Why isn't my fish listed in the calculator? This list of fish species comes from data published by the FDA. The list contains some of the most commonly eaten species of fish, but is limited to fish the government has tested. Do not assume that unlisted fish are low in mercury or safe to eat. The data used in the calculator are averages from the FDA, but individual fish can be higher or lower. Additionally, some seafood may be low in mercury, but might be high in other contaminants such as PCB's, cadmium or other metals, and other pollutants. For example, there are alarming reports regarding contamination of farm-raised salmon.

What about differences between canned tuna? Most canned tuna is labeled as either "albacore" or "chunk light". There are other species of canned tuna, as well. Albacore tuna, according to FDA data, contains 3 times as much mercury as chunk light (0.353 ppm vs. 0.118 ppm). Cans of chunk light tuna usually contain skipjack tuna, which is a smaller species and averages lower mercury levels. However, a Chicago Tribune investigation found that chunk-light canned tuna sometimes contains yellowfin tuna (0.325 ppm), but is not labeled correctly. Also, a study found that troll-caught albacore mercury levels are lower (0.14 ppm vs. 0.353 ppm) because they are younger than longline-caught albacore. Read canned tuna labels carefully to find out which species it contains! You can compare the mercury levels between all types of tuna in the mercury calculator above.

What about fresh and frozen tuna? Tuna is a popular seafood item. Tuna comes as steaks or, in Japanese cuisine, as sushi or sashimi. In restaurants and stores, tuna is labeled "ahi", the Hawaiian word for tuna, which could be either bigeye tuna (0.639 ppm) or yellowfin tuna (0.325 ppm), both high in methylmercury. Consumers should ask specifically what kind of tuna they are getting. For example, "maguro" and "toro" in sushi restaurants refer to the back or belly of the tuna, not the species. Sushi and sashimi can also be albacore or bluefin tuna.

What about salmon? Salmon has low average mercury levels, according to FDA data. However, the FDA found some salmon with mercury levels as high as 0.19 ppm.