For you fish eaters, the U.S. FDA stated in 2001 that fish with the highest mercury levels are tilefish, swordfish, mackerel, shark, white snapper, and tuna. The lowest levels are found in salmon, flounder, sole, tilapia, and trout (though these fish are high in other toxins, in some waters).
If you’re eating fish-oil pills hoping to avoid heart disease, consider that you might be getting cancer-causing toxins in the bargain. For instance, highly toxic PCBs have been found in Great Lakes fish. Family Practice News reported in 1989 that you’d have to drink water from the Great Lakes for 100 years to get the same amount of PCBs in one serving of trout or salmon from those waters. Similar findings link hydrocarbon pollution to fish in Puget Sound, Boston Harbor, and more. New Orleans has extraordinarily high rates of cancer, where residents eat fish and shellfish daily.
Farmed fish were thought to be safer, but recent data suggests other reasons to be concerned about the way fish bred in farms absorb toxins as well.
If you must eat fish, avoid those high in mercury. A less risky way to obtain Omega fats is to eat flaxseed or flax oil, as well as a diet rich in a variety of greens and seeds/nuts.