The Scientist: Susan Silbernagle, M.P.A., a researcher at the Gelfond Fund for Mercury Research & Outreach at Stony Brook University
The Answer: In general, you should not worry about consuming fish unless you eat excessive amounts all the time, or are particularly sensitive to mercury. But if you eat a lot of sushi, or fish in general—which you should, because it’s so good for you—it’s wise to be aware of mercury levels and opt for lower-mercury fish when you can.
The easiest way to figure out which fish are high in mercury is to look at where they stand (er, swim) in the food chain. Mercury accumulates in the food chain, so the higher up, the more mercury. Based on related logic, smaller, younger fish have likely built up less mercury, so if the whole fish fits on your plate, it’s probably safe. Salmon, a large, low-mercury fish, is an exception to this rule.
Some examples of the highest-mercury fish are: shark, swordfish, king mackerel, tilefish from the Gulf of Mexico, blue fin tuna (which is so threatened we shouldn’t really be eating it anyway), big eye tuna, marlin, orange roughy and some grouper species.
On the lower end of the spectrum: salmon, farmed trout, tilapia, herring, pollock and shellfish including clams, mussels, scallops and shrimp.
How often you can eat sushi without concern for elevated mercury levels depends on your body size, portion size and the mercury levels in the seafood you choose. A 132-pound person can eat a 6-ounce portion from the low mercury fish category (< 0.1 ppm) two to three times a week, or once a week from a moderate level fish like halibut (0.25 ppm) or yellow fin tuna, and still stay under the recommended blood mercury level. If you’re pregnant, or might conceive soon, you should not cut down on fish, which has important nutrients for the developing brain, but stick to low-mercury fish to protect the fetus.
In the same way that mercury accumulates in the food chain, it accumulates in your body. Methylmercury has a half-life of about 50 to 70 days in adults, so if you eat high mercury fish you should aim to spread out your seafood meals.