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EWG’s Consumer Guide to Seafood: Methodology
Federal nutrition guidelines advise Americans to eat more fish and shellfish but do not make it easy for consumers to figure out which kinds of low-mercury fish provide lots of omega-3s. EWG believes that the federal government’s proposed seafood guidance is flawed because it relies on a safe dose level established by the EPA in 2001 and now considered outdated and too high by many scientists.
EWG’s Seafood Calculator and Wallet Guide consider both the beneficial omega-3 fatty acid content and mercury levels in commercial species so that consumers can choose fish and shellfish better for their health.
Wallet Guide and Calculator Seafood Ratings
EWG classified seafood into five categories using the parameters described in the table below.
EWG Seafood Rating Category
Omega-3 content in 4-ounce serving*
|EWG Best Bets||At least 50% of Dietary Guidelines weekly recommendation||Eat up to 12 ounces weekly – Mercury = Less than or equal to 0.1 ppm||Best sustainability rating|
|Good Choices||At least 25% of Dietary Guidelines weekly recommendation||Eat up to 12 ounces weekly –Mercury = Less than or equal to 0.1 ppm||Any|
|Low Mercury, But Low Omega-3s||Less than 25% of Dietary Guidelines weekly recommendation||Eat up to 12 ounces weekly - Mercury = Less than or equal to 0.1 ppm||Any|
|Mercury Risks Add Up||Any||Can eat 4 to 8 ounces weekly – Mercury = 0.1 to 0.3 ppm||Any|
|Avoid||Any||Eat less than 4 ounces weekly – Mercury = greater than 0.3 ppm||Any|
* The Dietary Guidelines for Americans recommend that pregnant women and people with heart disease consume an average of 250 mg of DHA and EPA daily, or 1,750 mg a week (USDA 2010).
** EWG’s recommended weekly limit for mercury ensures that a woman weighing 166 pounds, the average weight for American women, ingests no more than 75 percent of EPA’s outdated safe level for mercury (EPA 2001) when she eats 12 ounces of seafood per week, based on the average mercury concentration in each species (Karimi 2012).
*** Sustainability ratings derived by Monterey Bay Aquarium Seafood Watch™ program.
To minimize the risk of exposing the developing fetus to too much mercury, EWG recommends that pregnant women and young children aim to keep their mercury intake well below the EPA guideline. EWG’s Seafood Calculator recommends that pregnant women and children not exceed 75 percent of EPA’s outdated mercury safe level (technically called the “reference dose”). For all other adults we base our calculations on EPA’s reference dose. Our analysis bases calculations on a woman weighing 166 pounds, the average for American women. EWG’s Seafood Calculator allows people to determine the appropriate serving size and frequency based on their bodyweight. Lighter adults and children should further limit their mercury intake by eating smaller portions or eating that species less frequently.
Portion size assumptions
EWG’s Seafood Calculator is based on a four-ounce portion, as specified in the Dietary Guidelines for Americans. Four ounces is roughly the size and weight of a deck of cards or equivalent to one tin of sardines or anchovies and four to six pieces of sashimi. Restaurants generally serve larger portions than these. Young children should eat smaller portions than adults. The FDA and the Dietary Guidelines for Americans do not specify a standard serving size for children. EWG’s calculator bases recommendations on the assumption that a child size portion is one ounce for every 20 pounds of weight.
Data sources for mercury concentrations in commercial fish and seafood
While mercury concentrations can vary widely within a particular species, it is possible to make some generalizations based on the average mercury content recorded in scientific monitoring studies. The FDA bases its assessment of mercury risks from seafood on incomplete data. Its recent benefit-risk assessment for children employs a study performed in 1978 and about 5,000 individual fish samples collected between 1990 and 2010 (FDA 2014b). FDA’s monitoring program has been criticized for sampling too few fish to account fully for variation in mercury concentrations (Karimi 2012).
To evaluate the amount of mercury in commercial fish and shellfish species, we used a large database of 300 studies, plus FDA’s monitoring data, assembled by Roxanne Karimi and Nicolas Fisher of Stony Brook University and Timothy Fitzgerald of the Environmental Defense Fund (Karimi 2012). These researchers found that in many cases mercury concentrations in their database exceeded measurements taken by FDA monitoring. They suggested that the FDA broaden its investigation of mercury in seafood and consider other data sources when determining safe consumption levels for seafood. Mercury estimates in the Stony Brook Seafood Hg Database were more than 20 percent higher than FDA estimates for 27 of 58 species examined, with notable differences in moderate- or higher-mercury species, including marlin, king mackerel, weakfish/seatrout, and freshwater trout. The researchers found relatively few monitoring studies from Asia and the Pacific Ocean even though substantial quantities of fish and shellfish from these regions reach the U.S. We use a weighted average of mercury concentrations for fish and shellfish species and sub-species using the name classifications from the Stony Brook Hg Database.
The Stony Brook Seafood Hg Database includes studies published through December 2010. To provide additional data for commercially important species, we added information from four more studies -- three studies of canned tuna (Consumer Reports 2011, Mercury Policy Project 2012, Gerstenberg 2009), and one study of mercury in thousands of samples of Great Lakes walleye – a regionally important fish for people residing in the inland U.S. (Monson 2011).
Data sources for omega-3 levels in seafood
EWG’s Seafood Calculator uses data reported by the USDA Nutritional Database last year (data release SR-26) to evaluate the concentrations of DHA and EPA in seafood (USDA 2013). We evaluated measurements from both raw and cooked fish samples.
Omega-3 data are less robust than mercury measurements. The USDA database contains far fewer measurements from each species and reports substantial variation among samples. Even so, it generally aligns with a global database of omega-3 fatty acids developed by the World Health Organization and the Food and Agricultural Organization in their 2010 report on seafood safety (WHO-FAO 2010). The USDA data are more relevant to the species sold commercially in the U.S. For that reason, EWG used the USDA dataset as the basis for our consumer recommendations. When both raw and cooked fish were analyzed we included data from both types. Omega-3 concentrations for popular species were in the ranges reported in independent monitoring efforts (Shim 2004). Scientific institutions should develop better data on omega-3 levels in common seafood species.
EWG’s Seafood Calculator bases its recommendations for omega-3 consumption on the recommendation in the Dietary Guidelines for Americans that pregnant women and people with heart disease consume an average of 250 mg of DHA+EPA daily or 1,750 mg per week (USDA 2010).
Challenges of providing consumer guidelines for seafood
Consumers’ efforts to purchase healthy seafood are complicated by several factors. Commercial and recreational fish are sold under many common or “market names”. Consumers seeking Coho salmon might find it labeled “silver salmon” at a fish counter. In other cases a fish labeled “ahi tuna” at a store or restaurant could actually be bigeye or yellowfin tuna, species with different mercury concentrations.
Fish are commonly mislabeled at the point of sale (GAO 2009). Advocacy group Oceana analyzed DNA from fish samples collected between 2010 and 2012 and found that many samples were mislabeled, most commonly mis-marketed were snapper and tuna. Only 7 of 120 samples of fish sold as “red snapper” were actually correctly labeled, the remaining 113 samples were another type of fish (Oceana 2013). In some cases consumers unknowingly purchase higher mercury varieties, including king mackerel sold as grouper and escolar labeled white tuna.
Our analysis bases portion size recommendations on the average mercury value from existing monitoring studies. However mercury concentrations vary widely among individual fish samples and among sub-species of the same fish. In order to provide straightforward advice we group together sub-species of fish like perch, seatrout, seabass and shark. However consumers who eat a particular sub-species of these fish may be at greater risk if it has higher mercury concentrations.
People who catch or eat non-commercial species may eat more mercury or other contaminants than reflected in our analysis of commercial species. The EPA has a clearinghouse of state and local fish and shellfish advisories for mercury and other contaminants in seafood (EPA 2014). The FDA/EPA draft advisory recommends that consumers limit non-commercial species to no more than six ounces per week. Children should eat only one to three ounces of any non-commercial fish and seafood per week.