Fish and shellfish are by far the biggest source of mercury in the human diet. A handful of large, predatory species carry the highest concentrations, while most other foods contain negligible amounts. Understanding which fish top the list, and which are safe to eat regularly, lets you get the nutritional benefits of seafood without unnecessary exposure.
Why Fish Contain More Mercury Than Other Foods
Mercury enters the ocean from both natural sources (volcanic activity, rock erosion) and human activity (coal-burning power plants, industrial waste). Once in the water, bacteria in ocean sediments convert inorganic mercury into methylmercury, a form that dissolves easily in living tissue. Tiny organisms like plankton absorb it first. Small fish eat the plankton, bigger fish eat the small fish, and at every step, the mercury concentration increases because it binds tightly to muscle protein and isn’t easily excreted.
This process, called bioaccumulation, means that the largest, longest-lived predators at the top of the food chain end up with the most mercury. A swordfish that has spent a decade eating smaller fish will have many times more mercury in its flesh than a sardine that feeds on plankton. That single biological principle explains almost every entry on the high-mercury and low-mercury lists below.
The Highest-Mercury Fish
FDA testing of commercial seafood ranks these species at the top for average mercury concentration, measured in parts per million (ppm):
- Tilefish (Gulf of Mexico): 1.123 ppm
- Swordfish: 0.995 ppm
- Shark: 0.979 ppm
- King mackerel: 0.73 ppm
- Bigeye tuna (fresh/frozen): 0.689 ppm
- Orange roughy: 0.571 ppm
Any fish averaging above 0.46 ppm falls into the FDA’s “Choices to Avoid” category for pregnant women, breastfeeding mothers, and young children. That includes all six species above. For the general adult population, occasional servings of these fish aren’t dangerous, but eating them frequently can push mercury intake to concerning levels over time.
Moderate-Mercury Seafood
A large middle tier of popular fish falls between roughly 0.15 and 0.46 ppm. These are fine to eat about once a week. Some notable examples:
- Grouper: 0.448 ppm
- Albacore tuna (fresh): 0.358 ppm
- Yellowfin tuna (fresh): 0.354 ppm
- Chilean sea bass: 0.354 ppm
- Canned albacore (white) tuna: 0.350 ppm
- Halibut: 0.241 ppm
- Mahi mahi: 0.178 ppm
- Snapper: 0.166 ppm
Halibut, snapper, and mahi mahi sit at the lower end of this range, so they’re closer to the “safe for frequent eating” threshold than grouper or fresh tuna.
Canned Tuna: White vs. Light
Canned tuna deserves its own mention because it’s one of the most commonly eaten fish in the U.S., and the two main types differ dramatically. Canned white (albacore) tuna averages about 0.407 ppm, while canned light tuna averages just 0.118 ppm. That’s roughly a 3.5-fold difference. Light tuna is typically made from skipjack, a smaller species that accumulates far less mercury. If you eat canned tuna regularly, switching to light tuna is one of the simplest ways to cut your mercury exposure.
Lowest-Mercury Seafood
Fish and shellfish with average mercury at or below 0.15 ppm are classified as “Best Choices,” safe for two to three servings per week even for pregnant women and children. This category includes many of the most popular and affordable options: salmon, sardines, anchovies, herring, shrimp, pollock, catfish, tilapia, cod, crab, scallops, and canned light tuna. Freshwater perch (0.150 ppm) and skate (0.137 ppm) also fall into this range.
The pattern is straightforward: smaller fish lower on the food chain, and shellfish that filter-feed or eat invertebrates, carry very little mercury. These species also tend to be the most affordable, making low-mercury eating the default for most households.
Non-Seafood Sources of Mercury
Rice is the most notable non-seafood source. Rice paddies are flooded during the growing season, and waterlogged soil creates the same oxygen-poor conditions that convert mercury into its more toxic methylmercury form. The rice plant can then absorb small amounts of methylmercury through its roots. U.S. Geological Survey researchers have documented this uptake in California’s Sacramento Valley rice fields. The concentrations are far lower than what you’d find in a piece of swordfish, but for people who eat rice as a dietary staple multiple times a day (common in parts of Asia), it can become a meaningful contributor to overall exposure.
Other foods contain only trace amounts. Root vegetables can pick up tiny quantities of mercury from contaminated soil, and some herbal supplements and traditional medicines have been found to contain mercury, but these are inconsistent and uncommon sources. For the vast majority of people, seafood accounts for the overwhelming share of dietary mercury.
Does Cooking Reduce Mercury?
It depends on the method. Baking and frying both reduce the amount of mercury your body can actually absorb from fish. In one study, baking lowered mercury concentration by about 10% compared to raw fish, and frying showed similar reductions. Steaming and marinating, on the other hand, actually increased mercury concentration by roughly 20%, likely because these methods cause moisture loss that concentrates the mercury remaining in the flesh. So while cooking method matters, the differences are modest. Choosing a lower-mercury species in the first place makes a much bigger impact than any cooking technique.
The Role of Selenium
Most ocean fish contain selenium, a mineral that directly counteracts mercury’s harmful effects. Selenium is essential for brain-protective enzymes, and mercury disrupts those enzymes. When a fish contains more selenium than mercury (on a molecular basis), eating it provides a net surplus of selenium that can offset the mercury risk. Most ocean fish, including many moderate-mercury species like yellowfin tuna and albacore, contain enough selenium to provide this protective buffer.
The few exceptions tend to be certain whale and dolphin meats consumed in some coastal communities, where mercury levels are so extreme they overwhelm the selenium content. For virtually all commercially available seafood, selenium provides at least partial protection, which is one reason large studies of fish-eating populations have not consistently found developmental harm at the mercury levels typical of a normal seafood diet.
How Much Is Safe to Eat
The FDA and EPA divide commercial fish into three tiers based on mercury concentration:
- Best Choices (0.15 ppm or less): 2 to 3 servings per week. Includes salmon, shrimp, pollock, catfish, canned light tuna, and most shellfish.
- Good Choices (0.15 to 0.46 ppm): 1 serving per week. Includes halibut, mahi mahi, snapper, canned white tuna, and yellowfin tuna.
- Choices to Avoid (above 0.46 ppm): Shark, swordfish, king mackerel, tilefish from the Gulf of Mexico, bigeye tuna, and orange roughy.
A serving is about 4 ounces (the size of your palm). These guidelines are designed primarily for pregnant and breastfeeding women and children ages 1 through 11, who are most vulnerable because methylmercury can interfere with brain development in a growing fetus or young child. Adults outside those groups have a wider safety margin, but the same rankings still apply for anyone trying to limit exposure.
The practical takeaway: you don’t need to avoid seafood. The healthiest approach is eating fish regularly while favoring species from the lower end of the mercury spectrum. Salmon, sardines, shrimp, and canned light tuna give you omega-3 fatty acids and protein with minimal mercury. Save swordfish and bigeye tuna for occasional indulgences rather than weekly staples.