The State of Circumpolar Walrus Populations

49
may cause individuals or groups to challenge ships, which could lead to injury or possibly
mortality if they are struck, trapped by ice, or entrained by propeller suction. In many areas,
walruses will have limited experience with ship passage, particularly in winter. Whether this
unfamiliarity will make them more or less apt to avoid ships is unknown. Marine mammal
observations from Arctic shipping as part of environmental impact assessment and monitoring
may provide the data needed to properly assess this threat.
Pollution
The effects of chemical contaminants on walruses are largely unknown because they are large,
isolated, and difficult to study experimentally (e.g., Wagemann and Stewart 1994; de March et al.
1998; Fisk et al. (ed.) 2003). Because walruses excavate much of their food from the bottom
sediment, they can accumulate naturally occurring cadmium and lead in their tissues at elevated
concentrations relative to other marine mammals in the same region (Outridge et al. 1997, 2002).
Levels of organochlorines in walrus tissues are generally low because they primarily feed low in
the food web (Norstrom and Muir 2000). Their levels are typically 4–10 times lower than those of
beluga whales (Delphinapterus leucas) from the same area, but with a similar pattern of residues.
The highest levels are found in individuals that are thought to eat seals, which accumulate these
contaminants in their blubber (Muir et al. 1995).
Pollution levels are too low to cause mortality or any impediment to reproduction in the Svalbard-
Franz Josef Land walrus population (Wolkers et al. 2006). The remains of a walrus from the
Kolguev Island area of the Barents Sea contained 7 to 70 times the total polychlorinated biphenyls
(ΣPCBs, 62 congeners; 1597.4 ng/g of lipid cf. 28.9-236 ng/g of lipid) found in fat tissue of Pacific
walruses from the Chukchi Sea (Semyonova et al. 2012). Further research is needed to assess the
risk from persistent organic pollutants to walruses in the Barents Sea.
The direct and indirect effects of petroleum on walruses have not been studied. Born et al. (1995)
believed that several aspects of the species’ ecology may make it vulnerable to oil pollution, in
particular, its gregariousness, which may spread oil from animal to animal, its preference for
coastal areas and loose pack ice where oil may be more likely to accumulate, and its reliance on
benthic molluscs which may accumulate petroleum hydrocarbons or succumb to the oil. Oil
fouling is unlikely to cause hypothermia due to their thick blubber but, like seals, walruses may
continue to use haulouts that have been fouled (Isaksen et al. 1998). This could lead to irritation
and damage to the eyes and skin and increased exposure to the deleterious effects of inhaling
aromatic hydrocarbons (neuronal damage). They would also be vulnerable to disturbance and
possibly stampedes during clean-up activities. Walrus populations may be most vulnerable to
harm from oil spills during the calving period, and calves may be the most vulnerable component