Oily fish are fish species with oil in soft tissues and in the coelomic cavity around the gut. Their fillets may contain up to 30% oil, although this figure varies both within and between species. Examples of oily fish include small forage fish such as sardines, herring and anchovies, and other larger pelagic fish such as salmon, trout, tuna, swordfish and mackerel.

Oily fish can be contrasted with whitefish, which contain oil only in the liver and in much less overall quantity than oily fish. Examples of whitefish are cod, haddock and flatfish. White fish are usually demersal fish which live on or near the seafloor, whereas oily fish are pelagic, living in the water column away from the bottom.

Oily fish meat is a good source of important fat-soluble vitamins such as Vitamin A and D, and is rich in omega-3 fatty acids (white fish also contain these nutrients but at a much lower concentration). For this reason the consumption of oily fish rather than white fish can be more beneficial to humans, particularly concerning cardiovascular diseases; however, oily fish are known to carry higher levels of contaminants (such as mercury or dioxin or POPs) than whitefish. Among other benefits, studies suggest that the omega-3 fatty acids in oily fish may help improve inflammatory conditions such as arthritis.

 

Note:   The above text is excerpted from the Wikipedia article Oily fish, which has been released under the GNU Free Documentation License.