Oliy fish is the name for several species of fish, which have a lot of fat (and oil) in their muscle tissue. Oily fish commonly have ten percent or more fat/oil in their tissue. Some species have over 30%. The other kind of fish is called whitefish: Whitefish only contain oil/fat in tissues around the liver. Common fish that are called "oily" include Atlantic herring, European sprat, Salmon, Atlantic mackerel, Tuna, Eel, Greenland halibut and different species of catfish. 