Oiled Wildlife Care Network

Photo: oiled pinniped
Oiled California sea lion

Impacts of Oil on Marine Mammals

Hair/Fur and Skin

Different classes of marine mammals experience different consequences from external oiling. Heavily furred animals, such as sea otters and fur seals, rely on a thick haircoat to maintain warmth and buoyancy. The fur traps a thin layer of air adjacent to the animal’s skin similar to the way feathers function in birds. This air layer insulates the body against the surrounding cold ocean water. When exposed to oil, the alignment of the hairs is altered, and the air layer is destroyed. Cold sea water is able to seep through the contaminated fur and causes the animal to rapidly become hypothermic.

For marine mammals without heavy haircoats (such as other species of seals, sea lions, dolphins and whales), problems associated with hypothermia are less of a concern because a thick blubber layer protects them from the cold. Young mammals are an exception to this, as they often have not yet developed this layer. Direct contact with the skin and mucous membranes puts all exposed animals at risk of painful chemical burns that also may become infected.

Internal Organs

Oiled marine mammals that spend a large amount of time grooming (e.g., sea otters) are at great risk of ingesting oil on the haircoat. Prey items may also be affected by oil spills and serve as a potential source of ingested petroleum products. Once ingested, oil can cause direct injury to the gastrointestinal tract, which can impair the ability to digest and absorb food. Metabolism of absorbed oil components by the kidney and liver can cause extensive damage to those organs as well. Volatile fumes released from oil in the environment, as well as on skin and hair, can irritate or injure the respiratory tract leading to inflammation and pneumonia.


Research by OWCN staff has shown that internal exposure to oil in mustelids (e.g., sea otters) can cause decreased birthrates even several generations past the initial exposure. In the field, oil spills have been shown to cause increased mortality in young animals, as well as pup abandonment in sea otters. In orca following the Exxon Valdez spill, two pods lost approximately 40% of their numbers. Since that time, the reproductive capacity of these pods has been reduced by the loss of females, with only about half of newborn calves surviving.

Effects on Populations

Southern sea otters are considered a keystone species for the nearshore kelp forest ecosystem. They range from Pt. Año Nuevo to Pt. Conception, making the species highly vulnerable to spills in this area. A significant offshore event has the potential to decimate this species, and by extension the kelp forest ecosystem.