Eating fish is an important part of a healthy diet. Fish provide high-quality protein and omega-3 fatty acids. However, nearly all fish contain traces of mercury, a toxic heavy metal. At high exposures, mercury can cause neurological problems and other health issues. This article reviews how much mercury is in different types of fish and whether mercury levels in fish are a health concern.
What is mercury and why is it in fish?
Mercury is a heavy metal that is found naturally in the environment. Mercury enters the waterways through:
- Natural processes like volcanic eruptions and weathering of rocks
- Human activities like coal burning and mining
- Mercury released from consumer products into landfills and water supplies
Once mercury enters the water, bacteria convert it to methylmercury, a highly toxic organic compound. Methylmercury builds up in animal tissues as it moves up the food chain. Predatory fish that eat other fish tend to have the highest levels.
The larger and older a fish is, the more mercury it may contain. Longer living predator fish like tuna can accumulate high amounts over their lifespan.
How is mercury regulated in seafood?
Several organizations provide guidelines for mercury in seafood:
- The EPA set a limit of no more than 0.5 parts per million (ppm) methylmercury in commercial fish.
- The FDA set an action level of 1 ppm for methylmercury in commercial fish.
- The Joint FAO/WHO Expert Committee on Food Additives recommends 0.5 ppm for methylmercury.
In the US, the FDA regulates mercury levels and warns about consuming fish high in mercury. It advises women who are pregnant, may become pregnant, or nursing and young children to avoid fish with high mercury levels.
Fish with the highest mercury levels
Fish that tend to have higher mercury concentrations include:
- King mackerel
- Shark
- Swordfish
- Tilefish from the Gulf of Mexico
- Bigeye and Ahi tuna (usually labeled yellowfin tuna)
- Marlin
- Orange roughy
The FDA and EPA recommend pregnant women and young children eat no more than one serving per month of fish with high mercury levels. A typical serving is 4 ounces.
According to FDA data, mercury concentrations in these fish average around 1 ppm. However, some samples may exceed 1.5 ppm.
Fish with moderate mercury levels
Many popular fish have moderate mercury concentrations, including:
- Canned albacore (“white”) tuna
- Grouper
- Chilean sea bass (aka Patagonian toothfish)
- Halibut
- Lobster
- Mahi mahi
- Bluefish
- Striped bass
The FDA recommends pregnant or nursing women and children eat no more than two to three 4-ounce servings per month of these fish.
Mercury levels in these fish average around 0.2–0.3 ppm. Some samples may exceed 0.5 ppm.
Fish with lower mercury levels
Many popular fish are low in mercury. These include:
- Salmon
- Pollock
- Tilapia
- Shrimp
- Canned light tuna
- Cod
- Catfish
- Clams
- Crab
- Flounder
- Haddock
- Herring
- Oysters
- Sardines
- Trout
- Whitefish
Women who are pregnant or nursing can safely eat up to 12 ounces (two average meals) of these fish per week. Other adults can eat them frequently as part of a healthy diet.
Their mercury levels are mostly under 0.1 ppm and virtually all samples contain less than 0.3 ppm.
Table of mercury levels in fish
| Fish | Mercury content (ppm) |
|---|---|
| Tilefish (Gulf of Mexico) | 1.450 |
| Swordfish | 0.995 |
| Shark | 0.979 |
| King mackerel | 0.730 |
| Bigeye tuna | 0.689 |
| Orange roughy | 0.571 |
| Marlin | 0.485 |
| Ahi tuna | 0.411 |
| Grouper | 0.268 |
| Halibut | 0.241 |
| Lobster | 0.189 |
| Albacore tuna | 0.176 |
| Bluefish | 0.144 |
| Chilean sea bass | 0.128 |
| Striped bass | 0.118 |
| Mahi mahi | 0.089 |
| Canned light tuna | 0.059 |
| Pollock | 0.042 |
| Salmon | 0.022 |
| Tilapia | 0.013 |
| Cod | 0.010 |
| Catfish | 0.009 |
| Clams | 0.003 |
| Crab | 0.006 |
| Shrimp | 0.001 |
| Scallops | 0.003 |
| Oysters | 0.012 |
Recommendations for eating fish
Based on their mercury content, here are recommendations for eating different types of fish:
- Avoid or limit to 1 serving per month: King mackerel, shark, swordfish, tilefish, bigeye tuna, orange roughy, marlin, ahi tuna
- Eat up to 2–3 servings per month: Canned albacore tuna, grouper, halibut, Chilean sea bass, lobster, mahi mahi, bluefish
- Eat up to 2 servings per week: Pollock, salmon, tilapia, cod, catfish, crab, shrimp, scallops, oysters, canned light tuna
- Eat 2–3 servings per week: Flounder, trout, sardines, herring, haddock, whitefish, clams
Pregnant women, those planning to become pregnant, nursing mothers and young children should avoid high mercury fish and limit moderate mercury fish. Other adults can eat from the moderate and low mercury fish lists regularly as part of a healthy diet.
Can you get mercury poisoning from eating too much fish?
Mercury poisoning from eating too much contaminated fish is rare. Mild symptoms may occur at exposures of 0.1 mg of mercury per kg of body weight per day. That would take eating a lot of high mercury fish on a regular basis.
Symptoms of excess mercury exposure can include:
- Impaired neurological development
- Numbness and tingling
- Impaired vision, hearing, and speech
- Lack of coordination
- Memory loss
- Headaches
At very high exposures over 0.6 mg/kg daily, mercury poisoning can be fatal. Symptoms include respiratory failure and severe neurological damage.
The types of fish most likely to cause mercury poisoning if eaten regularly are king mackerel, shark, swordfish, tilefish, bigeye tuna, marlin, and orange roughy.
Eating well below the recommended limits is prudent for these fish. The FDA recommends eating no more than one 4-ounce serving per month.
Who is most at risk of mercury poisoning?
The populations most vulnerable to mercury in fish are:
- Pregnant women: Mercury passes through the placenta and can affect the developing brain and nervous system.
- Nursing mothers: Mercury can be passed on through breast milk.
- Young children: The brain and nervous system continue developing through early childhood.
- Fetuses: Mercury can cross the placenta and cause brain abnormalities and developmental delays.
That’s why women who are pregnant, may become pregnant, or nursing and young children have stricter mercury recommendations.
However, at moderate fish intakes, mercury from fish doesn’t pose a health risk for most adults. Eating fish has many nutritional benefits that outweigh potential mercury concerns.
Tips for reducing mercury exposure from fish
You can reduce mercury exposure from fish by following these tips:
- Check local advisories about contaminated fish in lakes, rivers, and coastal areas. If high mercury fish are advised to avoid, follow those recommendations.
- Choose smaller fish which tend to be lower in mercury.
- Eat a variety of fish rather than just a few kinds.
- Limit high mercury fish to no more than one serving per month.
- Remove skin and fat from fish fillets, where some mercury may accumulate.
- Broil, grill, or bake fish to allow fat and mercury to drain away.
- Avoid or limit fish caught for sport in contaminated waters.
Following the guidelines for fish intake during pregnancy or for young children is also wise. In particular, avoid eating predatory fish that tend to be very high in mercury.
Conclusion
Eating fish provides important nutrients, so avoiding fish altogether due to mercury concerns is not advised. By following guidelines on the amount and types of fish to eat, adults can gain health benefits of seafood without excessive mercury exposure.
Special precautions apply for women who are or may become pregnant, nursing mothers, and young children. They should avoid high mercury fish completely and limit moderate mercury fish. Fish choices lower in mercury are recommended for regular consumption.
Overall, mercury poisoning from fish is uncommon. Eating fish in moderation as part of a balanced diet poses little risk for most people.
