The U.S. Food and Drug Administration (FDA) is concerned that the mercury levels in some fish and shell fish may be harmful to you, particularly for pregnant women and growing children, so they have published a chart classifying fish by mercury concentrations. Too much mercury can damage the brains of children and babies, and the nerves and brains of adults. Women can pass mercury to their babies during pregnancy or breast-feeding.

The general rule is that the older and larger the fish, the more time it has to accumulate mercury and the greater the chance for it to have larger amounts of mercury in it. Expect to find higher levels of mercury in long-living fish such as swordfish, king mackerel, tilefish, marlin, albacore tuna, and sharks. Smaller fish such as trout or salmon don’t live long enough to accumulate much mercury. Small tuna have far less mercury than large tuna, and most canned tuna contains smaller varieties. Consumer Reports analyzed mercury levels in canned tuna and found all samples tested were within FDA standards, but some cans had higher levels of mercury than others (Consumer Reports, Feb 9, 2023).

How Does Mercury Get Into Fish?
Mercury is found in coal, rocks, and soil and can be released into air when coal and other fuels are burned, soil erodes or is mined, or during forest fires or volcanic eruptions. The mercury then gets into the clouds and comes down when it rains or snows to fill streams, ponds, lakes and oceans, where it is converted to methyl mercury, the most poisonous form of mercury. Mercury is also released directly into water through wastewater and industrial pollution. In other food sources, mercury accumulates when it is taken up by plants in the soil that are then eaten by humans, animals, fish and shellfish. Mercury is not removed or inactivated by cooking.

FDA Listings of Seafood by Mercury Content
• Safest fish and shellfish: Anchovies, Atlantic croaker, Atlantic mackerel, Black sea bass, Butterfish, Catfish, Clam, Cod Crab, Crawfish, Flatfish (flounder, plaice, sole), Haddock, Hake, Lobster, Mahi mahi, Monkfish, Mullet, Oyster, Pacific chub mackerel, Perch, Pickerel, Pollock, Salmon, Sardine, Scallop, Shad, Shrimp, Skate, Smelt, Squid, Tilapia, most canned Tuna, Trout, Whitefish, Whiting
• Good choices: Bluefish, Buffalofish, Carp, Chilean sea bass, Grouper, Halibut, Herring, Rockfish, Sablefish, Sheepshead, Snapper, Spanish mackerel, Striped bass, Tilefish, most smaller Tuna, Weakfish, White croaker
• Avoid: King mackerel, Marlin, Orange roughy, shark, swordfish, Tilefish, larger Tuna

My Recommendations
I agree with the American Heart Association, which says that eating two servings of fish per week is good for you. However, since larger and older fish have the most mercury, try to eat primarily younger and smaller fish. See my report on Eat Fish Twice a Week.