Worldwide, the general trend may indeed be improving or, in U.S. waters, at least holding steady. Data from the Oil Spill Intelligence Report, which tracks oil spills worldwide, show a decline in the total volume of oil spilled from 1978 through 1980 compared to 1981 through 1990, with occasional megaspills inflating the total. In U.S. waters, Coast Guard data on oil spills from 1970-1986 show no clear increase or decrease in the number of spills, and the volume spilled remains relatively constant. However, there are many sources of oil pollution in the ocean other than tanker and oil rig accidents, which collectively account for about 12.5 and 1.5 percent of all spilled oil worldwide, respectively, according to a 1985 report from the National Academy of Sciences. Oil spills from tankers, drilling operations, and natural seeps cause a concentrated input of oil into the ocean, often occurring in a very short period of time, and thus can have serious local effects on animal and plant life and on land cherished for its recreational value.
Oil is actually a very complex mixture made up of hundreds of compounds, mostly hydrocarbons.
Lighter portions of the oil, though more toxic than crude oil, are more likely to evaporate or otherwise dissipate before they can reach the sensitive shoreline habitat. In fact, laboratory studies have shown that evaporation can reduce the volume of even a crude oil slick by 20-40 percent. Heavier oil that does not evaporate, although not as toxic as refined oil, can wash onto rocky and sandy shores and there can cause serious short-term harm to shellfish and plant life. In the early stages of an oil spill, large numbers of sea birds and mammals may be killed by coming in contact with the freshly spilled oil. The effects on finfish are less clear, but it appears that schools of fish may be able to swim away from the spill, although fish in ocean hatcheries, floating fish eggs, and larvae can be destroyed.