The walrus (Odobenus rosmarus) is a large pinniped marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. It is the only extant species in the family Odobenidae and genus Odobenus. This species is subdivided into two subspecies: the Atlantic walrus (O. r. rosmarus), which lives in the Atlantic Ocean, and the Pacific walrus (O. r. divergens). The Pacific walruses (Odobenus rosmarus divergens) are a conspicuous and important component of the Bristol Bay ecosystem and human social systems.
Adult walruses are characterised by prominent tusks and whiskers, and considerable bulk: adult males in the Pacific can weigh more than 2,000 kilograms (4,400 pounds) and, among pinnipeds, are exceeded in size only by the two species of elephant seals. Walruses live mostly in shallow waters above the continental shelves, spending significant amounts of their lives on the sea ice looking for benthic bivalve molluscs.
Physical Characteristics and Behavior
Walruses are strong bodied and have a very thick, tough hide that can be an inch thick. The most distinctive feature of walruses, both male and female, is their two ivory tusks, which are long upper canine teeth that grow throughout their life. Walruses also have hundreds of short, strong, highly sensitive whiskers that they use to search the seafloor for their food. Adult males, or bulls, are up 12 feet long and may weigh up to 2 tons; although females are smaller they can weigh more than 1 ton. Females begin breeding at 6-7 years of age and generally give birth every 2 years. Males are mature at 8-10 years of age, but generally cannot successfully compete against older, larger males for females until they are 15 years old. Walruses may live up to 40 years. Unlike the other Alaskan pinnipeds, walruses mate in the water during the winter months (January-March). However, delayed implantation of the fertilized egg is suspended from further development for several months, resulting in a total gestation period of 15 months, the longest of all the pinnipeds. Walrus calves are born on ice floes in late spring and weigh approximately 140 pounds.
Walruses are highly social animals that travel and rest in large groups. When sea ice is not available, walruses will commonly rest on rocks, sandy beaches, and even grassy hills. Resting on land allows walruses the opportunity to form large social haulouts that can contain tens of thousands of individuals.
Distribution and Migration
Pacific walruses range over the relatively shallow waters of the northern Bering and Chukchi seas, and are occasionally observed in the waters of the Eastern Siberian and Beaufort seas. In the Bering Sea, walruses are distributed from the Bering Strait to Bristol Bay in the east, and in the west their range extends as far south as the Kamchatka Peninsula. They also use island haulouts near St.
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The Pacific walrus population spends the winter on the Bering Sea pack ice before separating in the spring. Beginning in the spring, females with young migrate northward from the Bering Sea to the Chukchi Sea, often passively moving with the receding sea ice. Most adult males migrate to Bristol Bay where they rest on land haulouts between foraging bouts. By late fall, walruses begin their return migration southward from the Chukchi Sea to the Bering Sea, ahead of the advancing sea ice. The males that remained in Bristol Bay head north to meet the returning population in the waters near St. Lawrence Island.
Walrus Diet and Foraging Behavior
Walruses eat a wide variety of soft invertebrates found on or below the seafloor by using their sensitive whiskers to search for prey. Prey items include clams, snails, worms, sea cucumbers, and tunicates. Except for their tusks, walrus teeth are flat; as a result, walruses eat by sucking food into their mouths using the powerful suction created by pulling their piston-like tongue back quickly. Walruses suck out the soft parts of clams and snails leaving the shells to fall to the sea floor.
Walruses use their tusks to help pull themselves onto rocks or ice, to move ice to create an opening to dive through, and to fight off predators. The most common use of tusks is social. Females will threaten and jab each other for better position on haulouts and during the breeding season, male walruses will threaten and fight with their tusks to compete for females.
Typical Diet Composition
Walruses prefer molluscs - mainly bivalves such as clams. They also eat many other kinds of benthic invertebrates including worms, gastropods, cephalopods, crustaceans, sea cucumbers, and other soft-bodied animals. There are some rare but habitual seal-eating walruses. Their diet consists mainly of ringed and bearded seals. These are usually male walruses, recognizable because they are usually larger than other males, with powerful shoulder and chest muscles.
Foraging Habits
In the summer months, and during the southward migration in the fall, walruses spend most of their day foraging. They eat less on their northward migration in the spring. Food intake for mature male walruses dramatically decreases during the breeding season and probably for a shorter time for females in estrus. Walruses usually forage on the bottom within 80 m (262 ft.) of the surface. Because visibility is poor in deep and murky waters, walruses rely on their vibrissae to locate food. A walrus moves its snout along the bottom, rooting through the sediment and using its vibrissae to help detect prey. Soft-bodied invertebrates are usually not crushed or torn. Researchers have found numerous pebbles and small stones in the stomachs of walruses.
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Studying Walrus Diets in Bristol Bay
Bristol Bay is a biologically rich expanse of marine habitat over the continental shelf in the southeastern Bering Sea. Like most high-latitude ecosystems, it exhibits strong seasonal pulses of productivity that support complex food webs. Several upper trophic level predators including the Pacific walrus (Odobenus rosmarus divergens) inhabit Bristol Bay throughout the year. The walruses inhabiting Bristol Bay are an important traditional subsistence and cultural resource of area residents. Potential threats to walruses in Bristol Bay include human-induced disturbances at their haulouts and foraging areas, disruptions of prey resources by commercial trawl fishing operations, and contaminants released from prospective oil, gas, and mineral exploration and extraction activities proposed in the region. Yet, climate change and its effects on marine environments are potentially the greatest long term threat to the Bering Sea ecosystem.
Pacific walruses are typically benthic feeders, preying primarily upon bivalves, gastropods, crabs, and various worms. However, data regarding walrus diets in Bristol Bay are limited. Information comes from Traditional Ecological Knowledge of residents who hunt walruses and from four animals collected in southern Bristol Bay in February-April 1981. Elsewhere, diets of walruses are known to differ substantially across spatial scales comparable to these. Thus, previous understanding about the prey resources that support walruses in Bristol Bay at any time during the year has been sparse, and no information has been available on diets during summer and autumn.
Advancements in Diet Analysis: The Use of qPCR
Researchers have used quantitative polymerase chain reaction (qPCR) analysis of prey DNA in feces (scat samples) and gastrointestinal tracts to characterize the diets of walruses. Samples from feces and gastrointestinal tracts provide a representation of diet within the previous hours to less than a day due to rapid food passage times of walruses. The use of qPCR provides estimates of the relative contribution of different prey items in the diet and is superior to examination of stomach contents because the latter method is highly subject to biases associated with differential digestion times of prey species. In addition, walruses typically consume only soft tissues of their prey, leaving hard parts behind.
Dietary Diversity and Temporal Shifts in Bristol Bay
Diets were highly diverse at all locations, but with some variation in composition that may be related to the time of year that samples were collected (summer vs. autumn), or to spatial variability in the distribution of prey. Overall, polychaetes and tunicates had the highest frequencies of occurrence and relative abundances in 2014-15, but a major change in diet appears to have occurred by 2017-18. While some sample sizes were small, diets in these later years contrasted sharply, with a greater prevalence of sea cucumbers and mollusks, and reduced importance of decapods and fishes compared to the earlier years. Prey identified in scat samples from one collection site also contrasted sharply with those reported from the same location in 1981.
Diets of walruses in Bristol Bay were represented by a minimum of 36 genera or species (x ¯=9.10 ± 3.61 SD per sample; x ¯=22.4 ± 1.03 SE per site) detected in scat and GI tract samples over the years 2014-15 and 2017-18 combined. There was no significant difference between prey DNA in gastrointestinal tracts and DNA in scat samples from walruses at Hagemeister Island. Therefore, scat and gastrointestinal samples were lumped together for this location. Based on FO at all sites and years, benthic worms (primarily Lumbrineris and Priapulus spp.), and tunicates (Styela rustica) were among the most common prey in the diets of Bristol Bay walruses. Sea cucumbers were more common and fishes less common in the diets of walruses at Round Island in 2017-18 compared to Round Island in 2015 or any other location from 2014-15. Bivalves also generally appeared more often in samples from Round Island in 2017-18 compared to samples from there and other locations in 2014-15. Hermit crabs (Pagurus spp.) were the most common decapod consumed, but these were not seen in the diet of walruses at Round Island in 2017-18. Rather, the moon snail (Euspira pallida), horsehair crab (Erimacrus isenbeckii) and a variety of bivalves were widely detected in their diet in these later years.
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Conservation Status and Threats
Pacific walrus population size has varied substantially in the last 150 years, it increased between 1960 and 1980 and may have reached the carrying capacity of the marine environment. Potential threats to the Pacific walrus population include a reduction in sea ice, predation by killer whales and polar bears, and if traffic of large vessels increases there could be threats from pollution, ship strikes, and entanglement in trash. Specifically, recent changes in the thickness and duration of sea ice may affect walrus distribution and feeding opportunities as less ice is available. Walruses use sea ice as a stable, yet drifting, platform for resting between foraging trips, calving, and traveling. With less sea ice, walruses will likely spend more time on shore haulouts along the Russian and Alaskan coasts where foraging trips may be more limited to foraging grounds nearby.
Currently, two of the three walrus subspecies are listed as "least-concern" by the IUCN, while the third is "data deficient". The Pacific walrus is not listed as "depleted" according to the Marine Mammal Protection Act nor as "threatened" or "endangered" under the Endangered Species Act. The Russian Atlantic and Laptev Sea populations are classified as Category 2 (decreasing) and Category 3 (rare) in the Russian Red Book. Global trade in walrus ivory is restricted according to a CITES Appendix 3 listing.
Cultural Significance and Management
Walruses are an essential nutritional and cultural marine resource used by Alaska Natives throughout western and northern Alaska. Several thousand walruses are legally harvested in Alaska and Russia every year. The meat, blubber, skin, and organs provide a healthy and rich source of food, the hides can be processed into rope or used to cover boats, and the stomach lining is used to make traditional drums for Eskimo dances. Pacific walrus have appealed to and attracted much attention in literature, folklore, and artwork.
Management issues are jointly addressed by the USFWS and the Native co-management organization the Eskimo Walrus Commission (EWC). The EWC was established in 1978 to represent coastal walrus hunting communities in Alaska. Walrus hunts are regulated by resource managers in Russia, the United States, Canada, and Greenland, and by representatives of the respective hunting communities.