The seafood highest in mercury includes tilefish from the Gulf of Mexico, swordfish, shark, king mackerel, bigeye tuna, and orange roughy. These species all average above 0.5 parts per million (ppm) of mercury, with tilefish topping the list at 1.123 ppm. Understanding which fish carry the most mercury, and which are safe, helps you keep eating seafood without the risk.
The Highest-Mercury Seafood, Ranked
FDA testing of commercial fish over two decades gives us reliable average mercury concentrations for dozens of species. The worst offenders, measured in parts per million:
- Tilefish (Gulf of Mexico): 1.123 ppm, with individual samples reaching as high as 3.73 ppm
- Swordfish: 0.995 ppm average, with some samples hitting 3.22 ppm
- Shark: 0.979 ppm average, maximum recorded at 4.54 ppm
- King mackerel: 0.73 ppm
- Bigeye tuna (fresh/frozen): 0.689 ppm
- Orange roughy: 0.571 ppm
- Grouper: 0.448 ppm
- Fresh tuna (all species combined): 0.386 ppm
- Bluefish: 0.368 ppm
Any species averaging above 0.46 ppm lands in the FDA’s “Choices to Avoid” category for pregnant women and children. That clearly includes tilefish, swordfish, shark, and king mackerel. Bigeye tuna and orange roughy fall just above or near this line as well.
Why These Fish Accumulate Mercury
Mercury enters the ocean primarily from industrial emissions, volcanic activity, and coal burning. Bacteria in ocean sediment convert it into methylmercury, a form that living tissue absorbs easily but eliminates very slowly. Small organisms take it in first, and then it concentrates as it moves up the food chain, a process called biomagnification. By the time it reaches a top predator like swordfish or shark, the mercury concentration can be more than 10 times what’s found in the prey below it.
Two factors make a fish high in mercury: how high it sits on the food chain, and how long it lives. Swordfish, bluefin tuna, and sharks are both apex predators and long-lived species, giving mercury decades to build up in their muscle tissue. Orange roughy can live well over 100 years, which helps explain why a deep-sea fish that isn’t an aggressive predator still carries significant mercury levels. Research in the Gulf of Maine found that age alone could substantially increase mercury accumulation beyond what trophic level would predict.
The Tuna Question
Tuna is the most commonly eaten fish that raises mercury concerns, partly because there’s such a wide range between species. Bigeye tuna, the kind often served as sushi or seared steaks, averages 0.689 ppm. Canned white (albacore) tuna averages about 0.407 ppm. Canned light tuna, typically skipjack, averages just 0.118 ppm.
That’s roughly a six-fold difference between the highest and lowest tuna options. The reason is straightforward: skipjack are small, fast-growing fish that rarely exceed 8 pounds, while bigeye tuna can live over a decade and weigh hundreds of pounds. If you eat tuna regularly, choosing canned light over white or fresh cuts significantly reduces your exposure.
Where the Fish Comes From Matters
Geography plays a role. Tilefish from the Gulf of Mexico averages 1.123 ppm, making it the single highest-mercury commercial fish. King mackerel data also comes primarily from the Gulf of Mexico. Warmer waters with particular methylation conditions and regional industrial history can push mercury levels higher in local fish populations.
For salmon specifically, the farmed-versus-wild question doesn’t appear to matter much. A study comparing over 200 farmed and wild salmon from British Columbia found negligible differences in mercury between them. All samples ranged from 0.03 to 0.1 ppm, well below any safety threshold. Salmon, regardless of source, is consistently one of the lowest-mercury options available.
What Mercury Does to Your Body
Methylmercury targets the nervous system. At high levels, it causes tremors, impaired vision, difficulty with coordination, and sensory disturbances. These effects were first documented in severe poisoning cases, but research has since shown that even lower, chronic exposures carry risk, especially for developing brains. Children exposed to modest levels of methylmercury in the womb have shown measurable effects on IQ, language development, memory, motor skills, and concentration.
The EPA’s safety threshold for chronic exposure is 0.1 micrograms of mercury per kilogram of body weight per day. For a 150-pound person, that works out to roughly 6.8 micrograms per day. A single 4-ounce serving of swordfish contains roughly 113 micrograms of mercury, enough to exceed the daily reference dose for over two weeks. This is why occasional indulgence is treated differently from regular consumption.
Low-Mercury Seafood Worth Eating
Plenty of seafood gives you omega-3 fatty acids and protein with minimal mercury. The FDA’s “Best Choices” category includes fish averaging 0.15 ppm or less, safe to eat two to three servings per week. The list is longer than most people expect:
- Salmon: consistently under 0.1 ppm, farmed or wild
- Sardines and anchovies: tiny fish at the bottom of the food chain
- Shrimp, scallops, and oysters: shellfish are reliably low in mercury
- Cod, pollock, and haddock: common white fish with very low levels
- Herring and Atlantic mackerel: note that Atlantic mackerel is low-mercury, while king mackerel is one of the highest
- Canned light tuna (skipjack): at 0.118 ppm, it’s a best choice
- Catfish, tilapia, and trout: freshwater options that are all low
Crab, lobster, squid, flounder, and sole also fall into this safe category. The “Good Choices” group, averaging between 0.15 and 0.46 ppm, includes fish like albacore tuna, halibut, and snapper, safe at one serving per week.
Guidelines for Pregnancy and Children
The FDA recommends that pregnant and breastfeeding women eat 8 to 12 ounces of low-mercury seafood per week, split into two or three servings. One serving is 4 ounces. Sticking to the “Best Choices” list at that quantity keeps mercury intake well within the safety margin while providing the omega-3s that support fetal brain development.
For children, serving sizes scale with age: about 1 ounce for toddlers ages 1 to 3, 2 ounces for ages 4 to 7, 3 ounces for ages 8 to 10, and 4 ounces at age 11. Two servings per week from the “Best Choices” list is the recommendation. Children should avoid swordfish, shark, tilefish, and king mackerel entirely.
The mackerel distinction trips people up more than anything else on these lists. Atlantic mackerel is a small, short-lived fish and a best choice. King mackerel is a large predator averaging 0.73 ppm. If a menu or fish counter just says “mackerel,” ask which kind.