Oily fish have oil in their tissues and in the belly cavity around the gut.Their fillets contain up to 30% oil, although this figure varies both within and between species. Examples include small forage fish, such as sardines, herring and anchovies, and other larger pelagic fish, such as salmon, trout, tuna and mackerel.Oily fish can be contrasted with whitefish, which contain oil only in the liver, and much less overall than oily fish. Examples of whitefish are cod, haddock and flatfish.