Mercury in Seafood: Which Fish to Eat and Which to Avoid

Mercury is a naturally occurring heavy metal found in trace amounts in soil, water, and air. Industrial processes — particularly coal combustion — release additional mercury into the atmosphere, which eventually settles into rivers, lakes, and oceans where bacteria convert it into methylmercury (MeHg): the organic form that is absorbed into fish tissue and passes up the food chain.

The critical mechanism is bioaccumulation. Small fish absorb methylmercury from water and algae. Medium-sized fish eat many small fish. Large, long-lived predators eat many medium fish over decades. By the time you eat a swordfish or a large tuna, you're eating the accumulated mercury from thousands of smaller organisms. This is why species at the top of the food chain carry levels that can be 1–10 million times higher than the concentration in the surrounding water.

Once consumed, methylmercury is almost entirely absorbed from the gut into the bloodstream. It crosses the blood-brain barrier and the placental barrier with ease, which is why developing brains — in the fetus, in infants, in young children — are the most vulnerable targets.

EFSA's 2026 survey covered nearly 23,000 interviews across 29 European countries in two waves: 2023 and late 2024. The findings reveal a troubling trend.

Consumption of species at the highest mercury risk tier — those at or above the EU maximum level of 1 mg methylmercury per kilogram of fish wet weight — more than doubled among frequent eaters in just one year. Awareness of mercury in fish was relatively common, but that awareness was not translating into behaviour change. Respondents cited taste, price, and perceived health benefits as the main reasons they continued eating high-mercury species despite knowing the risks.

This knowledge-behaviour gap is precisely why the EFSA findings are significant. The people eating swordfish and large tuna most frequently often do so because they associate seafood with health. The irony is that for certain species, that association can work against them.

Around 1 in 3 people in Europe, including pregnant women, are consuming potentially unsafe amounts of fish species with the highest methylmercury contamination levels — EFSA, February 2026.

The FDA's monitoring data on commercial seafood provides the most comprehensive species-level picture available. Mercury content is measured in parts per million (ppm) of wet weight.

Highest mercury — avoid or strictly limit:

The FDA and EPA classify tilefish (Gulf of Mexico), swordfish, shark, king mackerel, bigeye tuna, orange roughy, and marlin as fish to avoid entirely for pregnant women, women who may become pregnant, breastfeeding mothers, and young children. For the general adult population, consumption should be strictly limited or avoided.

Moderate mercury — limit to 1 serving per week:

• Albacore (white) canned tuna
• Yellowfin tuna (fresh/frozen)
• Chilean sea bass / Patagonian toothfish
• Bluefish
• Grouper
• Spanish mackerel

Lower mercury — 2–3 servings per week generally considered safe:

• Salmon (wild Alaska: ~0.022 ppm)
• Sardines and anchovies
• Atlantic mackerel (not king mackerel)
• Herring
• Oysters, clams, scallops
• Shrimp and prawns
• Pollock, haddock, tilapia, flounder
• Canned light tuna (not albacore)

The contrast is stark. Wild Alaska salmon averages 0.022 ppm — about 45 times less mercury than swordfish. Switching from swordfish to salmon doesn't mean giving up seafood; it means eating a species that is simultaneously lower in mercury and higher in omega-3 fatty acids.

Methylmercury is a developmental neurotoxin. Its effects are most severe when exposure occurs during periods of rapid brain growth — which is precisely what happens in the fetal period and early childhood.

Research published in PMC/NIH has documented the effects of prenatal methylmercury exposure across multiple population studies. At higher exposure levels, effects include impaired cognitive function, delayed language development, reduced fine motor skills, and disrupted auditory processing. Critically, these impairments can occur at blood mercury levels that produce no symptoms in the mother herself — meaning a pregnant woman may feel completely well while her developing baby is being harmed.

The FDA and EPA joint guidance recommends that pregnant women, women who may become pregnant, breastfeeding mothers, and children aged 1–11:

• Eat 2–3 servings per week of fish from the "best choices" low-mercury list (salmon, sardines, shrimp, canned light tuna, pollock, tilapia)
• Limit to 1 serving per week of fish from the "good choices" moderate-mercury list
• Avoid entirely: shark, swordfish, king mackerel, tilefish (Gulf), bigeye tuna, orange roughy, marlin

This is a case where the guidance does not say "avoid all fish" — it says eat the right fish. The omega-3 fatty acids in low-mercury seafood, particularly DHA, actively support fetal brain development. The goal is to maximise the nutritional benefits while eliminating the species that carry meaningful risk.

For adults outside these vulnerable groups, the risk from moderate fish consumption is lower, but regular consumption of the highest-mercury species still warrants attention. Chronic low-level methylmercury exposure has been associated in some research with cardiovascular effects and subtle neurological changes in adults, though the evidence here is less definitive than for developmental effects.

Tuna deserves special mention because it is the most commonly consumed fish in many countries, and it is not a single category.

Canned light tuna (primarily skipjack) averages around 0.128 ppm — placing it comfortably in the low-mercury "best choices" tier. Most people can eat this 2–3 times per week without concern.

Canned albacore (white) tuna averages 0.350 ppm — more than twice as high. The FDA recommends limiting this to 1 serving per week for vulnerable populations.

Bigeye tuna (often served as sushi or tuna steak) averages 0.689 ppm — in the high-mercury avoid category.

Bluefin tuna — prized in high-end sushi — can reach levels comparable to swordfish.

The label matters enormously here. "Tuna" on a restaurant menu or a product label does not specify species, and the difference in mercury content between a can of skipjack light tuna and a portion of bigeye or bluefin is substantial.

A few principles make navigating this easier in practice:

Size and longevity are the key indicators. Large, long-lived predatory fish are almost always higher in mercury than smaller, shorter-lived species. A sardine lives 1–2 years. A swordfish can live 9 years and reach 650 kg. The accumulation difference is enormous.

Farmed vs. wild is not the primary concern here. Mercury content is determined mainly by what the fish eats and how long it lives, not whether it was farmed or wild-caught. Farmed salmon is low in mercury for the same reason wild salmon is: it's a relatively short-lived species that doesn't bioaccumulate to dangerous levels.

Cooking does not reduce mercury. Grilling, steaming, poaching, or frying a high-mercury fish does not reduce its methylmercury content. The only way to avoid mercury exposure from a given species is not to eat it.

Omega-3 supplements can substitute for high-mercury fish. If you want the cardiovascular and cognitive benefits of omega-3s without high-mercury species, fish oil supplements derived from small, low-mercury fish (anchovies, sardines, mackerel) or algae-based DHA supplements provide the nutrients without the exposure.

One of the more striking aspects of the EFSA 2026 data is that awareness was not the main problem. Mercury was the most commonly recognised chemical contaminant in fish — more people knew about mercury in seafood than about most other food safety issues. Yet that awareness was not changing eating habits.

The survey found that pregnant women, despite higher awareness of the relevant guidance, were not consuming significantly less high-mercury fish than the general population. Taste preferences, habitual choices, and the general perception of seafood as "healthy" were consistently more influential than specific safety knowledge.

This is a familiar pattern in food safety: knowing a risk exists and acting on that knowledge in your weekly shop are two different things. The information needs to be accessible at the point of decision — when you're looking at a menu, scanning a label, or choosing at a fish counter.

The nutritional case for eating fish is strong, and the Mayo Clinic and other major health authorities are emphatic that the benefits outweigh the risks — when the right species are chosen. The problem is not fish. The problem is a handful of large, long-lived predatory species that have accumulated mercury to levels that pose a measurable risk, particularly to developing brains.

The practical guidance is simple:

• Eat salmon, sardines, anchovies, herring, oysters, shrimp, and canned light tuna freely
• Limit albacore tuna and similar moderate-mercury choices to once a week
• Avoid swordfish, shark, king mackerel, bigeye tuna, tilefish (Gulf), orange roughy, and marlin — especially during pregnancy and when feeding young children

IngrediCheck can help you navigate these distinctions at the product level — flagging seafood products that contain high-mercury species, cross-referencing fish-based ingredients in processed foods (fish sauces, seafood snacks, supplements) against mercury risk tiers, and helping you make confident choices whether you're shopping for yourself, for a pregnant family member, or for your children.