Mercury is not evenly distributed across seafood. Large predators that live for years and eat many other fish tend to accumulate the highest methylmercury concentrations. That is why U.S. advice singles out particular species rather than warning people away from seafood as a whole.
The seven FDA/EPA Choices to Avoid
Mercury is not distributed evenly across fish. Large, long-lived predators eat many organisms over time and can accumulate methylmercury as they move through the food web. That is why species and typical size are more informative than flesh color or whether a fish is sold fresh, frozen, or canned. Two products that shoppers casually call “tuna” can come from different species and size classes with meaningfully different mercury profiles. The label matters because the biological identity of the fish, not the prestige of the product, drives the pattern.
For people who might become or are pregnant or breastfeeding, and for young children, FDA/EPA list king mackerel, marlin, orange roughy, shark, swordfish, tilefish from the Gulf of Mexico, and bigeye tuna as Choices to Avoid because they have the highest mercury levels in the chart.
The same agencies encourage lower-mercury seafood because fish also supplies nutrients important in healthy eating. During pregnancy or breastfeeding, their guidance is 2–3 servings totaling 8–12 ounces per week from Best Choices.
How large is the concentration gap?
Historical FDA monitoring data make the scale easier to see:
| Fish | Approx. mean mercury (ppm) |
|---|---|
| Swordfish | ~1.00 |
| Shark | ~0.98 |
| Canned albacore/white tuna | ~0.35 |
| Canned light tuna | ~0.126 |
| Sardine | ~0.013 |
| Shrimp | ~0.009 |
In this reference, average swordfish is roughly 75 times higher than sardine, although individual fish vary. The table explains the pattern; current local public-health advice should guide actual consumption decisions.
Why predators accumulate so much
The practical message is not to stop eating fish. Fish provides protein and, depending on the species, omega-3 fats and other nutrients; safety guidance is designed to preserve those benefits while reducing the highest exposures. Rotating species and choosing lower-mercury options more often is generally more useful than repeatedly eating one large predator. During pregnancy, breastfeeding, and childhood, species-specific official advice becomes especially important, because those recommendations are written around developmental sensitivity and should take priority over generic internet rankings.
Methylmercury enters aquatic food webs and remains in animal tissues. A predator consumes mercury already present in its prey, then keeps accumulating it over a long life. Size is therefore a useful clue, but species-specific guidance is more reliable than judging a fillet by how large it looks at the store.
Tuna is a good example of why species matters
FDA/EPA put canned light tuna in Best Choices, albacore/white and yellowfin tuna in Good Choices, and bigeye tuna in Choices to Avoid. FDA monitoring lists canned light tuna at about 0.126 ppm mean mercury versus roughly 0.35 ppm for canned albacore. The word “tuna” alone is not enough to describe exposure.
It is also worth resisting the temptation to copy a fish list from one country directly into another. FDA/EPA, European authorities, and national agencies may use different market names, locally available species, and consumption categories. A recommendation written around swordfish, shark, king mackerel, or specific tuna categories in the United States will not map perfectly onto every overseas seafood counter. When a local fish is missing from a foreign list, appearance is not enough to guess its mercury profile. Checking the scientific name or local health-authority advice reduces naming confusion while preserving variety in the diet.
The useful rule
Canned and frozen products still require attention to the name on the label. “Tuna” is an umbrella term, and U.S. guidance distinguishes categories such as canned light tuna and albacore because they do not have identical mercury levels. Other markets use different common names and regulatory lists. That detail is not bureaucratic trivia; it is an attempt to separate fish with different exposure patterns. For a food you buy often, identifying the species or category when possible and varying your choices is more informative than judging risk by price, color, or preservation method.
Keep seafood varied and favor the lower-mercury categories. If pregnancy, breastfeeding, or feeding young children makes mercury guidance especially relevant, use the FDA/EPA chart by species rather than trying to estimate risk from size, price, or whether the fish is fresh or frozen.
Sources
- FDA/EPA — Advice about Eating Fish — Current U.S. Best Choices, Good Choices, and Choices to Avoid categories.
- FDA — Mercury Levels in Commercial Fish and Shellfish (1990–2012) — Mean mercury measurements used to illustrate concentration differences.