There is no clear relationship between the amount of oil in the marine environment and the likely impact on wildlife. A smaller spill at the wrong time/wrong season and in a sensitive environment may prove much more harmful than a larger spill at another time of the year in another or even the same environment. Even small spills can have very large effects. Thus, one should not merely compare figures — the size of an oil spill is certainly not the only factor of importance in terms of what environmental damage can be caused by the oil.

In 1976, a spill estimated to have been less than 10 tonnes killed more than 60,000 long-tailed ducks wintering in the Baltic Sea and attracted to the seemingly calm water surface created by the oil slick. This could be compared to the effects on seabirds in Alaskan waters from the approximately 40,000 tonnes large Exxon Valdez oil spill in 1989, when an estimated 30,000 birds were oiled.

Another example from the waters off South Africa: "There is rather little correlation between the tonnages of oil released in spills and the impacts on the marine ecosystems. For example, a collision between two oil tankers in 1977 released 31,000 tonnes of oil and polluted 47 African Penguins, but in the Apollo Sea sinking of 1994, about 2,000 tonnes of oil impacted about 10,000 penguins. After the Apollo Sea, we generally believed that this was the maximum amount of penguin mischief that 2,000 tonnes of oil could achieve. However, when the Treasure sank on 23 June 2000, half this amount of oil threatened four times as many penguins! In round figures, 20,000 penguins were oiled, and 20,000 penguins were prevented from becoming oiled by removing them off their breeding colonies on Dassen and Robben Islands."

As summarized by the Australian Maritime Safety Authority (AMSA), important factors related to the impact of an oil spill on wildlife are:

• the spread of the oil slick,
• the type of oil spilled, its movement and weathering characteristics
• the location of the spill,
• the area of estuary, sea and foreshore impacted by oil,
• the sensitivity of the regional environment, eg proximity to bird breeding colony,
• the number of different habitats impacted, such as rock shore, beach, mangrove, wetland,
• the timing of the incident (during seasonal breeding, bird migration),
• the nature, toxicity and persistence of the oil; and
• the variety of species at the spill location.

In the words of the U.S. Environmental Protection Agency (EPA): "Most biological communities are susceptible to the effects of oil spills. Plant communities on land, marsh grasses in estuaries, and kelp beds in the ocean; microscopic plants and animals; and larger animals, such as fish, amphibians and reptiles, birds and mammals, are subject to contact, smothering, toxicity, and the chronic long-term effects that may result from the physical and chemical properties of the spilled oil."

Oil harms seabirds and marine mammals in two major ways:

• Physical contact — when fur or feathers come into contact with oil;
• Toxic contamination — some species are susceptible to the toxic effects of inhaled or ingested oil. Oil vapours can cause damage to an animal's central nervous system, liver, and lungs. Animals are also at risk from ingesting oil, which can reduce the animal's ability to eat or digest its food by damaging cells in the intestinal tract. Some studies show that there can also be long-term reproductive problems in animals that have been exposed to oil.

SEABIRDS

Oil may kill seabirds in several ways.

The first effect is often that oil destroys the structure of its protective layer of feathers and insulating down. The fat under the birds skin is an energy reserve as well as an extra layer of insulation. Cold water quickly penetrates into the down and reaches the skin. The amount of oil that a bird is smeared with is not important. In a cold climate an oil spot the size of 2-3 sq. centimetre can be enough to kill a bird. The insulating effect of the plumage is destroyed by the oil, and the bird freezes to death (hypothermia). If a bird gets smeared with a lot of oil it may clog the bird's feathers making it impossible for it to fly. The bird may also loose it buoyancy (its ability to float on the water surface) and actually drown.

In their efforts to clean themselves from oil and put their feathers in their original state, the birds may inhale or ingest oil. As many of the substances in oil are toxic, this may result in serious injuries/health effects such as pneumonia, congested lungs, intestinal or lung hemorrhage, liver and kidney damage. This poisoning is often as deadly as hypothermia, although the effects may not manifest themselves as quickly.

Oil may also affect the reproductive success of the birds as oil from feathers of a bird that is laying on eggs may pass through the pores in the eggshells and either kill the embryos or lead to malformations.

Seals, sea lions, walruses, polar bears, sea otters, river otters, beavers, whales, dolphins and porpoises, and manatees, are groups of marine mammals that may be affected by oil spills. Their sensitivity seems to be highly variable and appear to be most directly connected to how important their fur and blubber (layer of fat under the skin) are for keeping them warm. Thus, marine mammals living in cold climates (seals, sea lions, polar bears and otters) are likely to be more vulnerable than those living in temperate or tropical waters.

Effects of oil on marine mammals depend upon species may, in addition to hypothermia, include: toxic effects and secondary organ dysfunction due to ingestion of oil; congested lungs; damaged airways; interstitial emphysema due to inhalation of oil droplets and vapour; gastrointestinal ulceration and hemorrhaging due to ingestion of oil during grooming and feeding; eye and skin lesions from continuous exposure to oil; decreased body mass due to restricted diet; and stress due to oil exposure and behavioural changes.

Seals (true seals, sea lions, fur seals and walruses)

Seals are very vulnerable to oil pollution because they spend much of their time on or near the surface of the water. They need to surface to breathe, and regularly haul out onto beaches. During the course of an oil pollution incident, they are at risk both when surfacing and when hauling out.

Fur seals are more vulnerable due to the likelihood of oil adhering to their fur which will result in the fur losing its insulating ability (as they lack any blubber for additional insulation). Heavy oil coating on fur seals may result in reduced swimming ability and lack of mobility when the seals are on land.

Seals could also be damaged through the ingestion of oiled food or the inhalation of oil droplets and vapours. Oil, especially light oils and hydrocarbon vapours, will attack exposed sensitive tissues. These include mucous membranes that surround the eyes and line the oral cavity, respiratory surfaces, anal and urogenital orifices. This can cause corneal abrasions, conjunctivities and ulcers. Consumption of oil-contaminated prey could lead to the accumulation of hydrocarbons in tissues and organs.

Sea otters

Sea otters spend a lot of their time on the sea surface and are totally depending on their fur for isolation and for the ability to float. As a consequence, sea otters are regarded as being very sensitive to oil spills as oil may result in the fur losing its capacity to insulate the animals. However, inhaling hydrocarbons or ingesting oil when they groom themselves can damage their lungs, cause ulcers, and result in liver and kidney damage. Habitat loss and diminishing food resources constitute indirect effects on the otters. The Exxon Valdez incident is believed to have led to the death of 15,000 otters, mainly as a result of ingestion of oil.

Polar bears

Polar bears are depending on blubber, so called guard hair and a thick underfur for insulation. When grooming an oil contaminated fur they may swallow oil, something that is known to have resulted in the death of polar bears. There is also some evidence that the toxic effects of oil cause an inability of polar bears to produce red blood cells and lead to kidney damage.

Whales, including dolphins

Due to their migratory behaviour, there is little documented evidence of cetaceans (whales) being affected by oil spills. It would, however seem likely that baleen whales would be particularly vulnerable to oil while feeding. Oil may stick to the baleens while the whales "filter feed" in the vicinity of oil slicks. They plunge, take in huge quantities of water and then filter out their feed of plankton and krill. Sticky, tar-like residues are then particularly likely to foul their baleen plates. There are also indications that whales can inhale droplets of oil, vapours and fumes if they surface in slicks when they need to breathe. Exposure to oil in this way could lead to damage of mucous membranes, injuries in airways or even cause death.

Dolphins are smooth-skinned, hairless mammals, and as a consequence oil tends not to stick to their skin, but they can inhale oil and oil vapour. This is most likely to occur when they surface to breathe. This may lead to damages of the airway and lungs, mucous membrane damage or even death. A stressed or panicking dolphin would move faster, breathe more rapidly and therefore surface more frequently into oil which would increase exposure. Dolphins eyesight may also be affected by oil.

Manatees and dugongs

Manatees and dugongs live in warm waters and have a layer of blubber as insulation. Thus, the impact of oil on their body temperature might not be of importance. However, as all marine mammals they may be affected when they inhale volatile hydrocarbons when breathing on the water surface.

Sea turtles

Little information is available on the effects of oil on sea turtles. However, a number of effects have been suggested as possible.

If turtles surface in an oil slick to breathe, oil will affect their eyes and damage airways and/or lungs. Sea turtles could also be affected by oil through contamination of food supply or by absorption through the skin.

The nesting sites of sea turtles are typically located on sandy beaches. Oil contamination of such beaches can lead to several problems:

• Digestion/absorption of oil through food contamination or direct physical contact, leading to damage to the digestive tract and other organs;
• Irritation of mucous membranes (such as those in the nose, throat and eyes) leading to inflammation and infection;
• Eggs may be contaminated, either because there is oil in the sand high up on the beach at the nesting site, or because the adult turtles are oiled as they make their way across the oiled beach to the nesting site, and oiling of eggs may inhibit their development;
• Newly hatched turtles, after emerging from the nests, make their way over the beach to the water and may become oiled.

Fish and shellfish

Fish may ingest large amounts of oil through their gills. Fish that have been exposed to oil may suffer from changes in heart and respiratory rate, enlarged livers, reduced growth, fin erosion and a variety of effects at biochemical and cellular levels. If this does not kill them more or less directly, the oil may affect the reproductive capacity negatively and/or result in deformed fry.

Much less is known about the effects of oil on fish eggs and larvae. The large proportion of salmon eggs killed off by the Exxon Valdez spill indicate that the effects can be serious and long-term.

Very little is also known about the effects of oil on shellfish (except for the fact that contamination with hydrocarbons will make shellfish taste and smell bad and thus make it impossible to use them for food).

The negative effects of on oil spill may eventually fade away, but in many cases it will be matter of several years, even decades, before an area or ecosystem has fully recovered from a spill that caused extensive damages. Every situation is unique and depending on the particular conditions and circumstances in that area, and on the characteristics of the spill. Some areas might recover in a matter of weeks, others will need up to 20 years. The recovery of an ecosystem will also depend on the share of important populations being killed off or affected by acute poisoning.

The recovery of the affected habitats and species following an oil spill will to a large extent depend on the type of ecosystem , the vulnerability of the species and not least the climate of the region where the oil spill occurs. Generally, recovery will proceed faster in warmer climates and on rocky shores compared to cold climates and, for example, marshes. The long-term effects on deeper bottoms (i.e., if oil sinks and is absorbed in bottom sediments) is also a matter of concern.

The best documented evidence concerning the recovery of ecosystems affected by massive oil pollution are from the Persian Gulf and resulting from the discharges associated with the Gulf War in 1991. Studies (GESAMP) suggest that the chronic and acute releases that took place were rather rapidly accommodated by the system. Already at the end of 1992, researchers reported that many of the worst hit beaches in Saudi Arabia were almost clean of oil. It is believed that this may have been the result of the warm water of the Gulf and the fact that its bacterial populations were able to degrade and weather the oil much more quickly than previously believed to be possible.

The experience gained from the Exxon Valdez spill has been documented, and could serve as one example of what happens in the aftermath of a major spill in a sensitive area. See, for example, the web site of the Exxon Valdez Oil Spill Trustee Council, and NOAA Office of Response and Restoration ("NOAA biologists have been monitoring the long-term effects of the spill and cleanup efforts. Here are some of their reports, along with links to more information elsewhere").

• AMSA: What happens to wildlife in an oil spill? Australian Maritime Safety Authority.
• U.S. EPA: Understanding oil spills and oil spill responses (Chapter 5: Wildlife and oil spills). U.S. Environmental Protection Agency Oil Program: Oil Learning center.
• U.S. EPA: Effects of Oil Spills and Impact on species. U.S. EPA Oil Program: Oil Learning Center.
• U.S. NOAA: How oil harms fish and wildlife. See also Oil spills at the water surface. U.S. National Oceanic and Atmospheric Administration (NOAA) Office of Response and Restoration.
• GESAMP: Protecting the Ocean from Land-based Activities. Report 71, 2001 (p. 23). Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP):
• ITOPF: Effects of marine oil spills. International Tanker Owners Pollution Federation Limited (ITOPF).
• IPIECA: Publications on oil spills: • Guidelines on Biological Impacts of Oil Pollution. • Biological Impacts of Oil Pollution: Fisheries. International Petroleum Industry Environmental Conservation Association (IPIECA)
• IPIECA Publications on biodiversity: The Oil Industry: Operating In Sensitive Environments ("a set of case studies illustrating the petroleum industry's experience of operating responsibly in a diverse range of sensitive environments"). • See also a number of case studies online (further down on the same web page). • See also IPIECA Biodiversity page.
• Environment Canada: Canadian Wildlife Service. Oil Pollution and Birds.
• Society Promoting Environmental Conservation (U.S., British Colombia: "Oil Spills: Environmental Costs of Energy Use " (article, 2002). .
• University of California at Davis: Oiled Wildlife Care Centre. • See also list of links to organizations working with wildlife rescue. • See also 7th International Effects of Oil on Wildlife Conference (October, 2003) and new international alliance to improve oil spill wildlife rescue.
• International Bird Rescue Research Centre (IBRRC). See also IBRRC Success Stories.
• University of Cape Town, Avian Demography Unit, Department of Statistical Sciences: A 'Treasure' story (part of "The Treasure oil spill: Effects on the wildlife of Table Bay and particularly on the endangered African Penguin".) See also IBRRC: Treasure Spill Report.
• International Fund for Animal Welfare (IFAW).