Although only a small percentage of oil enters the ocean from major oil spills, it is from studies during those events that we understand the extreme toxicity oil has on marine animals. 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 tons 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 tons of the larger Exxon Valdez oil spill in 1989, when an estimated 250,000 birds died.
As summarized by the Australian Maritime Safety Authority, 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, e.g. 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
- The variety of species at the spill location
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 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 vapors. Oil, especially light oils and hydrocarbon vapors, 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 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 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.
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, vapors 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 vapor. 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.
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 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.
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, vapors 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 vapor. 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.
Exxon Valdez oil spill research has revealed unexpected persistence of toxic subsurface oil and chronic exposures. Even at sublethal levels, the 14 year old oil has continued to affect wildlife. Delayed population reductions and cascades of indirect effects has postponed recovery (see: http://www.afsc.noaa.gov/Publications/misc_pdf/peterson.pdf)
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