Seafood mercury levels: how 10 fish and shellfish sort on the FDA chart

A swordfish steak and a shrimp cocktail can differ widely in mercury, and the reason has little to do with flavor or price. It comes down to what each animal eats and how long it lives. The form of mercury that matters in food is methylmercury. It accumulates as it moves up the food chain, from tiny organisms to small fish to the large predators that eat them. A big, long-lived hunter collects the mercury stored in everything it consumed over its lifetime. Small fish and shellfish feeding lower on that chain tend to carry far less.

That pattern shapes the advice U.S. regulators give to people who eat fish. The Food and Drug Administration and the Environmental Protection Agency jointly publish a chart called Advice About Eating Fish. It sorts dozens of species into three groups. Best Choices can be eaten two to three times a week. Good Choices are limited to one serving a week. Choices to Avoid are the fish with the highest mercury.

The chart defines an adult serving as four ounces of fish, measured before cooking. Two to three servings of Best Choices therefore add up to eight to 12 ounces a week. The guidance is aimed mainly at women who are pregnant or might become pregnant, women who are breastfeeding, and young children. Children's servings are smaller than adult servings. Anyone else can use the same chart to compare seafood mercury levels across species. The agencies present it as a guide to choosing fish, not as a reason to stop eating it.

This guide covers 10 seafood choices drawn from that chart, not the full list. They are ordered from higher to lower mercury, with the avoid group first and Best Choices last. Three come from the avoid group, two from the once-a-week group and five from Best Choices. Each slide names the species and its place on the FDA/EPA chart. It also explains what sets that species apart, from the tuna labels on a grocery shelf to the reason sardines and shrimp carry so little mercury.

Illustration: Quartz

Swordfish sits in the Choices to Avoid group on the FDA and EPA's Advice About Eating Fish chart. That category holds the fish with the highest mercury. The agencies advise the people their guidance targets to skip swordfish entirely rather than limit how often they eat it.

The listing tracks the animal's size and diet. Xiphias gladius is a fast, wide-ranging predator found in tropical and temperate waters of the Atlantic, Pacific and Indian oceans. Adults can grow well past 1,000 pounds. The International Game Fish Association's all-tackle record is a 1,182-pound swordfish caught off Chile in 1953.

Swordfish hunt squid and other fish, including mackerel and hake. They slash at prey with the long, flat bill that gives the species its common name. A predator that eats other predators for years absorbs the methylmercury stored in each meal. That mercury builds up in its muscle, the part people eat.

Adult swordfish have no scales and no teeth. They also carry a specialized heater organ, formed from modified eye muscle, that warms the eyes and brain. That adaptation helps them hunt in cold, deep water. Swordfish often descend hundreds of meters during the day and rise toward the surface at night.

In markets and restaurants, swordfish is usually sold as thick steaks. The flesh is firm and dense. That texture holds up well on a grill or under a broiler. Commercial fleets catch most swordfish on longlines. Some fisheries still use harpoons.

The FDA and EPA direct the avoid advice mainly at pregnant and breastfeeding women, women who might become pregnant, and young children. Methylmercury can harm the developing brain and nervous system. That risk is why the agencies single out those groups.

Swordfish shares the avoid group with king mackerel, marlin, orange roughy, shark, bigeye tuna and tilefish from the Gulf of Mexico. Marlin is a fellow billfish. Like swordfish, it is a large ocean predator that feeds near the top of its food chain.

Illustration: Quartz

King mackerel sits in the Choices to Avoid group on the FDA and EPA's Advice About Eating Fish chart. It shares that list with swordfish, shark and bigeye tuna, the fish the two agencies flag for the highest mercury.

The species, known scientifically as Scomberomorus cavalla, lives in the western Atlantic Ocean and the Gulf of Mexico. It is a fast, migratory predator that hunts smaller fish and squid. In the U.S., it is caught along the Southeast coast and across the Gulf. Recreational anglers in Florida and the Gulf states commonly call it kingfish.

Its diet explains much of its place on the chart. A king mackerel feeds on fish that have already absorbed methylmercury from their own prey. Larger, older individuals tend to carry more than younger ones. Mercury readings can also vary considerably from one fish to the next.

The name causes confusion at fish counters and on menus. Mackerel is a common name shared by several different species. The FDA and EPA chart lists them separately rather than as a single fish. King mackerel is the one that appears in the avoid group. A package labeled only as mackerel does not say which species is inside.

Spanish mackerel, Scomberomorus maculatus, adds to the mix-up. It belongs to the same genus, lives in many of the same waters and has a similar silvery body. Young king mackerel are often mistaken for it. One visible difference is the lateral line. On a king mackerel it dips sharply beneath the second dorsal fin, while on a Spanish mackerel it slopes more gradually. Spanish mackerel also carries yellow-gold spots along its sides, which adult king mackerel lack.

For the groups the advice mainly targets, the chart's guidance is to leave king mackerel off the plate. Unlike a Good Choice, it comes with no weekly serving allowance.

The fish has oily, firm flesh. Cooks often grill it as steaks or smoke it. Smoked kingfish dip is a familiar dish in parts of Florida. Cooking and smoking do not lower its mercury, which binds to the muscle tissue rather than the fat or skin.

Checking the species name on the label, or asking at the counter, settles which of the chart's three groups a given mackerel falls into.

Illustration: Quartz

Bigeye tuna is the only tuna in the Choices to Avoid group on the FDA and EPA's Advice About Eating Fish chart. Every other tuna on the chart falls into a lower-mercury group. The species behind a tuna steak or sushi order decides where that meal lands on the chart.

The species, known scientifically as Thunnus obesus, takes its name from its large eyes. It lives in tropical and subtropical waters of the Atlantic, Pacific and Indian oceans. Bigeye can grow longer than six feet. They spend much of the day deeper than yellowfin, a closely related tuna. At those depths they hunt fish and squid.

That size and diet place bigeye near the top of the ocean food chain. Large, long-lived predators at that level tend to carry the most methylmercury. The FDA and EPA list bigeye in the same group as swordfish, shark and king mackerel, apart from the other tunas.

Most bigeye reaches diners fresh or frozen rather than canned. Hawaii's longline fishing fleet targets bigeye as its main catch. Its deep red, fatty flesh is a common choice for sashimi and sushi. It shares the sushi counter with yellowfin and bluefin. In Japanese markets it is sold as mebachi maguro. Restaurants also serve it as seared steaks, often left rare in the center.

Menus can make the species hard to identify. In Hawaii, the word ahi covers both bigeye and yellowfin. Poke bowls and seared ahi dishes may use either fish. A dish labeled only as tuna or ahi does not tell a diner which group on the FDA chart it belongs to.

The distinction matters because the chart treats the two fish differently. Yellowfin tuna appears in the Good Choices group, one step down from bigeye's group. Canned albacore sits in that same group. Canned light tuna is listed as a Best Choice, the chart's lowest-mercury group.

Asking a server or fishmonger whether the tuna is bigeye or yellowfin is one way to apply the FDA and EPA advice. Packaged frozen tuna steaks sometimes list the species on the label. The chart itself lists each tuna by name. That lets readers match a menu item or package to its group.

Illustration: Quartz

Canned albacore tuna sits in the Good Choices group of the FDA and EPA's Advice About Eating Fish chart. The agencies suggest one serving a week from that group. Canned light tuna, often stocked beside it, sits one tier lower in mercury among the Best Choices.

Albacore is the tuna sold in the U.S. as "white" tuna. The FDA's standard of identity for canned tuna, part of federal regulations, reserves the term "white" for albacore. Darker-fleshed species are labeled as light tuna instead. A can marked "solid white" or "chunk white" therefore contains albacore.

Under the same standard, "solid" means whole pieces of loin, while "chunk" means smaller pieces. Albacore's flesh is paler and milder in flavor than the meat in most other canned tuna.

The species also goes by longfin tuna, a name drawn from its long pectoral fins. Its scientific name is Thunnus alalunga. That places it among the true tunas in the genus Thunnus, alongside yellowfin and bigeye.

Albacore grows larger and lives longer than the skipjack used in most canned light tuna. A longer life gives albacore more time to build up methylmercury, the form of mercury the FDA and EPA track.

That difference creates a split inside the canned tuna aisle. The two products often sit side by side in cans of similar size, yet they fall into different groups on the chart.

The classification is not limited to the can. The chart lists albacore in the Good Choices group whether it is canned or sold fresh or frozen. Fresh and frozen albacore reaches stores as loins and steaks, which cooks often sear or grill.

Albacore shares the Good Choices group with yellowfin tuna, halibut, mahi mahi, snapper, grouper and Chilean sea bass. The chart frames the one-serving suggestion around the category as a whole rather than each fish separately. A reader who eats a fish from this group is advised to skip other fish that week. A week that includes an albacore tuna salad and a halibut fillet would therefore go past the guideline.

Illustration: Quartz

Mahi mahi sits in the Good Choices group of the FDA and EPA's Advice About Eating Fish chart. That places it in the chart's middle tier, with a suggested limit of one serving a week.

The fish goes by several names. Mahi mahi is the Hawaiian name. In much of the Atlantic and the Caribbean, the same species is sold as dolphinfish. Spanish-language menus often list it as dorado. Its scientific name is Coryphaena hippurus.

The dolphin label causes confusion. The fish has no relation to dolphins, which are marine mammals. Many U.S. restaurants use the Hawaiian name, which sidesteps that mix-up.

Mahi mahi lives in warm tropical and subtropical waters around the world, including the Atlantic, Pacific and Indian oceans. It is a fast-swimming, open-ocean fish with bright gold, green and blue coloring. Adult males are known for a tall, blunt forehead.

Its diet puts it among the ocean's predators. Mahi mahi hunts flying fish, smaller fish, squid and crustaceans. Predators take in the methylmercury stored in the animals they eat.

Mahi mahi also grows quickly and has a short lifespan compared with large ocean predators such as swordfish. A shorter life gives methylmercury less time to build up in a fish's tissue. The chart's groups reflect mercury measured in the fish themselves, and lifespan is one factor behind those levels.

In the kitchen, mahi mahi has firm, lean flesh that turns white when cooked and a mild, slightly sweet flavor. It holds together well on a grill. Common preparations include grilled fillets, blackened fish, fish sandwiches and fish tacos. It is a regular menu item in Hawaii and Florida.

The weekly limit matters most when mahi mahi shares a week with other fish. The FDA and EPA chart presents one Good Choices serving as an alternative to the Best Choices servings, not as an addition to them. Under that framing, a mahi mahi dinner covers the week's suggested fish intake.

Illustration: Quartz

Canned light tuna is in the Best Choices group of the FDA and EPA's Advice About Eating Fish chart. Canned albacore sits one group higher in mercury, under Good Choices. The gap comes down mostly to species. Most canned light tuna is packed from skipjack.

Skipjack is a smaller tuna than albacore. In predatory fish, size and lifespan tend to track with mercury. A shorter-lived tuna has less time to build up methylmercury from the fish it eats. That difference is the main reason the two cans land in different groups on the federal chart.

Skipjack, known scientifically as Katsuwonus pelamis, lives in tropical and warm-temperate waters of the Atlantic, Pacific and Indian oceans. It is the most heavily caught tuna species in the world. In Japan it is called katsuo. Smoked, dried and shaved, it becomes katsuobushi, the bonito flakes used to make dashi broth. In Hawaii it goes by aku.

The light tuna category is not limited to skipjack. Cans sold as light tuna can also contain other species, including yellowfin. The FDA and EPA chart lists yellowfin tuna among Good Choices, the same group as albacore. Many brands name the species in the ingredient list. The chart's Best Choices entry describes canned light tuna that includes skipjack. A can that lists skipjack matches that entry.

The packing liquid does not change the picture. Methylmercury binds to the protein in fish muscle rather than collecting in oil or water. Draining a can, rinsing the fish or cooking it does not remove the mercury it contains.

A place in Best Choices means the guidance suggests two to three servings a week. That puts canned light tuna alongside salmon, sardines and cod rather than with albacore, halibut and snapper. Individual cans still vary. The chart's groupings reflect typical levels, not a guarantee for every can. The advice is directed mainly at women who are pregnant or might become pregnant, women who are breastfeeding, and young children.

Farhad Ibrahimzade / Pexels

Cod sits in the Best Choices group on the FDA and EPA's Advice About Eating Fish chart. That group holds the chart's lowest-mercury fish.

Two species account for most of the cod sold. Atlantic cod, Gadus morhua, is fished in the North Atlantic, from New England and Atlantic Canada to Iceland and Norway. Canada closed its northern cod fishery off Newfoundland in 1992 after that stock collapsed. Pacific cod, Gadus macrocephalus, comes largely from the Bering Sea and the Gulf of Alaska. The chart lists cod as a single entry and does not split the two species into separate tiers.

The flesh is white, lean and mild. It separates into large flakes once cooked. Those traits make cod a common choice in home and restaurant kitchens. It is the traditional fillet in British fish and chips, where it is battered and deep-fried. Cooks also bake, poach and broil it, or simmer it in chowders and stews.

Salt cod has a longer history than the fresh fillet. The fish is split, salted and dried, then soaked in water before cooking. Portugal calls it bacalhau, and Spain calls it bacalao. Salted cod is also a staple of Caribbean cooking, in dishes such as Jamaica's ackee and saltfish. Fresh, frozen or salted, it is still cod, and it carries the same Best Choices listing.

Cod is a predator in its own right. It feeds on smaller fish, crustaceans and other invertebrates. Even so, its mercury lands in the chart's lowest group. Swordfish, a far larger ocean hunter, sits among the Choices to Avoid.

Two close relatives appear on the chart alongside cod. Haddock and pollock both belong to the cod family, Gadidae, and both are listed as Best Choices. Haddock is a common alternative to cod in fish and chips, especially in Scotland and northern England. Alaska pollock is widely used in frozen fish sticks and in surimi, the base for imitation crab.

Illustration: Quartz

Salmon is in the Best Choices group on the FDA and EPA's Advice About Eating Fish chart. That is the lowest-mercury tier of the guidance. It puts salmon alongside sardines, shrimp and cod, and below albacore tuna and swordfish.

The chart lists salmon as a single entry. It does not separate the fish by species, by ocean, or by whether it was farmed or caught wild. Shoppers in the U.S. still see several names on labels and menus. The Pacific species sold most often are chinook, also called king, along with coho, sockeye, pink and chum. Atlantic salmon is a separate species, and most of what is sold under that name is farmed.

Salmon's diet helps explain where it lands on the chart. Young salmon feed on insects and small invertebrates in rivers and lakes. At sea, adults eat krill, other small crustaceans and forage fish such as herring. That places salmon in the middle of the marine food chain rather than at the top. It does not hunt the large prey that swordfish and sharks pursue.

Its life cycle matters too. Salmon are anadromous. They hatch in fresh water, migrate to the ocean to grow, and return to fresh water to spawn. Most Pacific salmon die after spawning once. Pink salmon complete their entire life cycle in about two years. A shorter life leaves less time for methylmercury to build up in muscle tissue.

The krill and shrimp in a wild salmon's diet also give its flesh a pink to red color. Those crustaceans carry a carotenoid pigment called astaxanthin. Farmed salmon receive astaxanthin in their feed to produce a similar shade.

Salmon is sold in many forms, including fillets, steaks, smoked salmon, sushi and canned fish. Canned salmon is most often pink or sockeye. The chart's placement applies to the fish itself, whatever the preparation.

The advice is written mainly for women who are pregnant or might become pregnant, women who are breastfeeding, and young children. For those readers, salmon falls in the tier with the most generous weekly allowance. Good Choices carry a tighter limit, and the highest-mercury species are on the avoid list.

Illustration: Quartz

Sardines are in the Best Choices group on the FDA and EPA's Advice About Eating Fish chart. Their small size, plankton diet and short lifespan help explain why they carry little methylmercury compared with large ocean predators.

Methylmercury enters the ocean food web at its base. Mercury released into the environment settles into water. There, bacteria convert some of it into methylmercury. Tiny organisms such as plankton absorb it. Every animal that eats those organisms takes in their methylmercury too. The body clears it slowly.

Slow clearance is what lets it concentrate. A predator that eats many smaller fish over its life keeps much of the methylmercury from all of them. The concentration rises at each step up the food chain, a process called biomagnification.

Sardines sit near the opposite end of that chain. They feed largely by filtering plankton from the water. They are also small and short-lived. That leaves less time for methylmercury to build up in their tissue. The large, long-lived hunters on the chart's Choices to Avoid list reverse each of those factors.

The name "sardine" covers several small, oily species in the herring family rather than one fish. The European pilchard, Sardina pilchardus, and the Pacific sardine, Sardinops sagax, are two common examples. Labels vary by country and by canner. The term often works as a market name more than a strict scientific one. Herring and anchovies, close relatives, also appear in the chart's Best Choices group.

In the U.S., sardines are sold mostly in cans. They are packed in oil, water, mustard or tomato sauce. Canning softens the small bones enough to eat them. Eating the softened bones adds calcium to each serving. Many brands pack the fish whole or headless, with the skin left on.

Fresh sardines are more common in Mediterranean cooking, where they are often grilled whole. In Portugal, grilled sardines are a staple of summer street festivals, especially during Lisbon's Santo António festivities in June.