Oil slicks, droplets and “chocolate mouse” particularly affect organisms which come to the surface to breathe, dive into the water to find food, or aggregate at or near the water surface. Oiling can be specific to certain organs, and interfere physically with the organism’s movement, feeding and/or other actions. Oil slicks also have repercussions for floating solid waste, navigation buoys, vessels, fishing gear, shellfish farming, rafts, net pens and so on.

Of all that can be oiled or intoxicated, the victims which particularly concern us are two groups of organisms: seabirds and mammals. Their lives depend on regular contact with the water surface; in the case of seabirds to find food and rest between flights, and for mammals, to breathe at the water surface. Hydrocarbons on the water surface present a major risk of ingestion, inhalation and soiling of their fur or feathers.

The oiling of bird feathers causes a loss of thermal insulation, buoyancy and lift. For mammals that lick their coats, oiling leads to the risk of ingestion of oil. Other threats to wildlife are the risks of direct ingestion, irritation of the eyes and nostrils, inhalation of toxic vapours, suffocation by coating with oil as well as longer term toxic effects impairing the organism’s metabolism.

Oil particles dispersed in the water can accumulate on sensitive epithelial tissue (gills, mucous membranes, etc.), clog them up and lead to degeneration. Animals with affected filtering mechanisms can ingest enough oil to suffer a toxic effect whilst being incapable of feeding.

Organisms with oiled gills are incapable of ensuring their oxygenation, and soluble hydrocarbons can enter the bloodstream through the respiratory tract. These cases are particularly common in filtering molluscs and open water and bottom-dwelling fish, when natural or chemical dispersion causes high concentrations of oil to enter the water column and persist for several days.


Oiling in the open sea also affects the surface layers of plankton. Plankton is the first element in the food chain which large marine mammals feed on. Mammals are therefore also affected by the pollution, as are pelagic fish. The effects of the pollutant can be locally and temporarily significant for certain plankton populations which migrate from a few tens of metres deep at night, to near the surface during the day. However, the short duration of the cycle of these species and the high level of agitation of the water mean that the organisms lost due to the pollution are soon replaced, thus limiting the impact on the food chain.

Washing a guillemot in a care centre belonging to the "Ligue Pour la Protection des Oiseaux"

Oiling of birds’ plumage

(Source: IPIECA. A Guide to Oiled Wildlife Response Planning)

"The first, and often most important, effect on birds is external contamination of the feathers from contact with oil. This can cause a disruption of the delicate feather structure which traps warm air next to the body and keeps cold air and water away from the skin. Oil contact temporarily disrupts this intricate structure of barbs and barbules, thus interfering with the bird’s ability to thermo-regulate.

Most animals in these circumstances quickly become hypothermic (or hyperthermic) and will seek shelter to stay alive. Those reaching shore are often unable to find food, because of the individual’s inability to return to the sea to feed. They become dehydrated and hypoglycaemic and are prone to predation."