What is methylmercury and how does it get into fish?
Mercury is a naturally occurring element, but human activities like burning coal and industrial waste contribute significantly to its release into the environment. In water bodies, microorganisms convert this mercury into a more toxic form called methylmercury. This methylmercury is then absorbed by small organisms and works its way up the food chain, a process known as bioaccumulation and biomagnification. Fish absorb methylmercury from the water they live in and the food they eat. Since they excrete it very slowly, the amount of mercury accumulates in their muscle tissue over their lifetime. This is why larger, older, and predatory fish—those at the top of the food chain—tend to have the highest mercury levels.
The top contenders for highest mercury content
When identifying what has the highest mercury in fish, several species consistently appear on advisory lists from health organizations like the FDA, EPA, and environmental groups. These species are predominantly large, predatory fish.
- Tilefish (Gulf of Mexico): This fish often tops the list for average mercury concentration. Its long lifespan and predatory nature contribute to very high levels, leading to a strong recommendation to avoid it.
- Shark: As apex predators, sharks consume large quantities of other fish, leading to significant mercury bioaccumulation. All types of shark meat are considered to have high mercury content.
- Swordfish: Another large, predatory species, swordfish is known for high mercury levels. Pregnant women, those trying to become pregnant, and children are advised to avoid it entirely.
- King Mackerel: This fish also contains some of the highest mercury concentrations and is strongly recommended against for vulnerable populations.
- Bigeye Tuna: While all tuna has some mercury, bigeye tuna is one of the types with significantly higher levels and is included on the FDA's 'Choices to Avoid' list.
- Orange Roughy: This is a long-lived species that bioaccumulates mercury over many years. Its slow maturation and vulnerability to overfishing also make it an eco-unfriendly choice.
Health impacts of mercury exposure
For most healthy adults, occasional consumption of higher-mercury fish is not a major health concern, but the mercury can accumulate over time. The primary danger lies in long-term or high-dose exposure, particularly for developing nervous systems in unborn babies and young children. The most common form of mercury in fish is methylmercury, a potent neurotoxin. High exposure can lead to neurological damage, affecting cognitive thinking, coordination, memory, and motor skills.
Common symptoms of high methylmercury exposure include:
- A tingling sensation or 'pins and needles' in the hands, feet, and mouth.
- Impairment of speech, hearing, and vision.
- Lack of coordination or muscle weakness.
For those who consume a significant amount of fish, these risks are important to consider. The FDA and EPA provide clear guidelines to help consumers make informed choices and minimize their exposure.
High mercury vs. low mercury fish comparison
To better illustrate the differences in mercury content, consider this comparison based on FDA/EPA data.
| Feature | High Mercury Fish (Choices to Avoid) | Low Mercury Fish (Best Choices) |
|---|---|---|
| Example Species | Tilefish (Gulf), Shark, Swordfish, King Mackerel, Bigeye Tuna, Marlin, Orange Roughy | Salmon, Sardines, Shrimp, Catfish, Canned Light Tuna, Scallops, Anchovies |
| Life Span | Typically long-lived | Generally shorter-lived |
| Position in Food Chain | Apex predator; high trophic level | Lower trophic level, consumes smaller organisms or plankton |
| Bioaccumulation | Significant accumulation over a lifetime | Very low levels of accumulation |
| Consumption Advice | Avoid, especially for vulnerable groups like pregnant women and children | Safe to eat 2-3 servings per week |
How to make safer seafood choices
Fortunately, there are many fish and shellfish options that are low in mercury and rich in beneficial nutrients like omega-3 fatty acids.
- Diversify your fish intake: Don't rely on just one type of fish. By eating a variety of species, you can enjoy the health benefits of seafood while minimizing exposure to contaminants.
- Prioritize low-mercury options: Opt for choices from the FDA/EPA's 'Best Choices' list, which includes anchovies, Atlantic mackerel, catfish, clams, cod, salmon, sardines, scallops, shrimp, tilapia, and trout.
- Choose canned light tuna over albacore: Canned light tuna, made from smaller skipjack, has significantly lower mercury than albacore (white) tuna.
- Eat smaller, younger fish: Since mercury accumulates over a fish's lifespan, smaller and younger fish tend to have lower levels of contamination.
- Check local advisories: For fish caught recreationally, consult local advisories from your state or environmental protection agency, as mercury levels can vary by location.
- Understand the source: Ask your seafood supplier about the origin of their fish. For certain species like Atlantic Tilefish, the source location can dramatically impact its mercury content.
Conclusion
In conclusion, understanding what has the highest mercury in fish is key to making safe and healthy dietary choices. The highest mercury levels are found in large, predatory, and long-lived species such as Tilefish (from the Gulf of Mexico), Shark, Swordfish, and King Mackerel due to the process of bioaccumulation. Conversely, smaller, shorter-lived fish like salmon, sardines, and shrimp have much lower concentrations and can be enjoyed regularly as part of a healthy diet. By following guidelines from regulatory bodies like the FDA and EPA and diversifying your seafood intake, you can confidently enjoy the nutritional benefits of fish while minimizing your risk of mercury exposure, especially for sensitive groups like pregnant women and young children.
