Skipjack tuna consistently contains the least mercury of any commonly eaten tuna species. In studies comparing species side by side, skipjack comes in well below albacore, yellowfin, bigeye, and bluefin. If you are buying canned tuna at the grocery store, that translates to a simple rule: “chunk light” is almost always skipjack, while “solid white” or “albacore” carries roughly double the mercury or more. But the species on the label is just one variable. Where the fish was caught, how it was processed, and even which brand you choose all shift mercury levels enough to matter.
How Tuna Species Rank
Among the tuna you are likely to encounter at a store or restaurant, the species fall into a rough hierarchy from lowest to highest mercury. Skipjack sits at the bottom. A study comparing skipjack and albacore from the western Mediterranean found skipjack averaged about 0.07 mg/kg of mercury in muscle tissue, while albacore averaged roughly 0.20 mg/kg, nearly three times higher.1PubMed. Species-specific heavy metal concentrations of tuna species: the case of Thunnus alalunga and Katsuwonus pelamis in the Western Mediterranean Yellowfin generally falls in a middle range. Bigeye and bluefin sit at the top. Research using DNA barcoding to identify tuna sushi found that bigeye samples and bluefin akami (lean loin) carried significantly more mercury than yellowfin akami.2PubMed Central. DNA barcodes reveal species-specific mercury levels in tuna sushi that pose a health risk to consumers
The practical takeaway is that if mercury is your main concern, skipjack gives you the widest safety margin. Yellowfin is a reasonable middle ground, particularly for people who eat tuna occasionally rather than several times a week. Bigeye and bluefin are the species to limit, especially for pregnant women and young children.
Why Some Species Carry More Mercury Than Others
The biggest factor separating high-mercury tuna from low-mercury tuna is how deep the fish hunts. Mercury dissolved in seawater increases with depth, peaking in what oceanographers call the mesopelagic zone, roughly 200 to 1,000 meters down. Predators that forage in that zone eat prey that has already absorbed mercury from the surrounding water, and the mercury concentrates further at each step up the food chain.
Bigeye tuna are deep divers. They routinely hunt at mesopelagic depths, which exposes them to prey with higher mercury burdens. Research tracking mercury levels across a range of open-ocean predators found that total mercury climbed in lockstep with median foraging depth, and bigeye ranked among the deepest feeders.3PubMed Central. The influence of depth on mercury levels in pelagic fishes and their prey A study on Pacific bluefin tuna reinforced this pattern, finding that individuals spending more time foraging in deep water had higher methylmercury concentrations and slower growth rates.4PubMed. Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna
Skipjack, by contrast, is a smaller species that tends to stay closer to the surface. It also has a shorter lifespan, typically three to four years compared with a decade or more for bluefin. Fewer years of accumulation plus shallower feeding adds up to substantially less mercury in the flesh.
Body size matters as well. Larger marine predators bioaccumulate toxins over a longer lifespan and biomagnify them by eating prey that is itself high on the food chain.5Marine Pollution Bulletin. Mercury bioaccumulation and its relationship with trophic biomarkers in a Mediterranean elasmobranch mesopredator A mature bluefin can weigh several hundred kilograms; a skipjack rarely exceeds ten. That size gap corresponds directly to the mercury gap.
Where the Fish Was Caught Makes a Surprising Difference
Even within a single species, mercury concentrations can vary dramatically depending on catch location. A large-scale analysis of skipjack across the Pacific found a fivefold spatial gradient: the highest mercury levels appeared in the northwest Pacific near Asia, intermediate levels in the eastern Pacific, and the lowest values in the western, southwestern, and central Pacific.6PubMed Central. Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs The hotspot near Asia was linked to elevated atmospheric mercury from industrial emissions and river runoff carrying mercury onto coastal shelves.
A separate study of yellowfin and bigeye tuna found that the eastern equatorial Pacific stood out, with mercury concentrations elevated by roughly 0.2 parts per million above other sampled regions for both species. The reason appears to be oceanographic: that region has unusually shallow zones of low dissolved oxygen, which promote the microbial conversion of inorganic mercury into methylmercury, the organic form that enters the food chain.7Canadian Journal of Fisheries and Aquatic Sciences. Regional patterns in mercury and selenium concentrations of yellowfin tuna and bigeye tuna in the Pacific Ocean
This geography effect means that two cans of skipjack sitting next to each other on a shelf could differ substantially in mercury if one was caught off the coast of Japan and the other off Fiji. Unfortunately, canned tuna labels rarely tell you where the fish was caught, making this the hardest variable for a consumer to control.
Canned Tuna Labels and Brand Variation
At the supermarket, you generally see “chunk light” and “solid white” (or “albacore”). Chunk light is overwhelmingly skipjack, though some brands mix in yellowfin. Solid white is albacore by definition under U.S. labeling rules. Because albacore is a larger, deeper-feeding species, white tuna carries more mercury than light tuna as a category.
What most people do not realize is that brand choice matters too. A recent study measuring mercury in commercial canned tuna found a statistically significant effect of brand on mercury concentrations for both albacore and light tuna. Among albacore products, the highest-mercury brand was significantly higher than four of the five other brands tested. Among light tuna, two brands were significantly higher than the lowest-mercury brand.8PubMed Central. Variability of Mercury Concentrations Across Species, Brand, and Tissue Type in Processed Commercial Seafood Products The likely explanation is that brands source from different fisheries, in different ocean regions, and may use different species mixes or fish sizes.
Species mislabeling adds another layer of uncertainty. Tuna is especially susceptible to this problem because of high global demand, fast production rates, and a wide range of price points across species.9Journal of Food Quality. Comparison of DNA Extraction Methods for the Detection of Canned Tuna Species with DNA Mini-Barcoding A can labeled “light tuna” that actually contains bigeye would carry more mercury than expected. DNA barcoding studies have repeatedly uncovered mislabeling in both canned and restaurant tuna.
Does Canning Change Mercury Levels?
Canning does not remove mercury. In fact, it slightly concentrates it. When tuna is cooked and sealed in a can, it loses moisture, and the mercury stays behind in the flesh. A study of albacore found that mercury concentrations rose from an average of 0.17 ppm in raw samples to 0.21 ppm after canning, a roughly 23% increase. The fish lost about 13% of its weight through moisture loss, which explains most of the jump.10Food Chemistry. Effects of canning on total mercury, protein, lipid, and moisture content in troll-caught albacore tuna A similar pattern was confirmed for skipjack, where mercury in both light and dark muscle increased through each processing step, from raw to cooked to canned.11Journal of Food Composition and Analysis. Mercury content in the white and dark muscle of Skipjack tuna (Katsuwonus pelamis) along the canning process
One persistent myth is that draining the liquid from a can of tuna washes away some mercury. Research shows this is not the case. Mercury levels were not significantly different between tuna packed in oil versus tuna packed in water, and draining the fluid had no meaningful effect because the mercury binds to the protein in the flesh rather than dissolving into the packing liquid.12PubMed. Mercury in canned tuna: white versus light and temporal variation A broader review of canned tuna data reached the same conclusion: the packing medium and draining have limited or inconsistent influence on mercury in the edible portion.13PubMed Central. Total mercury exposure through canned tuna in oil sold in Quito Ecuador
Farmed Versus Wild Tuna
Tuna farming, mostly bluefin raised in Mediterranean and Australian sea pens, introduces another comparison. Farmed Atlantic bluefin tuna in one study averaged 0.6 mg/kg of mercury, well below the legal limit of 1.0 mg/kg, while wild bluefin from the same region averaged 1.7 mg/kg, exceeding the limit.14PubMed Central. Determination of Hg in Farmed and Wild Atlantic Bluefin Tuna (Thunnus thynnus L.) Muscle The farmed fish also had higher selenium concentrations and a much more favorable selenium-to-mercury ratio, making them safer from a toxicological standpoint.
The gap was smaller for Australian southern bluefin, where farmed fish averaged 0.31 mg/kg and wild fish 0.34 mg/kg.15PubMed. Dioxins, PCBs, metals, metalloids, pesticides and antimicrobial residues in wild and farmed Australian southern bluefin tuna The difference probably reflects the shorter fattening period used in Australian ranching operations. In general, farmed tuna tend to be harvested younger and fed controlled diets, which limits their total mercury accumulation compared with fully wild fish of the same species.
Selenium and Why It Complicates the Mercury Story
Mercury does not act in isolation inside your body. Selenium, an essential mineral also found in fish, appears to counteract some of mercury’s toxic effects. The ratio of selenium to mercury in a fish may matter as much as the absolute mercury level. When that ratio is well above 1, meaning there is much more selenium than mercury, the fish’s selenium may help your body manage the mercury load. When it drops below 1, as it does in certain high-mercury species like some sharks, the risk picture worsens.16PubMed Central. Selenium and mercury molar ratios in commercial fish from New Jersey and Illinois: variation within species and relevance to risk communication
Among saltwater fish, the selenium-to-mercury ratio varies enormously, from below 1 in some shark species to above 60 in small forage fish. The ratio tends to be driven more by mercury content than by selenium content, since selenium concentrations are relatively stable across species while mercury concentrations can differ by orders of magnitude.17PubMed Central. Selenium and mercury molar ratios in saltwater fish from New Jersey Tuna generally fare well on this metric. Atlantic bluefin, for instance, showed a positive selenium health benefit value across all tissues tested, meaning selenium was present in enough excess to offer meaningful protection.18PubMed Central. Tissue Distribution of Mercury and Its Relationship with Selenium in Atlantic Bluefin Tuna
This does not mean you can eat unlimited bluefin and trust selenium to neutralize the mercury. The science on exactly how much excess selenium you need is still unclear. But it does suggest that moderate tuna consumption is less risky than the raw mercury number might imply, and that tuna is a better choice than other high-mercury fish (like certain sharks) that have unfavorable selenium ratios.
Mercury Concerns During Pregnancy and for Young Children
Methylmercury can cross the placenta, and the developing brain is more sensitive to its effects than an adult brain. Studies have documented mild but measurable impacts on neurocognitive development when mothers consumed high-mercury fish during pregnancy.19PubMed Central. Fish consumption, methylmercury and child neurodevelopment That research underlies the common advisory that pregnant women should limit high-mercury seafood.
But the picture is not as simple as “avoid fish.” Fish also supplies omega-3 fatty acids, protein, and other nutrients that benefit fetal development. Evidence published since the original 2004 U.S. advisory suggests that the benefits of moderate fish consumption during pregnancy likely outweigh the mercury risk for most people, as long as the highest-mercury species are avoided.20PubMed Central. Fish Consumption During Pregnancy: An Opportunity, Not a Risk In practical terms, that means canned light tuna (skipjack) a few times a week is considered safe for pregnant women by most current guidelines. Bigeye, bluefin, and to a lesser extent albacore are the species to cut back on.
How Your Body Handles Methylmercury
Once you eat tuna, the methylmercury in it is absorbed efficiently through your gut. Your body does eventually eliminate it, but slowly. Estimates of the biological half-life, meaning the time it takes for blood levels to drop by half after exposure stops, vary across studies. One population-level analysis estimated a mean half-life of about 80 days, with wide individual variation.21PubMed Central. Estimation of the Biological Half-Life of Methylmercury Using a Population Toxicokinetic Model A controlled fish-consumption study measured faster clearance, finding an average half-life of about 44 days, with individual rates ranging from 28 to 62 days.22Toxicological Sciences. Variation in Methylmercury Metabolism and Elimination Status in Humans Following Fish Consumption A Japanese study measuring both blood and hair found half-lives of roughly 57 days in blood and 64 days in hair once background levels were accounted for.23PubMed. Hair-to-blood ratio and biological half-life of mercury: experimental study of methylmercury exposure through fish consumption in humans
The wide range between people is striking and has real implications. If you happen to be a slow eliminator, eating tuna twice a week will push your steady-state mercury level higher than someone who clears it faster. Part of the variation appears to involve gut bacteria. Research in both humans and mice has shown that intestinal microbes can demethylate methylmercury, converting it to an inorganic form that is less readily absorbed and more easily excreted.24PubMed Central. Assessing the Role of the Gut Microbiome in Methylmercury Demethylation and Elimination in Humans Certain bacterial groups, particularly sulfate-reducing bacteria and methane-producing archaea, appear to be especially active in this process.25PubMed. Impacts of Mercury Exposure Levels and Sources on the Demethylation of Methylmercury Through Human Gut Microbiota
None of this means you can change your gut flora to eat more bluefin safely. But it does help explain why two people eating the same amount of tuna can end up with very different mercury levels in their blood.
Rising Mercury in the Ocean
Mercury in tuna is not a fixed quantity over time. Industrial emissions, primarily from coal burning and gold mining, have increased the amount of mercury cycling through the atmosphere and oceans. A study using century-old museum specimens of an endangered Pacific seabird, which feeds at the same trophic level as tuna, found significantly higher methylmercury concentrations in feathers collected after 1940 compared with earlier periods, and another jump after 1990.26PubMed Central. Temporal increase in organic mercury in an endangered pelagic seabird assessed by century-old museum specimens These increases tracked historical global and regional anthropogenic emissions.
The tuna mercury hotspot near Asia documented in Pacific skipjack data is part of the same story: industrial activity on land drives mercury into the ocean, where microbes convert it into methylmercury, and fish absorb it.6PubMed Central. Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs International agreements like the Minamata Convention aim to reduce mercury emissions, but even if emissions drop, existing mercury stored in ocean sediments and deep water will continue cycling through marine food webs for decades. The mercury content of tuna is, in other words, a moving target, and one that depends on policy decisions happening far from the fishing grounds.
Low-Level Mercury Exposure in Adults
Most discussions of mercury and tuna focus on pregnancy, but there is evidence that chronic low-level exposure affects adults too. A study of people in communities with high fish consumption found dose-dependent effects on tremor, a neurological sign, at exposure levels below the threshold traditionally considered harmful. Younger adults appeared more sensitive, and the effects were detectable on clinical examination rather than just through lab tests.27PubMed. Low-level methylmercury exposure as a risk factor for neurologic abnormalities in adults The authors were cautious about interpreting those results because of confounding factors, but the finding reinforces the general principle that less mercury exposure is better even for people outside traditional risk groups.
For adults eating tuna regularly, the simplest risk reduction strategy remains species selection. Swapping one can of albacore for one can of chunk light skipjack roughly halves your mercury intake per serving, with no change in the amount of fish you eat. If you enjoy tuna steaks at restaurants, asking whether the fish is yellowfin or bigeye can make a meaningful difference. And if you eat tuna several times a week rather than occasionally, the species choice matters more, not less, because your body’s elimination system may not keep pace with the incoming load.