High exposure can affect brain development, learning, memory, and other neurological functions. Adults exposed to excessive amounts may also experience neurological symptoms.
For people who want to enjoy seafood with lower mercury exposure, salmon, trout, sardines, and other lower-mercury options are worth considering.
What to remember: Swordfish is not a good choice for people following pregnancy-related seafood guidance, and it should not be treated as an everyday fish.
3. Shark: Mercury Can Build Up Through the Food Chain
Shark meat is consumed in various parts of the world, but it can contain substantial amounts of mercury.
Sharks occupy high positions in marine food webs. Many species feed on other fish and marine animals that may already contain mercury. As sharks continue to feed over their lifetimes, mercury can accumulate in their tissues.
The amount varies by shark species, size, age, and where the animal was caught. Nevertheless, shark is classified by the FDA as a fish to avoid for pregnant or breastfeeding women and children.
Mercury exposure is particularly concerning because methylmercury can affect the nervous system. In developing children, excessive exposure may interfere with normal brain development and learning.
It is also worth remembering that cooking does not reliably remove mercury from fish. Grilling, frying, steaming, or boiling shark meat does not make high-mercury fish safe from mercury exposure.
What to remember: Avoid relying on cooking methods to reduce mercury. Choosing a lower-mercury species is the more effective approach.
4. Tilefish: Pay Attention to Where It Comes From
Tilefish is a less familiar seafood choice for many consumers, but it deserves attention because mercury levels can vary considerably depending on where the fish is caught.
In particular, tilefish from the Gulf of Mexico have been found to contain high mercury concentrations. The FDA places Gulf of Mexico tilefish in its category of fish to avoid for people who are pregnant, breastfeeding, or feeding young children.
However, not all tilefish have identical mercury levels. Tilefish from the Atlantic Ocean are treated differently in FDA guidance and are listed in a category with lower mercury levels than Gulf of Mexico tilefish.
This distinction matters when buying seafood. A product label that simply says "tilefish" may not provide enough information to determine its mercury risk.
If the origin is unclear, ask the seller or choose another species with a more established low-mercury profile.
What to remember: The location where tilefish was caught matters. Gulf of Mexico tilefish should be avoided by people following FDA pregnancy and childhood seafood guidance.
5. Bigeye Tuna: Not All Tuna Is the Same
Tuna is one of the most widely consumed seafood products in the world. It is available fresh, frozen, canned, and as sushi or sashimi.
But different tuna species can contain very different amounts of mercury.
Bigeye tuna is a large species that tends to have higher mercury levels than several other commonly consumed tuna varieties. The FDA includes bigeye tuna among its fish to avoid for pregnant or breastfeeding women and children.
By comparison, canned light tuna, which is generally made primarily from skipjack, is considered a lower-mercury option. Albacore, often sold as white tuna, generally contains more mercury than canned light tuna and is treated differently in consumption recommendations.
This is why the word "tuna" alone is not enough when assessing mercury exposure.
People who enjoy sushi should also pay attention to the species used. Bigeye tuna may appear on menus as a premium tuna option, so asking about the species can help consumers make informed choices.
What to remember: Bigeye tuna is a high-mercury choice. Canned light tuna is generally lower in mercury, while albacore and other tuna varieties require more careful consideration.
Why Does Mercury Accumulate in Certain Fish?
Mercury occurs naturally in the environment, but human activities such as coal burning, industrial processes, and mining can release additional mercury into the air and water.
In aquatic environments, microorganisms can convert some mercury into methylmercury, a form that can enter the food chain.
Small aquatic organisms absorb methylmercury. Small fish eat those organisms, larger fish eat the smaller fish, and predators at the top of the food chain consume many other fish over time.
As a result, mercury concentrations can increase at higher levels of the food chain. This is known as biomagnification.
Several factors influence how much mercury a fish contains:
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Species: Some fish naturally accumulate more mercury than others.
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Size: Larger fish of the same species may contain more mercury.
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Age: Older fish have had more time to accumulate mercury.
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Diet: Predatory fish that consume other fish may receive mercury from their prey.
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Location: Mercury levels can differ between marine environments and fishing areas.
This explains why a large predatory fish may contain substantially more mercury than a small fish, even when both come from the same ocean.
How Does Mercury Affect the Human Body?
The main concern associated with eating high-mercury fish is exposure to methylmercury.
After entering the digestive system, methylmercury is absorbed into the bloodstream and distributed throughout the body. It can cross the blood-brain barrier and the placenta, which is why fetal development receives particular attention in public health recommendations.
1. Effects on the developing brain
The developing brain and nervous system are especially sensitive to methylmercury.
High exposure during pregnancy may interfere with the development of the brain and nervous system. Depending on the level and timing of exposure, effects may include difficulties with learning, memory, attention, and coordination.