According to the Environmental Protection Agency, nearly all fish and shellfish contain trace amounts of elemental mercury. However, the EPA notes that mercury consumption should not be a health concern for most people. The people who should be most concerned with mercury in fish are pregnant women and small children. Fish with high levels of mercury can harm unborn babies and developing children. Therefore, eating fish low in mercury can be beneficial for pregnant women, nursing mothers, women who may become pregnant and children. Some of the most commonly eaten fish low in mercury include canned light tuna, salmon, pollock, catfish and shrimp. The FDA also notes that white albacore tuna has a higher mercury content than canned light tuna. Limit albacore tuna to one meal, or 6 ounces, per week.

Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organomercury compound. I suggest that you limit intake of the fish highest in mercury to a maximum of once per week. Pregnant and lactating women should avoid high mercury fish altogether.

Fish Highest in Mercury
King mackerel, Marlin, Orange Roughy, Shark, Swordfish, Tilefish (from the Gulf of Mexico), Tuna (Bigeye, Ahi), Bluefish and grouper: The National Resources Defense Council adds these to the list of those to avoid.

Fish with Lowest Mercury Levels
The Natural Resources Defense Council has a list of fish that contain the lowest levels of mercury. These include anchovies, catfish, flounder, hake, haddock, herring, salmon, trout, whitefish, pollock, mackerel, sardines and butterfish.