Mercury in fish is a toxic metal that accumulates in the flesh of fish as methylmercury, a form that your body absorbs easily and that can damage the nervous system at high levels. Nearly all fish contain at least trace amounts, but the concentration varies enormously by species. Swordfish averages about 1.0 parts per million (ppm) of mercury, while salmon and tilapia sit below 0.025 ppm. The type and amount of fish you eat determines whether mercury is a real concern for you.
How Mercury Ends Up in Fish
Mercury enters the environment from both natural sources like volcanic eruptions and human activities like burning coal. Once it reaches waterways, microorganisms in sediment and water convert it into methylmercury, a process that dramatically increases its toxicity. This conversion step is what makes mercury in aquatic environments so much more dangerous than mercury sitting in soil or rock.
Methylmercury crosses biological membranes easily, which means it gets absorbed quickly by algae and tiny organisms at the base of the food web. Small fish eat those organisms, and larger fish eat the small fish. At each step, mercury concentrations increase because the predator absorbs all the mercury its prey accumulated over a lifetime. This is why the biggest, longest-lived predatory fish carry the highest mercury loads. A swordfish that has spent years eating other fish has been stockpiling mercury with every meal.
Mercury Levels in Common Fish
FDA monitoring data shows a wide range of mercury concentrations across species. The differences are striking:
- Very low mercury (under 0.05 ppm): Salmon (0.02 ppm), tilapia (0.01 ppm), shrimp, sardines, anchovies
- Low to moderate mercury: Canned light tuna (0.13 ppm), skipjack tuna (0.14 ppm)
- Moderate to high mercury: Canned albacore tuna (0.35 ppm), yellowfin tuna (0.35 ppm)
- High mercury: Bigeye tuna (0.69 ppm), swordfish (1.0 ppm)
The pattern is consistent: small, short-lived fish that eat low on the food chain have minimal mercury. Large predators that live for years accumulate the most. This is why “eat smaller fish” is one of the simplest ways to reduce exposure. Canned light tuna, which is typically skipjack, contains roughly a third the mercury of albacore tuna.
What Mercury Does to Your Body
Methylmercury is a potent neurotoxin. At high exposure levels, it targets the nervous system and can cause loss of peripheral vision, tingling or “pins and needles” in the hands, feet, and around the mouth, difficulty with coordination and balance, impaired speech and hearing, and muscle weakness. These symptoms reflect damage to the brain and nerves, and they typically develop from sustained high exposure rather than a single meal.
The developing brain is far more vulnerable than an adult brain. When pregnant women consume high-mercury fish regularly, methylmercury crosses the placenta and can affect the baby’s cognitive development, memory, attention, language ability, and fine motor skills. This is why guidelines are strictest for women who are pregnant or breastfeeding and for young children.
For most adults eating a normal amount of fish, mercury exposure stays well below harmful levels. The concern is really about patterns: eating high-mercury species frequently over months or years, or eating large quantities during pregnancy.
How Much Fish Is Safe to Eat
The EPA and FDA jointly recommend eating two to three servings per week from their “Best Choices” category, which includes low-mercury options like salmon, tilapia, shrimp, sardines, pollock, catfish, and canned light tuna. Alternatively, you can eat one serving per week from the “Good Choices” list, which includes moderate-mercury fish like albacore tuna and yellowfin tuna. A serving is about 4 ounces for adults.
For women who are pregnant or breastfeeding, the Dietary Guidelines for Americans recommend 8 to 12 ounces of low-mercury seafood per week. Children should get about two servings per week, sticking to the lowest-mercury options. If you eat fish caught by family or friends in local waters, check state fish advisories for that body of water. When no advisory exists, the recommendation is to eat only one serving of that fish per week and skip other fish that week.
Can You Remove Mercury by Cooking?
Unlike some other contaminants that concentrate in fat or skin, mercury accumulates in the muscle tissue of fish, which is the part you eat. Trimming the skin, removing fat, or using different cooking methods does not reduce mercury content. The only way to lower your exposure is to eat less of the high-mercury species or switch to lower-mercury fish. This is an important distinction because many people assume that careful preparation can make any fish safe, and it simply doesn’t work that way with mercury.
Balancing the Benefits and Risks
Fish remains one of the best dietary sources of omega-3 fatty acids, high-quality protein, and several vitamins and minerals. The goal isn’t to avoid fish entirely. It’s to choose wisely. Swapping swordfish steaks for salmon fillets, or choosing canned light tuna over albacore, makes a meaningful difference in mercury exposure while preserving the nutritional benefits.
If you eat fish two or three times a week and stick primarily to low-mercury species, your exposure will stay well within safe limits. The people who need to pay closest attention are those who eat fish daily, favor large predatory species, or are pregnant, breastfeeding, or feeding young children. For everyone else, a little awareness about which fish land in the high-mercury category is usually enough.