What fish carry a lot of mercury? A Guide to High-Mercury Seafood

Studies have shown that mercury concentrations differ significantly among fish species, primarily due to a natural process called bioaccumulation. Knowing what fish carry a lot of mercury is essential for consumers who want to minimize their exposure to this neurotoxin while still enjoying the nutritional benefits of seafood.

Quick Summary

This guide details high-mercury species like shark, swordfish, and king mackerel, explaining why they accumulate more toxins. It also highlights safer, low-mercury alternatives and provides tips for informed seafood consumption.

Key Points

• Top Predators Accumulate More Mercury: Larger, longer-lived predatory fish like shark, swordfish, and king mackerel are highest in mercury due to biomagnification.

• Tuna Varies by Species: Canned light tuna (skipjack) is a low-mercury option, while larger species like bigeye and albacore contain higher concentrations.

• Vulnerable Groups Need Caution: Pregnant women, nursing mothers, and young children should limit or avoid high-mercury fish due to developmental risks.

• Choose Low-Mercury Fish Freely: Enjoy smaller, shorter-lived species such as salmon, shrimp, sardines, and pollock with less concern.

• Variety is Key: Eating a diverse range of fish helps minimize overall mercury exposure from any single source.

• Check FDA/EPA Guidelines: Follow the specific consumption advice from official health organizations for safe seafood intake.

The Science of Mercury: Bioaccumulation and Biomagnification

Mercury is a naturally occurring element that finds its way into waterways from both natural events, like volcanic eruptions, and human industrial activities, such as coal-burning power plants. In aquatic environments, bacteria convert elemental mercury into methylmercury, a highly toxic organic form that fish can easily absorb. This process is the foundation for the accumulation of mercury in fish.

How Mercury Moves Up the Food Chain

Methylmercury's levels increase dramatically through a process known as biomagnification. It works like this:

• Tiny organisms, like algae, absorb a small amount of methylmercury from the water.
• Small fish eat large quantities of these contaminated algae or plankton, accumulating the mercury in their tissues.
• Larger predatory fish eat many of these smaller fish, causing the mercury levels to concentrate even further up the food chain.

Because of this process, the highest concentrations of mercury are found in fish that are large, long-lived, and at the top of the marine food web. The FDA and EPA provide guidance on safe consumption levels, especially for sensitive groups.

Top Contenders: What Fish Carry a Lot of Mercury?

Several species are consistently flagged as high in mercury and should be consumed sparingly or avoided entirely, especially by pregnant women, nursing mothers, and young children.

Fish to Strictly Avoid or Severely Limit

• Shark: As an apex predator, shark meat, sometimes sold as "flake," has very high mercury levels.
• Swordfish: This large, predatory fish is known for carrying high concentrations of mercury and should be limited.
• King Mackerel: Found in both the Atlantic and Gulf of Mexico, this mackerel species contains significantly higher mercury than smaller mackerel varieties.
• Tilefish (Gulf of Mexico): This species has one of the highest mercury levels measured and should be avoided. The FDA differentiates it from Atlantic Tilefish, which has lower levels.
• Bigeye Tuna: This is a larger species of tuna, often used for sushi (ahi), and contains more mercury than its smaller counterparts like skipjack.
• Marlin: Similar to swordfish, marlin is a large, predatory fish with high mercury content that should be limited.
• Orange Roughy: This long-lived, deep-sea fish accumulates substantial mercury over its lifespan.

Fish with Moderate Mercury Levels

For most adults, these fish can be enjoyed in moderation, following agency guidelines. However, sensitive populations might need to limit them further.

• Albacore (White) Tuna: This type of canned tuna has higher mercury levels than canned light tuna (skipjack).
• Grouper: Different species of grouper have varying mercury levels, but generally fall into the moderate category.
• Chilean Sea Bass (Patagonian toothfish): This deep-sea fish contains moderate mercury levels.
• Bluefish: A large, predatory fish with higher mercury content.

A Safe Harbor: Low-Mercury Fish Options

Incorporating a variety of low-mercury fish into your diet is a great way to enjoy the health benefits of seafood with minimal risk. These are generally smaller, shorter-lived species.

• Salmon: A fatty fish rich in omega-3s, salmon is a consistently low-mercury choice.
• Shrimp: This popular shellfish has very low mercury levels.
• Canned Light Tuna (Skipjack): Made from smaller, younger fish, canned light tuna is a safer option than albacore.
• Catfish: Whether wild-caught or farm-raised, catfish is generally low in mercury.
• Sardines and Anchovies: These small, oily fish are very low on the food chain and contain minimal mercury.
• Pollock: Commonly used for fish sticks, pollock is a low-mercury fish.
• Haddock: This white fish is a safe, low-mercury choice.

Comparing Mercury Levels in Seafood

Seafood Type	Common Examples	Mercury Status	Mean Concentration (PPM)
Highest Mercury	Tilefish (Gulf of Mexico), Swordfish, Shark, King Mackerel	Avoid or very limited	0.73 - 1.123
High Mercury	Bigeye Tuna, Orange Roughy, Marlin	Limited consumption	0.485 - 0.689
Moderate Mercury	Albacore Tuna, Grouper, Halibut	Eat in moderation	0.241 - 0.448
Low Mercury	Salmon, Shrimp, Canned Light Tuna, Catfish	Eat 2-3 times per week	0.009 - 0.126

Who Should Be Most Cautious of High-Mercury Fish?

The developing nervous system is particularly sensitive to the neurotoxic effects of methylmercury. For this reason, specific groups should be especially cautious about their seafood consumption. The FDA provides targeted advice for these individuals:

• Pregnant or Nursing Women: Mercury can pass from the mother to the fetus or infant. High exposure can cause developmental issues.
• Women Planning to Become Pregnant: Methylmercury can persist in the body for over a year, so women trying to conceive should also follow special guidelines.
• Young Children: Infants and toddlers are more susceptible to mercury's effects on the developing brain and should only consume low-mercury fish in limited amounts.

Making Smarter Seafood Choices: Practical Tips

Making informed choices doesn't mean giving up seafood entirely. Fish remains an important source of nutrients, including omega-3 fatty acids, which are crucial for brain health. Here are some tips for safer consumption:

1. Vary Your Fish Intake: By eating a wide variety of fish from different categories, you reduce the risk of consuming too much mercury from a single source. Prioritize fish from the "low mercury" list.
2. Check the Source of Tuna: Be mindful of the type of tuna you purchase. For lower mercury content, opt for canned light tuna (skipjack) over canned albacore or fresh bigeye tuna steaks.
3. Read Local Advisories: Pay attention to local fish advisories, especially for recreationally caught fish. Mercury levels can vary depending on local water pollution.
4. Practice Moderation: Even for healthier fish, moderation is key. A balanced diet incorporates a range of proteins and nutrients.
5. Consider Alternatives: If you are highly concerned about mercury, consider other healthy protein sources or look for reputable fish oil supplements that are third-party tested for mercury purity.

Conclusion: Balancing Health and Nutrition

While high-mercury fish like shark and swordfish should be consumed with caution, particularly by vulnerable groups, the vast majority of seafood offers significant nutritional benefits with low risk. By understanding the process of biomagnification and making deliberate, varied choices, you can continue to enjoy fish as a healthy and delicious part of your diet. The key is to prioritize smaller, shorter-lived species and stay informed about consumption advisories from trusted sources like the FDA and EPA.

For more information on the safety of fish consumption, you can refer to the FDA's detailed guidance.

Frequently Asked Questions

Which specific fish should be completely avoided by pregnant women?

The FDA and EPA advise pregnant women and those planning to become pregnant to avoid consuming shark, swordfish, king mackerel, and tilefish from the Gulf of Mexico because they contain the highest levels of mercury.

Is all canned tuna high in mercury?

No, mercury levels differ based on the tuna species. Canned 'light' tuna, typically made from smaller skipjack tuna, is considered low in mercury. Canned 'white' or albacore tuna has higher mercury levels and should be limited.

What is the primary reason some fish have more mercury than others?

Larger, predatory fish that live longer are higher in mercury due to a process called biomagnification. They consume smaller fish that contain mercury, causing the toxin to accumulate and concentrate in their own tissue over time.

What are some examples of low-mercury fish?

Excellent low-mercury choices include salmon, sardines, anchovies, shrimp, pollock, and catfish. These are generally smaller and lower on the food chain.

How does mercury get into fish in the first place?

Mercury is released into the atmosphere from both natural and man-made sources. It settles into waterways where bacteria convert it into methylmercury, which is then absorbed by aquatic life and builds up in the food chain.

Can cooking or preparing fish reduce its mercury content?

No, cooking methods such as frying, baking, or grilling have no effect on reducing the methylmercury content in fish. The mercury is bound to the protein in the fish's tissue.

Are there any health benefits to eating fish despite the mercury risk?

Yes, fish is an important source of high-quality protein and beneficial omega-3 fatty acids. These nutrients support brain and heart health. For most people, the health benefits of eating low-mercury fish outweigh the potential risks of moderate mercury exposure.

What are the health risks of high mercury exposure?

High levels of methylmercury can act as a neurotoxin, potentially causing neurological and behavioral problems such as tremors, memory loss, and mood swings. It is particularly dangerous for unborn babies and young children.