If you could make your question more clear, that would help us to answer you.
If you just want to know what an oil spill is, usually it refers to a spill of crude oil by an oil tanker. An oil spill can be catastrophic to an ecosystem. The oil kills various types of animals and plants by sticking to them and interfering with their functioning; for example, it coats feathers of sea birds when they land in the water, and the birds can no longer fly. The oil washes up on shore, where it kills even more species.
If you do a search for Exxon Valdez, you will get detailed descriptions of what a large oil spill can do. http://www.eoearth.org/article/Exxon_Valdez_oil_spill
Oil spills can cause major damage to the marine environment system. Therefor occurrence of oil spills are detected using satellite and aircraft based surveillance using Synthetic Aperture Radar (SAR) system. Images taken by SAR systems show oil spill as dark areas. However, due to some natural phenomenon some other areas, which are not oil spill, also appear as dark dark ares and it becomes difficult to differentiate them from real oil spills. Such areas that look like oil spill on satellite images, but in reality are not oil spills, are called look-alikes.
I am not sure what exactly you are looking for. Do you mean that you want to know what an oil spill is or possible groups of fish or sea life that can look like oil spills?