Does Mackerel Have a Lot of Mercury?

Mackerel’s mercury content ranges from impressively low to genuinely concerning, and the answer depends almost entirely on which species you’re eating. King mackerel, a large predatory fish found in warm Atlantic and Gulf waters, averages close to 1 part per million (ppm) of mercury and sometimes exceeds federal action levels. Atlantic mackerel, the smaller species most commonly sold in European and North American markets, averages less than 0.05 ppm, putting it among the lowest-mercury fish you can buy. That gap is enormous, and it’s the reason blanket advice about “mackerel and mercury” tends to confuse more than it helps.

The Species Question Is the Whole Story

The word “mackerel” covers a family of fish that differ wildly in size, diet, habitat, and lifespan. Those biological differences translate directly into mercury differences, because mercury accumulates over time in fish tissue. A king mackerel can live over 20 years and grow past 30 pounds, spending most of its life eating other fish. A common Atlantic mackerel rarely exceeds two pounds and lives a much shorter life feeding on small crustaceans and plankton. The result is a mercury gap of roughly 20-fold between the two.

Measurements from southeastern U.S. waters found king mackerel mercury ranging from about 0.2 to 4.0 ppm, with mean levels of roughly 0.9 ppm on the Atlantic coast and 1.5 ppm on the Gulf coast. Spanish mackerel, a mid-sized species, fell in between, with means of about 0.3 ppm on the Atlantic side and 0.5 ppm on the Gulf side.¹ Atlantic mackerel from northern European waters, by contrast, averaged just 0.046 mg/kg wet weight, with a maximum of 0.36 mg/kg.² The Atlantic chub mackerel, another small species common in the eastern Atlantic, showed similarly low levels of about 0.11 mg/kg in recent sampling.³

When the FDA and EPA issue their fish-consumption advisories, they specifically name king mackerel on the “choices to avoid” list while placing Atlantic mackerel in the “best choices” category. If you’re shopping and the label just says “mackerel,” the species matters more than any other single factor. In practice, canned mackerel and most fresh mackerel fillets at grocery stores come from smaller species, but king mackerel steaks are common at fish counters in the southeastern United States and in recreational fishing catches.

Why Region Matters for the Same Species

Even within a single species, mercury levels can vary by where the fish was caught. King mackerel from Gulf of Mexico waters contained significantly more mercury than those from the Atlantic coast, with Gulf specimens averaging about 1.5 ppm compared to roughly 0.9 ppm for Atlantic fish.¹ The same pattern held for Spanish mackerel. Among Atlantic mackerel sampled in northern Europe, fish from the Skagerrak strait carried higher mercury than those from the Norwegian Sea or waters west of Scotland.²

The regional differences come down to local mercury availability in the water and sediment, which is shaped by industrial history, ocean currents, and the chemistry of the local food web. Research on marine food webs has found that the specific food sources a fish relies on can matter as much as, or even more than, its position in the food chain. A study examining mercury in marine fish found that what the fish ate and where it fed explained more of the variation in mercury levels than the fish’s trophic level alone.⁴ In plain terms, two fish of the same size and species can carry meaningfully different mercury loads depending on their local environment.

How Mackerel Stacks Up Against Other Common Fish

If you’re choosing between several popular fish at the store, the mercury comparison can be reassuring for the smaller mackerel species. An analysis of canned fish sold in the southeastern United States found that canned mackerel averaged about 36 micrograms of mercury per kilogram, roughly on par with canned salmon (about 33–36 µg/kg) and well below canned tuna, which averaged around 285 µg/kg. Sardines fell in between at about 107 µg/kg.⁵ A separate study of canned products confirmed that mackerel averaged about 55 parts per billion of mercury, compared to 188 ppb for tuna, with mackerel also delivering more omega-3 fatty acids (EPA and DHA) per serving than tuna did.⁶

A broader survey of fish sold in Poland confirmed tuna as the highest-mercury species on the market, with an estimated hazard quotient above 1, meaning regular consumption could pose a risk. Other fish, including mackerel, were deemed safe based on the same analysis.⁷ The practical takeaway is that if you’re already comfortable eating canned salmon, canned or fresh Atlantic mackerel sits at roughly the same mercury tier while offering a strong omega-3 profile.

Pregnancy, Infants, and Why the Stakes Are Higher

Mercury’s health risks are not equal across all people. The form of mercury found in fish, methylmercury, crosses the placenta and reaches the fetal brain, where it can interfere with neurological development. Evidence from major long-running studies in the Faroe Islands and Seychelles has shown that prenatal exposure through maternal fish consumption can produce subtle but measurable effects on cognitive development, and these effects have been observed even at mercury exposure levels below the tolerable weekly intake set by the European Food Safety Authority.⁸ The developing fetal brain is considerably more sensitive to mercury’s toxic effects than an adult brain.⁹

This creates a genuine dilemma for pregnant people, because fish consumption during pregnancy also benefits fetal brain development through omega-3 fatty acids. A U.S. cohort study found that higher fish intake during pregnancy was associated with better infant cognition on visual recognition memory tests, but higher maternal mercury levels were associated with lower cognition on the same tests. Each additional part per million of mercury in maternal hair corresponded to a roughly 7.5-point decrease in infant cognition scores.¹⁰ The implication is that the type of fish matters immensely. The research broadly supports eating fish during pregnancy while choosing low-mercury species, which is exactly where Atlantic mackerel, sardines, and salmon fit well, and where king mackerel does not.

Reviews of the pediatric evidence have consistently found mild but real neurodevelopmental effects from prenatal methylmercury exposure through maternal fish consumption, reinforcing current guidelines that recommend pregnant people eat two to three servings per week of low-mercury fish rather than avoiding fish altogether.¹¹

Mercury and Heart Health in Adults

For adults who aren’t pregnant, the main mercury concern has shifted toward cardiovascular effects. A well-known study published in the New England Journal of Medicine found that higher toenail mercury levels were directly associated with increased risk of heart attack, and that the mercury content of fish may partly offset the protective cardiovascular effects of the omega-3 fatty acids fish provides.¹² This finding complicates the simple “fish is good for your heart” message, because it suggests the benefit depends on which fish you’re eating. A species like Atlantic mackerel, with very low mercury and high omega-3 content, delivers the upside of fish consumption with minimal mercury downside. King mackerel flips that ratio unfavorably.

Does Selenium in Fish Offset Mercury Risk?

You may have heard that selenium, a mineral naturally present in fish, can protect against mercury toxicity. There’s a growing body of work supporting this idea. Selenium binds to mercury in the body and may reduce its harmful effects. Researchers have proposed that the ratio of selenium to mercury in a fish matters: when that ratio is above 1, meaning there’s more selenium than mercury on a molar basis, the fish may pose less risk than its raw mercury number suggests.¹³

Most ocean fish have selenium-to-mercury ratios well above 1, which is encouraging. But the picture isn’t straightforward. Research on bluefish found that the selenium-to-mercury ratio decreased as fish got larger, meaning bigger, older fish had less protective selenium relative to their mercury load.¹⁴ That pattern likely holds across species: the biggest, longest-lived fish (which already have the most mercury) also have the least favorable selenium ratio. Smaller mackerel species, with their low mercury content, almost certainly maintain a comfortable selenium surplus. The selenium angle is worth knowing about, but it doesn’t change the basic guidance: choose smaller fish when mercury is a concern.

Does Cooking Change the Mercury You Absorb?

Cooking mackerel won’t remove the mercury, but it may change how much of that mercury your body actually absorbs. Research on mercury bioaccessibility (the fraction that becomes available for absorption during digestion) found that boiling fish reduced bioaccessibility by about 40% compared to eating it raw, and frying reduced it by roughly 60%.¹⁵ The same study noted that co-ingestion of tea and coffee could further modify mercury bioaccessibility, likely because tannins and other compounds bind to mercury in the gut.

There’s a counterbalancing effect, though. Cooking drives off moisture, which concentrates the mercury remaining in the flesh. So the mercury concentration per gram of cooked fish is higher than in raw fish, even though a smaller fraction of that mercury may be bioaccessible.¹⁶ For practical purposes, these two effects partially cancel each other out. Cooking your mackerel is not going to dramatically change your mercury exposure in either direction, but if anything, frying or boiling is slightly more favorable than eating raw preparations like sashimi.

Seasonal Variation in Mercury Levels

If you eat mackerel regularly, timing might matter slightly. A study of chub mackerel (Scomber japonicus) found that mercury in muscle tissue peaked in winter and was lowest at other times of year. The same fish showed peak fat content in December and the lowest fat in April.¹⁷ The seasonal swing reflects changes in feeding intensity, growth rate, and fat metabolism throughout the year. When a fish is feeding heavily and growing, mercury gets diluted across new tissue. When growth slows, concentrations can creep up. For most consumers eating mackerel a few times a month, seasonal variation is negligible. For populations that eat mackerel almost daily, it’s one more variable that adds up.

Are Mercury Levels in the Ocean Getting Worse?

One reasonable worry is whether mercury in seafood is rising over time as industrial pollution continues. The global picture is mixed. Ocean modeling work estimates that surface ocean mercury concentrations have roughly quadrupled over the past 600 years due to human activity, primarily mining and fossil fuel combustion. Most of that anthropogenic mercury accumulates at depths of 500 to 800 meters in biologically productive regions.¹⁸ That long-term trend would suggest fish mercury should be rising.

But fish-level data tells a more nuanced story. Mercury in Atlantic chub mackerel from the Canary Islands dropped from about 0.23 mg/kg in 1973 to 0.11 mg/kg in 2021, a decline of more than half over roughly five decades.³ For king mackerel, the picture was different and more complex: comparisons of samples from before and after 1996 found a short-term decline in Spanish mackerel mercury from Gulf waters between 1990 and 2002, but levels remained higher than those recorded in the 1970s.¹ In short, global ocean mercury has increased over centuries, but regional regulations on mercury emissions and other factors mean that fish mercury doesn’t march uniformly upward everywhere. In some waters and for some species, it has actually decreased.

Mislabeling and What to Watch For

A practical concern that rarely gets discussed is fish mislabeling. Research in Peruvian seafood markets found that mislabeled fish could expose consumers to species with higher mercury than what they thought they were buying. Species substitution was especially problematic at market stalls, where a cheaper or more readily available fish might be sold under a different name.¹⁹ While this study focused on Peru, seafood mislabeling is a documented problem worldwide. For mackerel specifically, the risk is less about being sold a different species entirely and more about ambiguous labeling. “Mackerel” on a menu or at a counter could refer to Atlantic mackerel, Spanish mackerel, or king mackerel, and the mercury difference between those three spans an order of magnitude. If mercury is a concern for you, ask which species you’re getting. Canned products are more reliably labeled because species identification happens at the processing stage and is subject to regulatory oversight.

If you’re buying fresh mackerel and want the low-mercury option, look for labels specifying Atlantic mackerel (Scomber scombrus), chub mackerel (Scomber japonicus), or Pacific mackerel. These are all small, short-lived species. King mackerel fillets and steaks tend to be thick, pale, and firm, with portions much larger than what you’d get from an Atlantic mackerel, so size alone is a decent clue at the fish counter.