CCWHC Wildlife Health Topics

Petroleum Oils and Wildlife

Last Updated: May 2000

Author: F. A. Leighton

Reviewers: P. Albers, J. Mazet, D. Jessup

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Table of Contents
What is "Oil"?
The Process of "Weathering"
Species Affected by Spilled Petroleum Oils
Plants Invertebrates Fish Birds and Mammals
Physical Contact with Oil
Oil as Poison
Sources of Petroleum Oil Pollution
Responding to Oil Spills
Reporting the Occurrence of an Oil Spill in Canada
Coping with Oil-contaminated Wildlife
Further Reading

What is "Oil" ?

Petroleum oils include all liquids derived from crude petroleum. The environmentally-important petroleum oils are those produced and transported in large quantities which can, thus, be spilled in large quantities. These include crude oil (the stuff that comes out of the ground), and major fuels produced by distillation of crude oil: gasoline, kerosine/jet fuel, diesel/Fuel Oil No. 2, Fuel Oil No. 1, bunker c oil (fuel oil No 6) and Intermediate Fuel Oil 180, the latter two being the major fuels of marine shipping).

Crude oils usually are named after the geographic location of the oil well: South Louisiana, Hibernia, Kuwait, Prudhoe Bay. These names say nothing about the chemical composition of the oil. Crude oils vary markedly in chemical composition and even the oil from one well may differ in chemical composition over time. The fuel oils produced from crude oils also differ in chemical composition. Thus, two No 1 fuel oils can be very different chemically, although their combustion properties will be similar.

Petroleum oils are complex mixtures of chemicals: hydrocarbons and hydrocarbon-like chemicals that include atoms of nitrogen, oxygen, or sulphur (often called N-S-O compounds). There also are metals in oil, such as vanadium and mercury. Hydrocarbons occur in two major chemical subdivisions: those made up of benzene rings (called aromatic hydrocarbons or aromatic N-S-O compounds) and compounds without benzene rings that consist of straight, branched or circular carbon-hydrogen sequences. Every petroleum oil consists of hundreds of different hydrocarbons and N-S-O compounds, and no two oils are exactly the same in chemical composition. For example, some may contain 30% or more of aromatic hydrocarbons and others virtually no aromatic hydrocarbons at all.

Some aspects of the toxicity of petroleum oils depend on the chemical composition of the oil. Where this has been studied, most toxicity has been found associated with the aromatic compounds in oil . Note that "aromatic" means "composed of benzene rings"; it does not mean "easily evaporates" or "volatile" or "of low molecular weight". In fact, where this has been studied, it has been the larger, non-volatile aromatic compounds that have been the most toxic compounds found in petroleum oil. On the other hand, aquatic organisms are at higher risk from smaller, lighter molecules because these are more readily dissolved in water and thus reach higher concentrations in water than do the relatively insoluble larger molecules

The Process of "Weathering" - Changes in Oil After It Is Spilled

When oil is spilled into the environment, its chemical composition immediately begins to change. The rate of change varies with environmental conditions. The processes by which oil changes in composition after it is spilled is referred to collectively as "weathering". Weathering does not necessarily make oil less toxic. Weathering is a continuous process that, on average, is completed in about one year in temperate open waters, but which can take much longer in cold environments, under ice or when oil becomes buried in sediments or enters porous beach material. Initial changes in the spilled oil occur fairly quickly, and the rate of change then slows over time. The end result of weathering of oil spilled into water is conversion of the original oil into hydrocarbons dispersed in the air or water and to masses of residual asphalt called tar balls, which are cast onto the shore or sink.

The main processes of weathering are evaporation, formation of emulsions of oil-in-water or of water-in-oil (this latter produces a sticky mess fondly called "mousse") and dispersion of those emulsions, the dissolving of oil components into water, photo-oxidation of compounds, sedimentation, and microbial conversion of hydrocarbons to carbon dioxide and water. At any point in the process, the oil may become buried in soil or sediments, and the weathering process may be suspended for years until the oil is re-exposed when the beach or sediment is disturbed, as by a storm.

The concentration of potentially poisonous chemicals in oil is most likely to increase during weathering, especially during the first few weeks. Volatile components evaporate quickly, leaving behind the less volatile molecules. The less-volatile aromatic hydrocarbons and related N-S-O-compounds are the group of chemicals in petroleum oils most likely to include serious poisons. As noted above, solubility also is an important criterion of toxicity for aquatic organisms exposed only to dissovled petroleum compounds. It often is implied in papers and reports that weathered oil is less toxic than is fresh oil. There is no scientific basis for this assumption; the opposite also may be true.


Most oil spills occur in water near shorelines. Thus, marine and aquatic animals and plants are most often affected by oil. All animal and plant taxa can be affected by oil spilled into their habitat. Petroleum oils spilled into the environment can affect organisms by direct physical coating, by altering essential elements of the habitat and by direct toxic effects of chemicals in the oil. The environmental impact of spilled oil always is a complex mixture of these effects applied to all living organisms in the affected area.

Oil is an unusual pollutant. When spilled into water, it does not quickly become diluted but remains in a concentrated mass on the surface which is only slowly changed and degraded by the process of weathering. Thus, oil has its most pronounced effects on organisms that make use of the water surface or that inhabit shorelines. This effect extends to benthic organisms when large quantities of oil are incorporated into sediments or is dispersed in shallow water.


High mortality in marsh grass, mangrove, and intertidal plant communities has occurred due to spilled oil, and these habitats may require one or two decades to recover. Floating algae may be killed or may grow more abundantly in response to oil, depending on conditions and on the concentration of the oil. Variable responses to oil by plants can result in major shifts in the relative abundance of plant species in polluted environments. Terrestrial plants also may be killed by oil or suffer reduced growth and reproductive rates due to combinations of physical coating, altered soil chemistry and toxic effects of petroleum components.


Invertebrates in intertidal zones affected by spilled oil often are virtually eliminated. Again, the affects of oil include physical smothering, loss of food and direct chemical toxicity. Invertebrates may be eliminated from sediments contaminated by oil for many years, since the oil can persist in sediments for long periods of time.


Eggs and larvae of fish are very sensitive to toxic chemicals in oil. For example, Pacific Herring and Pink Salmon larvae suffered high mortality and high rates of physical deformity when exposed to concentrations as low as 0.4 to 1.0 part per billion of dissolved, multi-ring aromatic hydrocarbons derived from weathered oil. Such levels can occur in water in oil-spill areas. Adult fish are less sensitive and generally can avoid oil; they are exposed only to droplets of oil dispersed into the water column or to dissolved compounds. In shallow waters, adult fish can be exposed to higher concentrations of oil and oil-derived chemicals, and fish kills have occurred in such circumstances. Fish also can be affected by altered food resources and habitat, especially in near-shore areas, streams and estuaries.

Birds and Mammals

Birds often are killed in large numbers in oil spills. Aquatic birds that spend much time on the water surface or feed by diving are particularly vulnerable, and are exposed directly to concentrated oil that is spilled into their habitat. Birds often are the most evident victims of oil spills and much effort is expended in cleaning and releasing oiled birds. The impact of oil on marine mammals generally is much less than on birds, with the exception of mammals such as Sea Otters and other species that rely on fur, rather than blubber, for thermal insulation. The impact of oil on these fur-dependent species can be severe and similar to that on birds.

Physical Contact with Oil:

Physical contact is the effect of oil that kills large numbers of birds and fur-dependent aquatic mammals, and is by far the most important effect of oil on these animals. Birds rely on their feathers for water-proofing, buoyancy, insulation and aerodynamic contours. Water is excluded by feathers because the narrow slits between feather barbules are too narrow to permit water, with its high surface tension, to pass through. Oil has a low surface tension and readily passes through feathers. Oil-contaminated Muskrat and Northern Shoveler
  Muskrat and Northern Shoveler: Victims of a spill of bunker C oil into the South Saskatchewan River at Saskatoon

Feathers thus become matted by oil immediately upon contact, and all the protective and functional properties of feathers are lost. Birds on cold water quickly die of cold; others are prevented from feeding or leaving the oil spill area, and die of exposure, starvation and dehydration. Similarly, the fur of aquatic fur-bearing mammals excludes water because the spaces between guard hairs are too small to admit water. Oil penetrates such fur easily and the fur becomes matted with oil. Death from cold and exposure follows quickly, particularly in animals who must live in and feed in cold water, such as Sea Otters.

Oil as Poison: Petroleum oils contain potentially poisonous compounds. Animals may be exposed to these by inhalation, direct contact with the skin or ingestion, and bird embryos can be exposed to oil compounds when the external surface of egg shells is contaminated with oil.

Inhalation of Volatile Components - During the earliest stages of an oil spill, evaporation may result in high concentrations of volatile compounds in the local atmosphere. This could lead to suffocation and/or to inhalation of toxic concentrations of volatile components such as hexane and benzene. These high atmospheric concentrations do not last long, perhaps a few hours. There is one report of birds dying under conditions that suggested the cause was inhalation exposure. Probably this is not a major cause of death or illness in wildlife. Respiratory disease has been noted in birds rescued from oil spills and this may be caused or exacerbated by inhalation of toxic volatile components of oil.

Direct Contact with the Skin - Some oils appear to contain irritant compounds or to affect fatty components of tissues in such a way as to cause chemical damage and inflammation when they contact body surfaces such as the skin or the corneal or conjunctiva of the eye. Affected areas may become secondarily infected with bacteria and may distract animals from feeding or preening.

Ingestion of Oil - Animals contaminated externally with oil will attempt to clean themselves by licking and preening, and will ingest oil in the process. Oil also can be ingested when their water or food is contaminated. It is virtually impossible to separate the effects of external, physical oil exposure from the effects of ingested oil when animals are exposed in real oil spills. The effects of ingested oil are known only from experiments in which animals were fed oil without also being contaminated externally. The results of such experiments over the past 40 years are quite confusing, since many different oils fed to many different species at many different doses and frequencies have been reported. The chemical composition of the oils used in these experiments seldom was determined. Nothing helpful is known about the toxicity of ingested oil to relevant mammalian species. Three toxic effects of oil ingestion by birds have been well-documented. The first is that ingestion of petroleum oil appears to induce a stress response that is additive, perhaps synergistic, with other stress-inducing stimuli to which birds are exposed at the same time. This may be of great importance in the wild, where multiple stresses occur and affect survival in multiple ways. The second effect is a negative impact on reproduction. Several different oils fed to several different species have resulted in reduced reproduction with effects on both male and female birds. The third effect is damage to red blood cells, with anemia as the result. This has been documented for only one crude petroleum oil with a high content of aromatic compounds that was ingested at the high end of expected doses in oil spill situations.

Oil on Bird's Eggs -

Tiny quantities ( a drop or less) of oil applied to birds' eggs during the first half of incubation can cause almost 100% mortality of the embryos inside. This high sensitivity of avian embryos to compounds present in petroleum oils has been documented in the field as well as the laboratory. Adult birds with lightly-contaminated feathers can deliver lethal doses of oil to their eggs when they return to the nest to incubate. Fortunately, this period of high vulnerability is limited to the few short weeks of the breeding season for most bird species.

oiled egg
  Ten microlitres (10 one-millionths of a litre) of crude oil on a hen's egg


It has been estimated that the some 3-4 million metric tons of petroleum oil contaminate the world's oceans annually. Most oil pollution occurs along marine coastlines, but spills also occur inland. There are many sources from which petroleum oils can reach the environment. Most result from human activity, but there also are natural sources. The largest source of petroleum contamination is discharge from routine shipping operations, for which petroleum-derived oil is the main fuel. Accidental spills associated with shipping or with off-shore oil production facilities have led to the largest oil spills.

Offshore Oil Rig
  Offshore Oil Platform, Conception Bay, Newfoundland

Oil pump on Sk wetland

Inland locations such as western Saskatchewan and Alberta have vast numbers of oil wells, storage facilities and pipelines. Many of these are in or near wetland habitat.

Thus, there is potential for contamination of prairie aquatic environments by oil .

Oil pump on a Saskatchewan wetland  

Petroleum contamination of the environment occurs in two very different forms. Most petroleum arrives in the environment from numerous small sources, none of which would be classified as a significant "spill". Only up to about 15% of annual petroleum contamination occurs in the form of "spills" - large volumes of oil released into the environment from a single source. Practically nothing is known about the environmental effects that may be due to diffuse, low-intensity oil pollution. Almost everything that is known about the impact of petroleum pollution has been learned in the context of real or simulated oil spills. Thus, the information presented here pertains to the effects of classical oil spills.


1. Reporting the Occurrence of an Oil Spill in Canada

Oil spills, chemical spills and other environmental emergencies should be reported first to the Environment Canada Regional Environmental Emergency Coordinator closest to the site of the oil spill or other emergency. The following is contact information for reporting such emergencies across Canada:

[Information last updated: 25 October 1999]

National Environmental Emergencies Centre
(collect, 24 hours a day)


(819) 997-3742



(819) 953-5361



Environmental Emergencies Branch

Regional Environmental Emergency Coordinators:


(902) 426-2576


(514) 283-2345


(416) 739-5908


(780) 951-8753


(604) 666-6496


Environmental Technology Centre


(613) 998-9622

A policy document from the Canadian Wildlife Service regarding response to oil spills is available on the Internet at:

2. Coping with Oil-contaminated Wildlife

Wildlife agency personnel may be required to respond to oil spills. The aim of such responses usually is to limit damage to the environment and to do something with wild animals coated with oil. Often there is intense public interest in the welfare of oil-contaminated birds and mammals. Environment Canada and most provinces have emergency response plans that include oil spills (see above). Part of any response will involve handling oil-contaminated wildlife, and agencies should be fully prepared to respond. Rehabilitation of oiled wildlife is best accomplished in cooperation with organizations with technical skill and experience specifically in the rehabilitation of oil-contaminated wildlife. The basic process of washing oiled wildlife is simple, but the logistics of doing it effectively with large numbers of animals are horrific. Cleaned birds must be held in captivity for days. Pre-cleaning and post-cleaning management of the animals are the most difficult and the most important aspects of the rehabilitation process. Failure to do the job properly will make the work totally ineffective. One private and highly professional organization that can provide advice, training, and on-site assistance, and that also can recommend other experienced groups, is listed below.

Tri-State Bird Rescue and Research, Inc.

Canadian Representative: Tom Dunbar :

Telephone: 519-524-4272


Emergencies Only (pager): 519-575-7299

Main Office: 110 Possum Hollow Road

Newark, Delaware 19711 USA


If you are involved in an oil spill which threatens to contaminate or has contaminated wildlife, you may call the following numbers 24 hours a day, 365 days a year:

800 - 710 - 0695 (Pager) If your call is not returned, call

800- 710 - 0696 (Pager) Use these numbers ONLY to report an oil spill.

(Enter your area code and telephone number when asked to "leave a numerical message".)

For non-emergency situations contact:

TSBR Main Office: 302-737-7241 Fax: 302-737-9562

The State of California has an extensive spill response program. The program office can recommend actions and organizations competent to assist in an oil spill response.

Office of Spill Prevention and Response

1416 Ninth Street; P.O. Box 944290

Sacramento, CA 94244-2090 USA

OSPR 24-hour Communications Center (916) 445-0045


The Oiled Wildlife Care Network, a unit of the Wildlife Health Center, School of Veterinary Medicine, University of California (Davis) is another source of information on responding to spilled oil. Rehabilitation protocols for oiled wild animals, and other information and publications are available on their website

Oiled Wildlife Care Network

Wildlife Health Center

University of California,

Davis , California 95616-8615

Telephone: (530) 752-4167


Your Name

Product spilled

Spill Location





Time/Date Government Agencies notified / involved

Responsibility regarding spill

Responsible Party, if known

Next steps you plan to take

Telephone and fax numbers

Wildlife involved  


Albers, P.H. 1995. Petroleum and individual polycyclic aromatic hydrocarbons. In: Handbook of Ecotoxicology (D.J. Hoffman, B.A. Rattner, C.A. Burton and J. Cairns, Jr. eds.). Lewis Publishers, Boca Raton. p. 330-355.

Jessup, D.A. and F.A. Leighton. 1996. Oil pollution and petroleum toxicity to wildlife. In: Noninfectious Diseases of Wildlife, 2nd Ed. (A. Fairbrother, L.N. Locke, G.L. Hoff, eds.). Iowa State University Press, Ames, Iowa. p. 141-156.

Gilardi, K.V.K. and J.A.K. Mazet. 1999. Oiled Wildlife Response in California: A Summary of Current Knowledge of Populations at Risk and Response Techniques. Oile Wildlife Care Network, Davis , CA. 124 pp. (This can be downloaded free of charge from the Oiled Wildlife Care Network Website

Albers, P. H. 1998. An Annotated Bibliography on Petroleum Pollution. Version 2000.1. USGS Patuxent Wildlife Research Center, Laurel, MD. [This bibliography is updated annually. It is not available on line, but may be down-loaded in .pdf or non-pdf formats. To download, locate it on the Internet Website and follow directions on screen


If you recognize sickness or death in Canadian wildlife, report this to local wildlife officials or make a report directly to the CCWHC:

Call Toll-Free (in Canada): 1-800-567-2033


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