Understanding How Mercury Gets into Seafood
Mercury is a naturally occurring element, but industrial activities release it into the atmosphere, where it can fall into oceans, lakes, and rivers. In aquatic environments, bacteria convert elemental mercury into methylmercury, a highly toxic organic form. This methylmercury is absorbed by small aquatic organisms and then passed up the food chain, becoming more concentrated at each step—a process known as biomagnification.
The Process of Biomagnification
- Small organisms: Plankton and other microorganisms absorb trace amounts of methylmercury from the water.
- Small fish: These fish eat the contaminated microorganisms, accumulating the mercury in their tissues.
- Larger predatory fish: These species consume many smaller fish over their long lifespans, causing the mercury to become highly concentrated in their bodies.
- Humans: When we eat these large predatory fish, we can absorb the accumulated methylmercury.
Why Large, Long-Lived Fish are Highest in Mercury
The key factors determining a fish's mercury content are its size, diet, and lifespan. Large, predatory fish like swordfish and sharks are at the top of the food chain and can live for many years, giving them the most time to accumulate high levels of mercury. In contrast, smaller, shorter-lived fish and shellfish that feed on plankton generally have much lower mercury levels.
The FDA and EPA's 'Choices to Avoid' List
The Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) have issued clear guidance on which seafood has a lot of mercury, particularly for at-risk populations like pregnant or breastfeeding women and young children. Their 'Choices to Avoid' list is comprised of fish with the highest mercury concentrations.
- King Mackerel: A large, fast-swimming predatory fish, it has high mercury content.
- Marlin: Another large, predatory species, marlin is on the highest-mercury list.
- Orange Roughy: A long-lived deep-sea fish, its age contributes significantly to its high mercury levels.
- Shark: As apex predators with long lifespans, sharks consistently test high for mercury.
- Swordfish: This large, predatory fish is known for having some of the highest mercury levels.
- Tilefish (Gulf of Mexico): This specific tilefish population is notorious for its exceptionally high mercury content. (Note: Atlantic Tilefish are a 'Good Choice' with lower mercury).
- Bigeye Tuna: Larger than other tuna species, bigeye (also known as ahi) tuna accumulates more mercury than canned light tuna.
A Comparison of Mercury Levels in Common Seafood
The following table compares some popular seafood choices, indicating their typical mercury concentration and the FDA/EPA consumption category.
| Seafood Type | Common Characteristics | FDA/EPA Category | Typical Mercury Level (PPM) |
|---|---|---|---|
| Shark | Large, long-lived predator | Choice to Avoid | ~0.979 |
| Swordfish | Large, predatory | Choice to Avoid | ~0.995 |
| Bigeye Tuna | Large tuna, predatory | Choice to Avoid | ~0.689 |
| Albacore Tuna (canned) | Larger than skipjack | Good Choice (limited) | ~0.35 |
| Sablefish | Moderate size | Good Choice | ~0.361 |
| Halibut | Large, bottom-dwelling | Good Choice | ~0.241 |
| Tuna (canned light) | Smaller skipjack tuna | Best Choice | ~0.022-0.05 |
| Salmon | Varied, smaller size | Best Choice | ~0.022 |
| Shrimp | Small crustacean | Best Choice | ~0.009 |
| Sardines | Small, plankton-eating | Best Choice | ~0.013 |
Making Safer Seafood Choices
For most people, the nutritional benefits of eating seafood outweigh the risk of mercury exposure, provided they choose wisely and consume in moderation. The key is to select a variety of fish and prioritize those on the FDA's 'Best Choices' list, which includes options rich in beneficial omega-3 fatty acids like salmon. For example, swapping a single meal of swordfish for two meals of salmon or canned light tuna over a week significantly reduces your mercury intake while still providing valuable nutrients.
For those who are pregnant, nursing, or feeding young children, following the FDA's recommendations is especially important to protect the developing nervous system from potential harm caused by high methylmercury exposure. A useful resource for more detailed guidelines is the FDA's official advice on eating fish: https://www.fda.gov/food/consumers/advice-about-eating-fish.
Strategies to Reduce Mercury Risk
- Prioritize variety: Eat a range of fish from both the 'Best' and 'Good' choices to ensure a broad spectrum of nutrients.
- Control portion sizes: Adhere to recommended serving sizes, especially for those in sensitive groups.
- Check local advisories: If consuming locally caught fish, consult local health departments for specific guidance, as mercury levels can vary regionally.
Conclusion
Understanding which seafood has a lot of mercury is vital for making informed dietary choices. By recognizing that large, long-lived predators like shark, swordfish, king mackerel, and bigeye tuna are the primary sources of concern due to biomagnification, consumers can easily moderate their intake. Choosing a variety of low-mercury options, such as salmon, shrimp, and canned light tuna, allows individuals to reap the significant nutritional benefits of seafood while effectively minimizing health risks associated with methylmercury exposure. For at-risk groups, following the FDA and EPA's comprehensive guidelines is the safest path to a healthy diet.
