What are oily fish
Oily fish have oil in their tissues and in the belly cavity around the gut. Their fillets contain up to 30 percent oil, although this figure varies both within and between species. Examples include small forage fish, such as sardines, herring, whitebait and anchovies, and other larger pelagic fish, such as salmon, trout 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. Whitefish 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 are a good source of vitamins A and D, and are rich in omega-3 fatty acids (whitefish contain the same nutrients but at a much lower level). For this reason the consumption of oily fish rather than whitefish can be more beneficial to humans, particularly concerning cardiovascular diseases, but oily fish are known to carry higher levels of contaminants (such as mercury or dioxin) than whitefish. Amongst other benefits, studies suggest that the Omega 3 fatty acids in oily fish may help improve inflammatory conditions such as arthritis.