FAQ: How Oil Harms Fish and Wildlife

Q. How do oil spills affect animal and plant life?

A. On our website, you can find out what our team of marine biologists has learned about the effects of the Exxon Valdez oil spill on the plants and animals of Prince William Sound, Alaska. To see that information, try out the links on our Spotlight on the Exxon Valdez page. In particular, take a look at Prince William Sound: an Ecosystem in Transition and Mearns Rock Time Series.

Good starting points for your research elsewhere on the web include Oil, Water, and Chocolate Mousse, a report on oil spills and their effects from Environment Canada; the website of the Oiled Wildlife Care Network at the University of California at Davis; Tri-State Bird Rescue and Research; and the International Bird Rescue Research Center.

Q. In an oil spill, does the type of oil matter? If it does, what type of oil causes the most harm?

A. The type of oil definitely matters, because different types of oil behave differently in the environment, and animals and birds are affected differently by different types of oil. However, it's not so easy to say which kind is worst.

We distinguish between "light" and "heavy" oils. Fuel oils, such as gasoline and diesel fuel, are very "light" oils. Light oils are very volatile (they evaporate relatively quickly), so they usually don't remain for long in the environment (typically no longer than a few days). If they spread out on the water, as they do when they are accidentally spilled, they will evaporate relatively quickly. However, while they are present, they present two significant hazards. First, some can ignite or explode. Second, many light oils, such as gasoline and diesel, are also considered to be toxic. They can kill animals or plants that they touch, and they also are dangerous to humans who breath their fumes or get them on their skin.

In contrast, very "heavy" oils (like bunker oils, which are used to fuel ships) look black and sticky and evaporate slowly. If this kind of oil washes ashore, it make a big mess that can last for a long time (weeks, months, or even years). While these oils can be very persistent, they are generally significantly less acutely toxic than light oils. Instead, the threat from heavy oils comes from their ability to smother organisms. Also, if heavy oils get onto the feathers of birds, the birds may die of hypothermia (they lose the ability to keep themselves warm). We observe this same effect if sea otters become oiled. After days or weeks, some heavy oils will harden, becoming very like an asphalt road surface. In this hardened state, heavy oils will probably not harm animals or plants that come in contact with them.

In between light and heavy oils are many different kinds of medium oils, which will last for some amount of time in the environment, and will have different degrees of toxicity.

We can't tell you which oil is "worst," because the effects of any oil depend on where it is spilled, where it goes, and what animals and plants, or people, it affects.

Q. Do you know where I can find pictures of animals harmed by oil spills?

A. You'll find just a couple of photos on our website showing oiled wildlife, in the Exxon Valdez Spill image gallery. Elsewhere on the web, you can find photos in the Hinterland Who's Who report, Oil Pollution and Birds, at Tri-State Bird Rescue and Research (this site has a special kid's section and lots of pictures), and in the Smithsonian's "Ocean Planet" website. You're welcome to download and use our photos, but be sure to check or ask for permission before downloading photos from other websites.

Q. What creatures are most affected by oil spills?

A. Since most oils float, the creatures most affected by oil are animals like sea otters and seabirds that are found on the sea surface, or on the surface of beaches if the oil comes ashore. During most oil spills, seabirds are harmed and killed in greater numbers than other kinds of creatures. Sea otters can easily be harmed by oil, since their ability to stay warm depends on their fur remaining clean. If oil remains on a beach for a while, other creatures, such as snails and clams, may suffer. To learn more details about this topic, check an online report on oiled wildlife from the University of California at Davis (especially Chapter 3).

Q. How are fish affected by oil spills?

A. Most often, fish either are unaffected by oil, or are affected only briefly. However, fish can be substantially affected in some circumstances, especially when oil spills into shallow or confined waters. We have observed fish kills caused by spills of light oils and petroleum products (such as diesel fuel, gasoline, and jet fuel) into shallow water, and we've found that fish eggs in shallow water, such as salmon eggs in a streambed, can be wiped out by an oil spill. We also have seen fish kills in contained areas, such as lakes, lagoons, and some shallow-water nearshore areas, where spilled oil naturally concentrates. For example, in 1994, territorial reef fishes in nearshore areas off Puerto Rico were greatly affected by No. 6 fuel oil spilled from the Barge Morris J. Berman.

The type of oil and the timing of the release influence the severity of oil's effects on fish. Light oils and petroleum products (like gasoline) can cause acute toxicity to fish, but the toxic event is generally over fairly quickly. Heavier oils may not affect fish at all, or, in the cases of fish in larval or spawning stages, may be quite detrimental.

If there's reason to believe that fish in the vicinity of an oil spill have been contaminated by oil, fish from that area cannot be sent to markets or sold to people who might eat them. Only when testing shows that fish are no longer contaminated would they be allowed to be sold for human consumption.

Project Idea: If you would like to further research this topic yourself, check our reports on oil spills and other incidents in which fish were among the "resources at risk." These reports explain about the hazards to fish, and what the responders did to try to lessen those hazards. To see those reports, in our Historical Incidents Database, from the menu of data fields, choose "RAR" (resources at risk). In the box next to the menu, type "fish," then press Start Search. You'll see a list of links to those spill reports.