The U.S. Fish and Wildlife Service 

is the federal agency responsible for 

many of the nation’s fish and wildlife 

resources. Having this responsibility 

has established the agency as one of the 

primary trustees for fish, wildlife and 

habitat at oil spills.

Oil spills affect wildlife and their 

habitats in many ways. The severity 

of the injury depends on the type and 

quantity of oil spilled, the season and 

weather, the type of shoreline, and the 

type of waves and tidal energy in the 

area of the spill.

Oil can be categorized into five groups, 

ranging from very light to very heavy 

oils. Most oil has a density less than 

water, so it floats. Oil tends to spread 

into a thin layer on the water surface as 

a sheen. Once in the water, oil undergoes 

weathering, a process that describes 

the physical, chemical, and biological 

changes that occur when oil interacts 

with the environment. 

Weathering reduces the more toxic 

elements in oil products over time, as 

exposure to air, sunlight, wave and 

tidal action, and certain mircoscopic 

organisms degrades and/or disperses 

oil. Weathering rates depend on 

factors such as type of oil, weather, 

temperature, and the type of shoreline 

and bottom that occur in the spill area. 

Types of Oil 

Very light oils (jet fuel, gasoline) are 

highly volatile and evaporate quickly. 

Very light oils are one of the most 

acutely toxic oils and generally affect 

aquatic life (fish, invertebrates, and 

plants) that live in the upper water 

column. 

Light oils (diesel, light crude, heating 

oils) are moderately volatile and can 

leave a residue of up to one third of the 

amount spilled after several days. Light 

oils leave a film on intertidal resources 

and have the potential to cause longterm contamination.

Medium oils (most crude oils) are less 

likely to mix with water and can cause 

severe and long-term contamination to 

intertidal areas. Medium oils can also 

severely impact waterfowl and furbearing marine mammals.

Heavy oils (heavy crude, No. 6 fuel oil 

and Bunker C) do not readily mix with 

water and have far less evaporation 

and dilution potential. These oils tend 

to weather slowly. Heavy oil can cause 

severe long-term contamination of 

intertidal areas and sediments. Heavy 

oils have severe impacts on waterfowl 

and fur-bearing marine mammals. 

Cleanup of heavy oil is difficult and 

usually long-term.

Very heavy oils can float, mix, sink, or 

hang in the water. These oils can become 

oil drops and mix in the water, or 

accumulate on the bottom, or mix with 

sediment and then sink. 

Wildlife and Habitat

Oil causes harm to wildlife through 

physical contact, ingestion, inhalation 

and absorption. Floating oil can 

contaminate plankton, which includes 

algae, fish eggs, and the larvae of 

various invertebrates. Fish that feed 

on these organisms can subsequently 

become contaminated. Larger animals 

in the food chain, including bigger 

fish, birds, terrestrial mammals, and 

even humans may then consume 

contaminated organisms. 

Initially, oil has the greatest impacts on 

species that utilize the water surface, 

such as waterfowl and sea otters, and 

species that inhabit the nearshore 

environment. Although oil causes 

immediate effects throughout the entire 

spill site, it is the external effects of oil 

on larger wildlife species that are often 

immediately apparent. 

Plants

Marine algae and seaweed responds 

variably to oil, and oil spills may result 

in die-offs for some species. Algae 

may die or become more abundant in 

response to oil spills. Although oil can 

prevent the germination and growth 

of marine plants, most vegetation, 

including kelp, appears to recover after 

cleanup.

Effects of Oil Spills on 

Wildlife and Habitat 

Alaska Region

Pool of oil on a heavily impacted 

beach, Prince William Sound, AK. 

NOAA

Sediment plume and sheens of 

refloated oil, Prince William 

Sound, AK. NOAA
sensitive to oil toxins. 

Birds and Mammals

Physical contact with oil destroys the 

insulation value of fur and feathers, 

causing birds and fur-bearing mammals 

to die of hypothermia. In cold climates, 

an inch diameter oil drop can be enough 

to kill a bird. Heavily oiled birds can 

loose their ability to fly and their 

buoyancy, causing drowning. 

In efforts to clean themselves, birds 

and sea otters ingest and inhale oil. 

Ingestion can kill animals immediately, 

but more often results in lung, liver, and 

kidney damage and subsequent death. 

Seals and sea lions may be exposed to 

oil while breathing or resting at the 

water’s surface or through feeding on 

contaminated species. 

Long-term or chronic effects on 

birds and marine mammals are less 

understood, but oil ingestion has been 

shown to cause suppression to the 

immune system, organ damage, skin 

irritation and ulceration, damage to the 

adrenal system, and behavioral changes. 

Damage to the immune system can 

lead to secondary infections that cause 

death and behavioral changes may affect 

an individual’s ability to find food or 

avoid predators. Oil also affects animals 

in non-lethal ways such as impairing 

reproduction.

Avian and mammalian scavengers 

such as ravens, bald eagles, and arctic 

foxes are also exposed to oil by feeding 

on carcasses of contaminated fish and 

wildlife. Direct mortality and reduced 

reproduction in bald eagles were 

attributed to the Exxon Valdez oil spill. 

Habitat

Oil has the potential to persist in the 

environment long after a spill event and 

has been detected in sediment 30 years 

after a spill. Oil spills may cause shifts in 

population structure, species abundance 

and diversity, and distribution. Habitat 

loss and the loss of prey items also have 

the potential to affect fish and wildlife 

populations. 

Oil remains in the environment long 

after a spill event, especially in areas 

sheltered from weathering processes, 

such as the subsurface sediments 

under gravel shorelines, and in some 

soft substrates. However, pelagic and 

offshore communities are fairly resilient 

and rebound more quickly than inshore 

habitats. Although oil is still present 

in the sediment and coastal areas 15 

years after the Exxon Valdez oil spill 

in Prince William Sound, Alaska, some 

wildlife populations have recovered. It 

is believed that continued effects will 

most likely be restricted to populations 

that reside or feed in isolated areas that 

contain oil. 

The U.S. Fish and Wildlife Service 

responds to spills to minimize impacts 

to trust resources and continues its 

work long after a spill event. The 

Service continues assess and monitor 

the damage to habitat and wildlife and 

attempts to minimize the long-term 

effects on new generations of wildlife. 

U.S. Fish & Wildlife Service

1 800/344 WILD

http://www.fws.gov

For more information, contact:

U.S. Fish and Wildlife Service

Regional Spill Response Coordinator

1011 E. Tudor Road Anchorage, Alaska 99503

907/786 3598

Visit the Contaminant Program home page:

http://alaska.fws.gov/fisheries/

contaminants/index.htm

December 2004

Oily sheen in the intertidal habitat 

with aquatic plants following the 

Exxon Valdez spill. 

Oiled loon, Prince William Sound, AK.

Invertebrates

Oil can be directly toxic to marine 

invertebrates or impact them through 

physical smothering, altering metabolic 

and feeding rates, and altering shell 

formation. These toxic effects can be 

both acute (lethal) and chronic (sublethal). Intertidal benthic (bottom 

dwelling) invertebrates may be 

especially vulnerable when oil becomes 

highly concentrated along the shoreline. 

Additionally, sediments can become 

reservoirs for the spilled petroleum. 

Some benthic invertebrates can survive 

exposure, but may accumulate high 

levels of contaminants in their bodies 

that can be passed on to predators. 

Fish

Fish can be impacted directly through 

uptake by the gills, ingestion of oil or 

oiled prey, effects on eggs and larval 

survival, or changes in the ecosystem 

that support the fish. Adult fish may 

experience reduced growth, enlarged 

livers, changes in heart and respiration 

rates, fin erosion, and reproductive 

impairment when exposed to oil. Oil 

has the potential to impact spawning 

success, as eggs and larvae of many fish 

species, including salmon, are highly 

Oiled sea otter, Prince William sound, 

AK USFWS