Large fish that tend to live longer, such as swordfish, tilefish, shark and king mackerel, typically contain the highest amounts of mercury, advises the U.S. Environmental Protection Agency. Because all fish contain some mercury, limiting dietary intake of other fish reduces the risk of mercury building up in the blood.
The predators in the ocean generally contain the most mercury and methylmercury because of the quantity of fish they consume. Tilefish usually have the highest levels, with fish measuring as many as 1.45 parts per million of mercury. Other fish meeting the threshold of more than 0.5 parts per million, or the highest category of mercury, include king mackerel, shark, swordfish, marlin, orange roughy and both bigeye and ahi tuna.
Because mercury levels build up in the blood over time, it is best to eat other large fish with high levels of mercury sparingly. These fish include grouper, Spanish mackerel, Chilean sea bass, yellowfin tuna and canned albacore tuna. Eating three servings or less of these fish per month prevents side effects from mercury.
Fish with the lowest mercury levels include smaller species of both fish and shellfish. These varieties include catfish, clams, fresh and canned salmon, scallops and tilapia. Freshwater trout, shrimp, oysters and flounder are other safe choices. People can eat these as often as desired unless local advisories warn of higher-than-normal mercury levels.