Fish contain mercury because of a chain of events that starts with pollution, moves through bacteria, and concentrates up the food chain. Mercury released into the air from coal plants, mining, and volcanoes eventually settles into oceans, lakes, and rivers. Once in the water, microorganisms convert it into a form that fish absorb and can’t easily get rid of, so it builds up in their tissue over their entire lives.
How Mercury Gets Into the Water
Mercury enters the environment from both human activity and natural processes, but industrial sources dominate. Coal combustion is the single largest contributor, responsible for roughly 35% of human-caused mercury emissions worldwide, releasing about 810 metric tons into the atmosphere each year. Small-scale gold mining operations, where liquid mercury is used to extract gold from ore, account for another 17%. Volcanoes and geothermal activity contribute a much smaller share, around 90 metric tons annually.
Once airborne, mercury can travel thousands of miles on wind currents before settling onto land and water. Rain washes it from the atmosphere and soil into streams, rivers, and eventually the ocean. This is why mercury shows up in fish caught far from any factory or mine. A coal plant in one country can contribute to mercury levels in fish on the other side of the world.
Bacteria Turn Mercury Into a Deadlier Form
The mercury that lands in water is mostly inorganic, a form that doesn’t accumulate well in living tissue. The critical step happens at the bottom of lakes, rivers, and coastal waters, where certain bacteria transform inorganic mercury into methylmercury. This organic form is far more easily absorbed by living organisms and far harder to eliminate.
The bacteria responsible are anaerobes, microorganisms that thrive in low-oxygen environments like lake-bottom sediments and waterlogged soils. Sulfate-reducing bacteria are the group most strongly linked to this conversion, though iron-reducing bacteria and methane-producing microbes do it too. They all share a specific gene pair that encodes the proteins needed to attach a carbon group to mercury, creating methylmercury. The process happens most actively in zones where oxygen levels shift, such as the boundary between oxygen-rich surface water and oxygen-depleted depths during summer stratification.
Environmental conditions matter. In freshwater systems, variations in water acidity can account for up to 70% of the variation in mercury levels found in fish. Temperature, dissolved organic carbon, and sulfur levels also influence how much methylmercury bacteria produce. Fluctuating water levels, like those caused by dam operations or seasonal flooding, increase production by cycling sediments between oxygen-rich and oxygen-poor states.
Why Bigger Fish Have More Mercury
Once bacteria produce methylmercury, it enters the food chain at the very bottom. Plankton and tiny organisms absorb it directly from the water. Small fish eat those organisms and retain the methylmercury in their tissues. Larger fish eat many smaller fish, and the mercury from every one of those meals accumulates. This process, called biomagnification, means mercury concentrations increase dramatically at each step up the food chain.
The magnification factor across an entire food web ranges from roughly 1.2 to 28 times at each level. A large predatory fish like a swordfish eats hundreds of smaller fish over its lifetime, each one delivering a dose of methylmercury that the swordfish’s body stores but barely eliminates. The older and larger the predator, the higher its mercury load. This is why the fish with the highest mercury levels are consistently the biggest, longest-lived species that sit at the top of their food chains.
Which Fish Have the Most Mercury
FDA testing of commercial seafood reveals a wide range. The highest-mercury species, measured in parts per million (ppm), are:
- Gulf of Mexico tilefish: 1.123 ppm
- Swordfish: 0.995 ppm
- Shark: 0.979 ppm
- King mackerel: 0.73 ppm
- Bigeye tuna (fresh/frozen): 0.689 ppm
These species share common traits: they’re large-bodied predators, they live long lives, and they feed high on the food chain. A swordfish can live over a decade and weigh hundreds of pounds, accumulating mercury with every meal for years.
Which Fish Are Lowest in Mercury
Smaller, shorter-lived species that feed lower on the food chain carry far less mercury. The EPA and FDA jointly maintain a “Best Choices” list of seafood safe to eat two to three servings per week. Among the lowest-mercury options are anchovies, sardines, salmon, shrimp, tilapia, pollock, catfish, trout, scallops, squid, oysters, clams, and crawfish. These species tend to be smaller, grow quickly, and don’t live long enough to accumulate significant mercury loads.
For children, the guidance is more conservative: two servings per week, chosen from the lowest-mercury fish on the Best Choices list. The species specifically recommended for children include anchovies, Atlantic mackerel (not king mackerel), catfish, clams, crab, crawfish, flounder, haddock, mullet, oysters, pollock, salmon, sardines, scallops, shrimp, sole, squid, tilapia, trout, and whiting.
What Mercury Does in Your Body
When you eat fish containing methylmercury, your gut absorbs it efficiently. It crosses into the bloodstream and distributes throughout the body, including the brain. The body does eliminate it, primarily through feces (about 31% of a dose over 70 days) and to a lesser extent through urine and hair. But the process is slow. The average half-life of methylmercury in the human body is about 80 days, meaning it takes that long to clear just half of a single dose. With regular exposure from frequent fish meals, levels can build up faster than the body clears them.
The EPA’s safety threshold is 0.1 micrograms of methylmercury per kilogram of body weight per day. For a 150-pound person, that works out to roughly 6.8 micrograms daily. A single serving of swordfish can contain well over 50 micrograms, which is why high-mercury species are a concern even when eaten occasionally. The developing brain is especially vulnerable, which is why guidance for pregnant women and young children is stricter.
Why Mercury Levels Vary Between Fish of the Same Species
Two tuna steaks from different fish can have very different mercury levels. Geography plays a role: fish from waters near industrial areas or regions with heavy coal use tend to carry more mercury. Water chemistry matters too, since acidic lakes produce more methylmercury than neutral ones. A fish’s age and size within its species also create variation. A large, old tuna has had more years to accumulate mercury than a younger, smaller one from the same waters. Diet differences, local bacterial activity, and water temperature all contribute to the range. This is why FDA data reports averages, and why any individual fish could be higher or lower than the listed value.