Oil spills are the greatest anthropogenic threat to Sea Otter (Geraci and Williams 1990). Sea Otters become hypothermic when oiled because oiled Sea Otter fur loses its insulative property and Sea Otters have no blubber layer, oil can be ingested while grooming, leading to gastrointestinal disorders, other ailments and death and volatile components of oil inhaled by Sea Otters can cause lung damage. Estimates of sea otter mortality following the Exxon Valdez spill in Prince William Sound ranged from 2,650 (Garrott et al. 1993) to 3,905 (DeGange et al. 1994).

Significant numbers of Sea Otters drowned in gill and trammel nets in California from the mid-1970s to the early 1980s (Estes 1990). Recent population declines in California’s Sea Otters may be incidental to summer commercial fisheries. Estes et al. (2003) found that Sea Otter mortality was elevated in the summer months and that commercial fin fish landings in the coastal live trap fishery increased. Recent analyses indicated annual Sea Otter carcass recoveries and reported fishery landings were significantly correlated.

Thomas and Cole (1996) found 10% of southern Sea Otters they examined to be emaciated without specific cause of mortality. Severe weather and periodic climatic events such as El Niño can disrupt foraging behaviour and food availability, and increase pup loss. Under these circumstances, Sea Otters may find it difficult to meet their high metabolic needs, leading to malnutrition or starvation. Serious tooth wear in older Sea Otters may also contribute to mortality (Riedman and Estes 1990). Recent studies have found infectious disease to be an important mortality factor in California Sea Otter populations. Around 280 Sea Otters found dead have been linked “to a pair of protozoan parasites, Toxoplasma gondii and Sacrocystis neurona, that are known to breed in cats and opossums (Conrad et al. 2005, Johnson et al. 2009). In Alaska, Streptococcal endocarditis, encephalitis and/or septicemia, referred to as Strep. syndrome has been identified in forensic-level necropies of northern sea otters (Unusual mortality event working group) as well as trauma from boat strikes. Goldstein et al. (2009) found northern sea otters from the Alaska Peninsula, Kodiak and Kachemak Bay area infected with phocine distemper.

Killer Whales (Orcinus orca), Great White Sharks (Carcharodon carcharias), Bald Eagles (Haliaeetus leucocephalus), Coyotes (Canis latrans), and Brown Bears (Ursus arctos) have been documented as predators of Sea Otters (Riedman and Estes 1990). Predation by Killer Whales is one factor that is believed to have caused Sea Otter population declines across the Western Gulf of Alaska and Aleutian Islands (Doroff et al. 2003, Estes et al. 1998, Hatfield et al. 1998). Significant declines in preferred prey species populations - Northern Fur Seals (Callorhinus ursinus), Harbour Seals (Phoca vitulina), and Steller Sea Lions (Eumetopias jubatus) are believed to have caused Killer Whales to prey switch and consume Sea Otters (Estes et al. 1998).

Studies in Alaska and Washington and elsewhere have shown that Sea Otter predation on sea urchins may indirectly enhance the growth of kelp and kelp-associated communities. Shellfish are important to commercial, recreational, and tribal fisheries throughout the species range and predation by Sea Otters can be significant and result in localized fisheries and economic issues.