower ice concentrations from the mostly high ice concentrations that were available to them (quartiles: 76%, 93%, and 99%). Areas with high average predicted walrus site selection generally coincided with areas of high organic carbon input identified in other studies. Projected decreases in sea ice in the St. Lawrence Island polynya and the potential for a concomitant decline of bivalves in the region could result in a northward shift in the wintering grounds of walruses in the northern Bering Sea. - Funding: Financial support was provided to PIs Grebmeier and Cooper through the U.S. National Science Foundation (ARC-082290) and the North Pacific Research Board (BSIERP Project B67). Walrus tagging data in 2006 was obtained from a separate project with financial support from U.S. Fish & Wildlife Service, U.S. Geological Survey, and North Pacific Research Board (NPRB #632). Tagging data in 2008 and 2009 was obtained specifically for this project with ship and aerial support from the U.S. National Science Foundation and financial support from the U.S. Geological Survey, Ecosystems Mission Area, Wildlife Program, and North Pacific Research Board (NPRB #B67). This work was assigned the following project numbers: BEST-BSIERP Bering Sea Project publication number 136 and NPRB publication number 480. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: TLMC & FH are employees of Western EcoSystems Technology (WEST), Inc., contracted by USGS to assist with analysis of the walrus telemetry data. There are no patents, products in development or marketed products to declare. This does not alter the authors adherence to all the PLOS ONE policies on sharing data and materials. Knowledge of how a population selects resources enables a better understanding of how changes in the availability of those resources may affect the populations distribution and abundance. Evidence of selection is the use of a resource at a level that is disproportionate to the resources availability. Many factors can contribute to selection, including competition, predator density, prey density and quality, and spatial patterns of habitat [1]. Arctic marine mammals are experiencing substantial changes to their sea ice habitat. The potential negative effects of sea ice loss to marine mammals have been recognized [2,3,4], including recent observations of changes in Pacific walrus areas of use in response to sparse sea ice [5] and projections of worsening sea ice conditions in coming decades [6]. Walruses require a substrate to rest upon between foraging trips, and when possible, adult females and young use sea ice throughout their seasonal ranges [7]. However, the persistence and extent of the sea ice habitat of the Pacific walrus is undergoing rapid change from climate warming. Although the loss of sea ice habitat is most acute in summer and fall in the Chukchi Sea [6], the southern boundary of sea ice in the Bering sea is projected to shift northward during the 21st century [8,9]. Heterogeneity or patchiness of resources occurs over multiple temporal and spatial scales [10,11,12,13] and an animals selection of resources can be thought of as a hierarchical process [14,15]. Selection hierarchy can be defined wherein a species first-order selection is the choice of geographic range, second-order selection is the choice of home ranges within the geographic range, thirdorder selection is the choice of habitat components within a home range, and fourth-order selection is the choice of specific resources within a habitat component [15]. The geographic range of the Pacific walrus (Odobenus rosmarus divergens) (i.e., first-order selection) occurs over the continental shelf, mainly within the Chukchi Sea from the eastern East Siberian Sea to western Beaufort Sea and the Bering Sea from Kamchatka to Bristol Bay [16]. A very small population of walruses occupying the Laptev Sea, and separated from the Chukchi Sea walruses by the East Siberian Sea, was recently assigned to the Pacific subspecies (O. r. divergens) [17]. Pacific walruses forage on the seafloor, primarily for infaunal invertebrates, and the continental shelves of the Chukchi and Bering Seas provide extensive areas with high benthic biomass [18,19,20] and shallow waters where diving to the seafloor is energetically feasible [21]. Walruses consume a wide range of organisms, from small crustaceans to seals [7], but their predominant prey in the Chukchi and Bering Seas are bivalves, gastropods, and polychaetes [22]. The seasonal home range of the Pacific walrus (i.e., secondorder selection) is delimited by the continental shelves of the Chukchi and Bering Seas in summer and the Bering Sea in winter. In winter, walruses form three main breeding concentrations: a small concentration in the western Bering Sea (Anadyr Gulf in Russia), a moderate concentration in the southeastern Bering Sea (from Nunivak and the Pribilof Islands to Bristol Bay), and a large concentration in the northern Bering Sea [7]. The northern Bering Sea walrus concentration occurs in the region of the St. Lawrence Island polynya (cf. St. Lawrence Island Polynya, South in Stringer and Groves [23]). The St. Lawrence Island winter polynya typically extends ,25 km southward of the island or further depending upon local winds [24]. Hydrographic studies indicate that locally high nutrient concentrati (...truncated)