Walruses in a Time of Climate Change
K. M. Kovacs1, P. Lemons2, J. G. MacCracken2, C. Lydersen1
1Norwegian Polar Institute, Tromsø, Norway
2U.S. Fish and Wildlife Service, Anchorage, AK, USA
November 25, 2015
- Sea ice deterioration due to global climate change is thought to be the most pervasive threat to ice-associated marine mammals in the Arctic, including walruses.
- Current population trajectories of some stocks of walruses are also influenced greatly by hunting levels, including those of the distant past in some areas, which results in a mosaic that includes an exponential increase in walruses in the region (Svalbard) that has experienced the fastest and most profound regional sea ice losses over recent decades.
- Habitat loss will be exacerbated for walruses by additional climate-change related factors such as ocean acidification, increased shipping and increasing development in the North, including oil and gas extraction, as well as increased disease and contaminant risks.
Concern has been raised regarding the impacts of climate change on the conservation status of ice-affiliated marine mammals in the Arctic since the first suggestions that the planet's climate was warming. It is generally thought that global climate change is already the most pervasive threat to arctic pinnipeds (seals and walruses) (Hoffman 1995; Tynan and DeMaster 1997; Laidre et al. 2008, 2015; Huntington 2009; Kovacs et al. 2011, 2012; Jay et al. 2011; MacCracken 2012). But, there is significant regional variation in the rates of change of key environmental features within the Arctic, including the extent and seasonal duration of sea ice. Further, mammalian population trajectories are influenced by a host of factors, including a species' adaptive capacity (evolutionary potential, dispersal ability, genetic diversity, breadth of feeding niche, tolerance of various environmental conditions, behavioral plasticity, etc.; see Gilg et al. 2012 for a summary) and in the case of many marine mammals, human harvest levels past and present.
Walruses make an interesting case study in this time of rapid climate change. They are broadly distributed in the Arctic but occur as two distinct subspecies within disparate ranges. Odobenus rosmarus rosmarus occurs in the North Atlantic Region (including the Barents Sea and adjacent seas to the east) while O. r. divergence occurs in the North Pacific region (Chukchi, Bering and western Beaufort seas) westward through to the Laptev Sea (Lindqvist et al. 2008). Both subspecies are benthic (bottom) feeders whose diet is dominated by bottom-dwelling invertebrates (e.g., Sheffield and Grebmeier 2009; Skoglund et al. 2010), so their foraging areas are located in shallow waters.
Walruses give birth on sea ice in the late spring and mate along ice edges in the drifting pack-ice during the winter. They also use ice extensively as a haul-out platform throughout much or all of the year, depending on sex, season and general availability of sea ice in areas that afford feeding opportunities. Sea ice also provides shelter from storms and from some predators. The rates of change in sea ice occurring in the ranges of the two subspecies are, in general terms, very different, with the rate of seasonal-ice-cover losses in the Atlantic region being faster than the rate in the Pacific region (Laidre et al. 2015). But, straightforward predictions for walrus trends on the basis of this important birthing, breeding and haul-out habitat do not work particularly well for assessing current or near-future states because varied hunting regimes in the distant past (100s of years ago), recent past (decades) and present, across the range of walruses heavily affect the population trajectories (Fig. 9.1). In addition geographic features - including the location of current summer sea ice margins compared to possible (shallow, benthic) feeding areas and also possible locations for terrestrial (summer) haul-out in comparison to feeding grounds - influence the energetics of accessing food differently on a regional basis.
Pacific Walrus Population
The latest research indicates that the Pacific walrus population in the Bering and Chukchi seas likely declined throughout the period from about 1980 to 2000 (MacCracken et al. 2014, Taylor and Udevitz 2015). The weight of evidence suggests that this population had actually approached the carrying capacity of their environment in the late 1970s - early 1980s, due to restrictions on subsistence harvests (Fay et al. 1989, 1997, Hills and Gilbert 1994). But, population models suggest a subsequent decline of approximately 50% (Taylor and Udevitz 2015), likely due to changes in vital rates associated with a population at or near carrying capacity. This decline has likely been exacerbated by declines in sea ice, which are associated with global climate change that are reducing the carrying capacity of the environment for walruses (Garlich-Miller et al. 2011, Taylor and Udevitz 2015). Hypothesized mechanisms include (1) the retreat of sea ice to a position over the deep Arctic Ocean basin, forcing walruses to use land-based haulouts where trampling events result in increased mortality to young animals (Jay and Fischbach 2008, Udevitz et al. 2012) and (2) the decline in sea ice reducing walruses' access to prey, which could affect adult female body condition, ultimately reducing calf survival and recruitment (Jay et al. 2011, Taylor and Udevitz 2015). While the use of land-based haulout areas is not novel for walruses, females with dependent young typically utilize sea ice for hauling out (Fay 1982), which allows them to avoid particularly large land-based haulouts where crowding and trampling events can result in large mortality events of dependent young (Fischbach et al. 2009). Unregulated subsistence harvests in the United States and subsistence and commercial harvests in the Russian Federation (commercial harvests ended in 1990) have contributed to declines of Pacific walruses in the past (Fay et al. 1989, Fay and Bowlby 1994). However, since 1992, harvest of this subspecies has been limited to subsistence takes by communities in Alaska and Chukotka (Garlich-Miller et al. 2006) and is currently not considered a threat to the population (USFWS 2011). However, a major remaining concern is the effects of declining sea ice on future energetics of females and young animals that must now make feeding trips from coastal haulouts to areas of high prey abundance (180 km one-way), rather than utilizing nearby ice edges for resting as they did in the past. Current research will hopefully soon shed light on this potential stressor. The status of the Pacific walrus stock in the Laptev Sea is currently unknown (Laidre et al. 2015).
Atlantic Walrus Population
Atlantic walrus abundance, similar to the situation in the Pacific, has largely been dictated in the past by hunting intensities through time (Stewart et al. 2014a, 2014b). Although some stock boundaries are still uncertain, seven eastern Canada/west Greenland stocks are generally recognized in addition to the east Greenland stock. The subpopulation that occupies the Barents Sea and adjacent areas to the south and east in Russia, are also included within the Atlantic subspecies range. The higher degree of population sub-structure in the Atlantic subspecies is likely a product of the extensive archipelago systems, continuous versus discontinuous regions of sea ice enhancing, or limiting connectivity, and a few deep water areas that promote isolation among groups.
Historically, walrus hunting increased as bowhead (Balaena mysticetus) whaling declined in both the northwest and northeast Atlantic. Accessible stocks were heavily depleted before protective measures came into place in the early- (Canada) and mid-1900s (Norway and Russia), and, in the case of Greenland, much more recently, with quotas being established in the early 2000s. Stewart et al. (2014b), Witting and Born (2014) and Gjertz et al. (1998) have explored Canadian, Greenlandic and Barents Sea hunting histories, respectively, within the limits of available data. All conclude that landed catches were far too high to be sustainable and that depletions certainly occurred throughout most of the range of the Atlantic subspecies, even in isolated areas with heavy ice cover such as northeast Greenland and the Frans Josef Land Archipelago. A recent example of a significant reduction is the decline that took place in the west Greenland/Baffin Bay stock in the period 1900-1960, when this stock was reduced by 80%. But, management interventions (i.e., controlling human harvesting; see Wiig et al. 2014) have resulted in signs of recovery in this and some other previously depleted North Atlantic stocks (Witting and Born 2014). The total abundance of Atlantic walruses is not known, but it is likely that they number in excess of 25,000 animals when all of the various stock numbers are combined. This number is not markedly different from the estimates that have been made for this subspecies over several decades, though the dynamics of individual stocks have shown varied trends and some areas have never been surveyed. Protective measures recently put in place in Greenland are likely to go a long way towards ensuring more stable population numbers within the Atlantic subspecies.
A particularly noteworthy case with respect to trying to detect climate change impacts on arctic pinniped populations, among other stressors, is the situation for walruses in Svalbard, Norway. Svalbard is an Arctic hot-spot that is experiencing dramatic sea ice declines and warming ocean and air temperatures (Beszczynska-Möller et al. 2012; Nordli et al. 2014; Laidre et al. 2015), and yet walrus numbers in the archipelago are increasing exponentially (Lydersen et al. 2008; Kovacs et al. 2014). This situation arises because of the extreme historical overexploitation of the walruses in this area that took place over several hundred years up until the 1950s. When walruses did finally become protected in Svalbard in 1952, there were at best a few hundred animals occupying a few sites. But, after 60 years of complete protection from hunting, with some special no-go reserve areas, recovery is taking place. More females with calves are documented during surveys and historically used sites are being reoccupied as walruses continue to expand through the archipelago. These changes are occurring despite the fact that overall carrying capacity of the region for walruses is almost certainly declining because of sea ice declines. Studies are currently taking place to determine whether seasonal movement patterns are being affected by the changing sea ice conditions. This includes the use of remote cameras to study occupancy patterns at several haul-out sites, exploring the potential impacts of various sources of disturbance. For instance, the impact of rapidly expanding marine tourism activities is being investigated, via assessments at visited and non-visited sites.
Potential Threats to Walruses
Because walruses will make use of terrestrial sites for haul-out, extinction due to climate change impacts on sea ice is unlikely to occur for this species. But, it is certain that land-based sites alone will not support the same number of walruses that the mixed seasonal use of sea ice and land has permitted in the past (Jay et al. 2012; Kovacs et al. 2012). Additionally, documented indeclines in the northern Bering Sea among dominant clam populations that are critical prey for walruses, associated with reductions in sea ice declines (e.g., Grebmeier et al. 2010), provide cause for concern; such ecosystem changes are clearly important for walruses and other animals. It is also expected that other climate-change related factors such as acidification, increased shipping, increasing development in the North including oil and gas extraction, disease and contaminant risks, will all represent increasing threats to walruses in the future (e.g., Kovacs et al. 2012; MacCracken 2012, MacCracken et al. 2013).
Ocean acidification. Global warming has already led to increased acidification (lowered pH) of the world's oceans, particularly in the Arctic (AMAP 2013; Mathis 2011). Ocean acidification reduces the saturation state of carbonate ions in the water, which can affect the growth, development and survival of calcifying invertebrates that are the major prey of walruses. However, the response of species to lowered pH is highly variable depending on the species, life stage, duration and level of exposure, adaptive capacity, and evolutionary history. To date, there is no evidence that ocean acidification is affecting walrus prey. It appears that carbonate saturation states are still adequate, though tipping points might be reached by as early as 2020 in the Arctic Ocean (Freely et al. 2009). This could have negative implications for bivalve populations, on which walruses feed.
Commercial shipping. Commercial shipping is increasing across the Arctic, especially through the Northern Sea Route as sea ice reductions have taken place. Associated with this increased activity is increasing noise and concerns about shipping accidents that might release oil or other contaminants. Most of this traffic within the range of the Pacific walrus has been confined to Russian waters. While no large accidents have been reported, oiled wildlife was found 2012 in the vicinity of St. Lawrence Island, albeit with no identified source. In the North Atlantic, fisheries are thriving, as is the tourist industry, adding to the movement of goods. Ships striking walruses appears to be a minor concern as they are able to avoid large vessels, but the disruption of subsistence hunts has been reported. The International Maritime Organization (IMO) recently adopted the voluntary Polar Code, which provides guidelines for safe operations in the Arctic. In addition, several groups, including the Arctic Council's PAME (Protection of the Arctic Marine Environment) team, are working to identify ecologically significant areas for incorporation into the IMO process and also working to identify sensitive areas for marine protected area planning (e.g. PAME 2015).
Oil and gas exploration/development. Oil and gas exploration in the Chukchi Sea in the range of the Pacific walrus population had a burst of activity starting around 2008, with numerous seismic surveys conducted in the US and Russia. This was followed by exploratory drilling, which occurred in 2012 and 2015 in the US. However, by 2015 several companies with leases in US waters had indefinitely suspended exploratory operations, eliminating this potential stressor for the foreseeable future for this subpopulation. In the North Atlantic, seismic surveys and plans for northward expansion of oil platforms continue. In waters of the Pechora and Kara seas development has already taken place in key walrus habitats in the southern parts of their range (Lydersen et al. 2012), with little or no impact assessment work related to marine mammals preceding development. These activities are deemed to be "highly hazardous" to walruses in the southeastern Barents Sea (Boltunov et al. 2010) and are likely to be a threat to this benthic feeding pinniped throughout its range if development is not well-managed.
Disease and contaminants. Increased disease risks associated with climate change have no direct elements that are specific to walruses. Instead, the risk is associated with the impact increased contact with temperate species might have on all of the ice-affiliated marine mammals that have lived in cold environments, with few disease vectors during recent evolutionary time frames (Altizer et al. 2013). Similarly, contaminants risks are likely to be associated with increased risks due to multiple stressors, rather than the actual contaminant burdens in walruses, given their generally low trophic feeding position in food webs (Robarts et al. 2009). However, possible trends toward increased seal predation by walruses (see Seymour et al. 2014) could dramatically alter the situation regarding contaminants exposure (Wolkers et al. 2006).
Walrus Harvesting and Management
Hunting has been the major source of mortality driving walrus population dynamics and distribution for the Atlantic and Pacific subspecies in the past (Garlich-Miller et al. 2006, Stewart et al. 2014b). The US Fish and Wildlife Service predicted a few years ago that sea ice losses would eventually result in a Pacific walrus population decline and that the subsistence harvests of some 4,000-5,000 animals per year would become unsustainable. However, sea ice losses/conditions have restricted the ability of Alaskan hunters to harvest walruses, due to a variety of factors. Consequently, the US walrus harvest has declined to less than 1,400 animals per year in 2013 and 2014. Canadian hunts are on the order of only a few hundred animals per year over the last decade due to declining community dependence on this species. Greenlandic hunters have taken a few hundred walruses annually (range 121-404 in the last decade); quotas have recently been established in Greenland to attempt to achieve sustainable harvest levels (Wiig et al. 2014). The Russian harvest of Pacific walruses is the largest hunt currently, with approximately 1,800 and 1,500 animals harvested in 2013 and 2014, respectively (see Shadbolt et al. 2014 for summary statistics and sources). Although this level of harvesting is thought to be sustainable currently, there are concerns that if climate change induced alterations to the environment/ecosystem continue that this level of harvesting could pose a threat to Pacific walruses (USFWS 2011).
Accurate reporting of harvests, including struck and lost rates, as well as updated population estimates are essential tools for proper management of walruses given the additional risks faced by this species at this time related to climate change and concomitant ecosystem changes. Accurate assessment of risk is also dependent on an increased understanding of the effects that climate change is actually having on walruses. After all, climate change-driven alteration of the environment, caused by high levels of greenhouse gas emissions, is thought to be the ultimate driver of changes that will determine the future abundance of walruses. Mitigation via protection of terrestrial haul-out sites and other stressors are also likely going to be important conservation tools within the adaptive management system that will be required to sustain viable populations of this charismatic arctic endemic species.
Altizer, S., R. S. Ostfeld, P. T. J. Johnson, S. Kutz, and C. D. Harvell, 2013: Climate change and infectious diseases: from evidence to a predictive framework. Science, 341, 514-9.
AMAP, 2013: Arctic Ocean Acidification Assessment: Summary for Policy Makers. AMAP, Oslo, Norway.
Beszczynska-Möller, A., E. Fahrbach, U. Schauer, and E. Hansen, 2012: Variability in Atlantic water temperature and transport at the entrance to the Arctic Ocean, 1997-2010. ICES J. Mar. Sci., doi:10.1093/icesjms/fss056.
Boltunov, A. N., S. E. Belikov, Y.A. Gorbunov, D. T. Menis, and V. S. Semenova, 2010: The Atlantic walrus of the Southeastern Barents Sea and adjacent regions: review of the present-day status. WWF Russia and the Marine Mammal Council, Moscow.
Fay, F. H., 1982: Ecology and Biology of the Pacific Walrus, Odobenus rosmarus divergens Illiger. North American Fauna, 74, 1-279.
Fay, F. H. and C. E. Bowlby, 1994: The Harvest of Pacific Walrus, 1931-1989. U. S. Fish and Wildlife Service Technical Report MMM 94-2.
Fay, F. H., B. P. Kelly and J. L. Sease, 1989: Managing the exploitation of Pacific walruses: A tragedy of delayed response and poor communication. Mar. Mamm. Sci. 5, 1-16.
Fay, F. H., L. L. Eberhardt, B. P. Kelly, J. J. Burns, and L. T. Quakenbush, 1997: Status of the Pacific walrus population, 1950-1989. Mar. Mamm. Sci., 13, 537-565.
Fischbach, A. S., D. H. Monson, and C. V. Jay. 2009: Enumeration of Pacific walrus carcasses on beaches of the Chukchi Sea in Alaska following a mortality event, September 2009. U. S. Geological Survey Open-File Report 2009-1291.
Freely, F. A., S. C. Doney, and S. R. Cooley, 2009: Ocean acidification: present conditions and future changes in a high-CO2 world. Oceanography, 22, 36-47.
Garlich-Miller, J. L., L. T. Quakenbush, and J. F. Bromaghin, 2006: Trends in age structure and productivity of Pacific walruses harvested in the Bering Strait region of Alaska, 1952-2002. Mar. Mamm. Sci., 22, 880-896.
Garlich-Miller, J. L., J. G. MacCracken, J. Snyder, J. M. Wilder, M. Myers, E. Lance, and A. Matz, 2011: Status review of the Pacific walrus (Odobenus rosmarus divergens). U.S. Fish and Wildlife Service, Anchorage, Alaska, USA.
Gjertz, I., Ø. Wiig, and N. A. Øritsland, 1998: Backcalculation of original population size for walruses Odobenus rosmarus in Franz Josef Land. Wildl. Biol., 4, 223-229.
Gilg, O., K. M. Kovacs, J. Aars, J. Fort, G. Gauthier, D. Gramillet, R. A. Ims, H. Meltofte, J. Moreau, E. Post, N. M. Schmidt, G. Yannic, and L. Bollache, 2012: Climate change and the ecology and evolution of Arctic vertebrates. Ann. N. Y. Acad. Sci., 1249, 166-190.
Grebmeier, J. M., S. E. Moore, J. E. Overland, E. E. Frey, and R. Gradinger, 2010: Biological response to recent Pacific Arctic sea ice retreats. EOS Trans. Am. Geophys. Union, 91, No. 18, 4.
Hills, S., and J. R. Gilbert, 1994: Detecting Pacific walrus population trends with aerial surveys. Trans. 59th N. Am. Wild. Natur. Resources Conf., 59, 201-210.
Hoffman, R. J., 1995: The changing focus of marine mammal conservation. TREE, 10: 462-465.
Huntington, H.P., 2009: A preliminary assessment of threats to arctic marine mammals and their conservation in the coming decades. Marine Policy, 33, 77-82.
Jay, C. V., and A. S. Fischbach, 2008: Pacific walruses response to Arctic sea ice losses. U. S. Geological Survey Fact Sheet 2008-3041.
Jay, C. V., B. G. Marcot, and D. C. Douglas, 2011: Projected status of the Pacific walrus (Odobenus rosmarus divergens) in the twenty-first century. Polar Biol., 34, 1065-1084.
Jay, C. V., A. S. Fischbach, and A. A. Kochnev, 2012: Walrus areas of use in the Chukchi Sea during sparse sea ice cover. Mar. Ecol. Prog. Ser., 468, 1-13.
Kovacs, K. M., S. Moore, J. E. Overland, and C. Lydersen, 2011: Impacts of changing sea-ice conditions on Arctic marine mammals. Mar. Biodiv., 41, 181-194.
Kovacs, K. M., A. Aguilar, D. Aurioles, V. Burkanov, C. Campagna, N. Gales, T. Gelatt, S. Goldsworthy, S. J. Goodman, G. J. G. Hofmeyr, T. Härkönen, L. Lowry, C. Lydersen, J. Schipper, T. Sipilä, C. Southwell, S. Stuart, D. Thompson, and F. Trillmich, 2012: Global threats to pinnipeds. Mar. Mamm. Sci., 28, 414-436.
Kovacs, K. M., J. Aars, and C. Lydersen, 2014: Walruses recovering after 60+ years of protection at Svalbard, Norway. Polar Res., 33, 26034.
Laidre, K. L., I. Stirling, L. Lowry, Ø. Wiig, M. P. Heide-Jørgensen, and S. Ferguson, 2008: Quantifying the sensitivity of arctic marine mammals to climate-induced habitat change. Ecol. Appl. 18, S97-S125.
Laidre, K. L., H. Stern, K. M. Kovacs, L. Lowry, S. E. Moore, E. V. Regehr, S. H. Ferguson, Ø. Wiig, P. Boveng, R. P. Angliss, E. W. Born, D. Litovka, L. Quakenbush, C. Lydersen, D. Vongraven, and F. Ugarte, 2015: Arctic marine mammal population status, sea ice habitat loss, and conservation recommendations for the 21st century. Conserv. Biol., 29, 724-737.
Lindqvist, C., L. Bachmann, L. W. Andersen, E. W. Born, U. Arnason, K. M. Kovacs, C. Lydersen, A. V. Abramov, and Ø. Wiig, 2008: The Laptev Sea walrus Odobenus rosmarus laptevi: an enigma revisited. Zool. Scripta, 38, 113-127.
Lydersen, C., J. Aars, and K. M. Kovacs, 2008: Estimating the number of walruses in Svalbard from aerial surveys and behavioural data from satellite telemetry. Arctic, 61, 119-128.
Lydersen, C., V. I. Chernook, D. M. Glazov, I. S. Trukhanova, and K. M. Kovacs, 2012: Aerial survey of Atlantic walruses (Odobenus rosmarus rosmarus) in the Pechora Sea, August 2011. Polar Biol. 35, 1555-1562.
MacCracken, J. G., 2012: Pacific walrus and climate change: Observation and predictions. Ecol. Evol., 2, 2072-2090.
MacCracken, J. G., J. Garlich-Miller, J. Snyder and R. Meehan. 2013. Bayesian belief network models for species assessments: an example with the Pacific walrus. Wildl. Soc. Bull., 37, 226-235.
MacCracken, J. G., P. R. Lemons III, J. L. Garlich-Miller, and J. A. Snyder, 2014: An index of optimum sustainable population for the Pacific walrus. Ecol. Indicators, 43, 36-43.
Mathis, J. T., 2011: The extent and controls on ocean acidification in the western Arctic Ocean and adjacent continental shelf seas. In Arctic Report Card: Update for 2011, http://www.arctic.noaa.gov/report11/ocean_acidification.html.
Nordli, Ø., R. Przybylak, A. E. J. Ogilvie, and K. Isaksen. 2014. Long-term temperature trends and variability on Spitsbergen: the extended Svalbard Airport temperature series, 1898-2012. Polar Res., 33, 21349.
PAME, 2015: Arctic Council Arctic Marine Strategic Plan 2015-2015. PAME International Secretariat, Akureyri, Iceland.
Robarts, M. D., J. J. Burns, C. L. Meek, and A. Watson, 2009: Limitations of an optimum sustainable population or potential biological removal approach for conserving marine mammals: Pacific walrus case study. J. Environ. Manage., 91, 57-66.
Seymour, J., L. Horstmann-Dehn, and M. J. Wooler, 2014: Proportion of higher trophic-level prey in the diet of Pacific walruses (Odobenus rosmarus divergens). Polar Biol., 37, 941-952.
Shadbolt, T., T. Arnbom, and E. W. T. Cooper, 2014: Hauling out: international trade and management of walrus. TRAFFIC and WWF-Canada, Vancouver, B. C.
Sheffield, G., and J. M. Grebmeier, 2009: Pacific walrus (Odobenus rosmarus divergens): differential prey digestion and diet. Mar. Mamm. Sci., 25, 761-777.
Skoglund, E. G., C. Lydersen, O. Grahl-Nielsen, T. Haug, and K. M. Kovacs, 2010: Fatty acid composition of the blubber and dermis of adult male Atlantic walrus (Odobenus rosmarus rosmarus) in Svalbard, and their potential prey. Mar. Biol. Res., 6, 239-250.
Stewart, D. B., J. W. Higdon, R. R. Reeves, and R. E. A. Stewart, 2014b: A catch history for Atlantic walruses (Odobenus rosmarus rosmarus) in the eastern Canadian Arctic. NAMMCO Sci. Publ., 9, 219-314.
Stewart, R. E. A., K. M. Kovacs, and M. Acquarone, 2014a: Walrus of the North Atlantic. NAMMCO Sci. Publ., 9, 7-12.
Taylor, R. L., and M. S. Udevitz, 2015: Demography of the Pacific walrus (Odobenus rosmarus divergens): 1974-2006. Mar. Mamm. Sci., 31, 231-254.
Tynan, C. T., and D. P. DeMaster, 1997: Observation sand predictions of Arctic climate change: potential effects on marine mammals. Arctic, 50, 308-322.
Udevitz, M. S., R. L., Taylor, J. L. Garlich-Miller, L .T. Quakenbush, and J. A. Snyder, 2012: Potential population level effects of increased haulout-related mortality of Pacific walrus calves. Polar Biol., 36, 291-298.
USFWS (United States Fish and Wildlife Service), 2011: Endangered and threatened wildlife and plants; 12-month finding on a petition to list the Pacific walrus as endangered or threatened. Federal Register (U. S. A.), 76, 7634-7679.
Wiig, Ø, E. W. Born, and R. E. A. Stewart, 2014: Management of Atlantic walruses (Odobenus rosmarus rosmarus) in the arctic Atlantic. NAMMCO Sci. Publ., 9, 315-344.
Witting, L., and E. W. Born, 2014: Population dynamics of walruses in Greenland. NAMMCO Sci. Publ., 9, 191-218.
Wolkers, H., B. van Bavel, I. Ericson, E. Skoglund, K. M. Kovacs, and C. Lydersen, 2006: Congener-specific accumulation and patterns of chlorinated and brominated contaminants in adult male walruses from Svalbard, Norway: indications for individual-specific prey selection. Sci. Total Environ., 370, 70-79.