Flounder is one of the lowest-mercury fish you can eat. The FDA and EPA jointly place it in their “Best Choices” category, the safest tier for mercury, meaning you can eat two to three servings per week without concern. With an average mercury concentration of just 0.056 parts per million (ppm), flounder falls far below the 0.3 ppm threshold that environmental agencies use as a general safety benchmark and nowhere near the FDA’s action level of 1.0 ppm.
How Flounder Compares to Other White Fish
FDA testing of commercial seafood from 1990 to 2012 measured flounder (grouped with other flatfish like sole and plaice) at a mean of 0.056 ppm mercury across 71 samples. That puts it roughly on par with haddock (0.055 ppm) and well below cod (0.111 ppm). Tilapia edges out nearly everything at 0.013 ppm, but flounder remains solidly in the low-mercury range. For comparison, high-mercury fish like swordfish and king mackerel typically measure above 0.7 ppm.
Mercury Varies by Flounder Species
Not all flounder is identical. A study of flatfish caught off the New Jersey coast found meaningful differences between species. Winter flounder averaged just 0.06 ppm mercury, while summer flounder (also called fluke) came in at 0.14 ppm. Windowpane flounder was slightly higher at 0.18 ppm. All three still fell comfortably below the 0.3 ppm advisory threshold, but if you’re choosing between varieties and want to minimize exposure as much as possible, winter flounder is the lowest option.
These differences come down to diet and size. Larger, longer-lived flatfish accumulate more mercury over time. Summer flounder grow bigger than winter flounder, which partly explains the gap.
Serving Limits for Pregnant Women and Children
Because flounder sits in the FDA’s “Best Choices” tier, pregnant and breastfeeding women can safely eat 8 to 12 ounces per week (roughly two to three servings). Children can have two servings per week. In fact, flounder is on the short list of fish that the Dietary Guidelines for Americans specifically names as safe for children even at higher intake levels, alongside options like salmon, shrimp, tilapia, and pollock.
The key recommendation for all groups is variety. Eating different types of low-mercury seafood each week provides a broader range of nutrients while spreading out any trace contaminant exposure across multiple sources.
What About PCBs and Other Contaminants?
Mercury isn’t the only contaminant people worry about in fish. A U.S. Geological Survey study looked at both mercury and PCBs (industrial chemicals that persist in the environment) in summer flounder and found both were relatively low. Male flounder tended to carry higher PCB levels than females, while mercury levels were essentially the same regardless of sex. Overall, the researchers concluded the concentrations posed low risk to consumers and supported the continued operation of the summer flounder fishery.
Flounder’s Nutritional Profile
Beyond its low contaminant levels, flounder is a lean, protein-rich fish. A 100-gram serving of raw flounder provides about 12 grams of complete protein with only 2 grams of total fat and 0.4 grams of saturated fat. It contains zero carbohydrates and no sugar. Flounder is also a good source of selenium, a mineral that plays a role in thyroid function and may help the body counteract mercury’s effects, and provides iodine at about 14.3 micrograms per 100 grams.
Flounder is not an especially rich source of omega-3 fatty acids compared to fattier fish like salmon or sardines. If omega-3 intake is your primary goal, those options deliver more per serving. But as an everyday, low-calorie protein source with minimal mercury risk, flounder is hard to beat.