Oily fish (oil-rich fish, pelagic fish) are those fish which have oils throughout the fillet and in the belly cavity around the gut, rather than only in the liver like white fish. Oily fish generally swim in mid-waters or near the surface (the pelagic zone). Oily fish are a good source of Vitamins A and D as well as being rich in Omega 3 fatty acids. For this reason the consumption of oily fish has been identified as more beneficial to humans than white fish. Amongst other benefits, studies suggest that the Omega 3 fatty acids in oily fish may help sufferers of depression, reduce the likelihood of heart disease and improve inflammatory conditions such as arthritis. However, oily fish are predators, high up in the food chain, and thus more likely to contain toxic substances than white fish.