What is Mercury and How Does it Get into Fish?
Mercury is a naturally occurring element that enters aquatic ecosystems from both natural sources (such as volcanoes and forest fires) and human activities (like coal-fired power plants and mining). In water, microorganisms convert this inorganic mercury into a highly toxic form called methylmercury. Fish and shellfish absorb methylmercury from the water as they feed, causing it to accumulate in their muscle tissue.
The process of bioaccumulation and biomagnification means that mercury levels increase as you move up the food chain. For instance, small fish that eat algae will have low levels of methylmercury. However, larger, predatory fish that eat those smaller fish will accumulate higher concentrations. This is why species like shark, swordfish, and bigeye tuna consistently have higher mercury levels than smaller fish like salmon or sardines.
Does Freezing Remove Mercury from Fish?
No, freezing does not remove or reduce the mercury content in fish. The chemical bond between methylmercury and the muscle proteins is very stable and is not broken down by the cold temperatures of a freezer. This fact is critical for consumers to understand. Whether a fish is sold fresh, frozen, or canned, its mercury level remains the same based on its species, size, and origin.
Studies comparing fresh, frozen, and canned fish have consistently found that freezing does not alter the mercury concentration. The benefit of freezing is that it preserves the fish's nutritional value and freshness by halting bacterial and enzymatic activity, allowing it to be shipped globally and stored for longer periods. The decision to buy fresh versus frozen should therefore be based on factors like cost, convenience, and desired quality, not on the misguided belief that one is safer due to mercury content.
Which Frozen Fish are High in Mercury vs. Low in Mercury?
When navigating the seafood aisle, whether fresh or frozen, it is helpful to be aware of which species typically contain higher and lower levels of mercury. The FDA and EPA provide guidance to help consumers make informed choices.
Best Choices (Low Mercury):
- Salmon (canned and fresh/frozen)
- Shrimp
- Sardines
- Tilapia
- Catfish
- Pollock
Good Choices (Moderate Mercury):
- Canned albacore tuna
- Cod
- Mahi Mahi
- Snapper
Choices to Avoid (High Mercury):
- Shark
- Swordfish
- King Mackerel
- Bigeye Tuna
- Orange Roughy
- Tilefish (from Gulf of Mexico)
Making Informed Seafood Choices
For a balanced diet, including a variety of seafood is recommended due to its high protein content and beneficial omega-3 fatty acids. However, particularly for vulnerable populations, such as pregnant women, nursing mothers, and young children, making low-mercury choices is especially important.
Comparison of High-Mercury and Low-Mercury Fish
| Feature | High-Mercury Fish (e.g., Shark, Swordfish) | Low-Mercury Fish (e.g., Salmon, Shrimp) |
|---|---|---|
| Trophic Level | Higher on the food chain, large predators | Lower on the food chain, smaller fish and shellfish |
| Lifespan | Long-lived species | Generally shorter lifespan |
| Mercury Accumulation | High levels due to biomagnification | Low levels due to smaller size and diet |
| Consumption Advice (FDA/EPA) | Limited or avoided entirely, especially for at-risk groups | Recommended for regular consumption, 2-3 servings per week |
| Form | Available fresh or frozen | Available fresh, frozen, or canned |
| Risk for Vulnerable Groups | Potentially harmful to the developing nervous system | Very low risk, beneficial for development |
Conclusion
The mercury content of fish is a natural characteristic determined by its species, size, age, and environment, not by whether it is fresh or frozen. Freezing is a preservation method that locks in a fish's quality and nutritional value at the time it was processed, without altering its mercury level. Therefore, when choosing fish for a healthy nutrition diet, consumers should focus on selecting species known to be low in mercury, regardless of their market form. By following guidance from health authorities like the FDA and EPA, individuals can confidently reap the many nutritional benefits of seafood while minimizing mercury exposure. The key is mindful consumption, favoring smaller, younger, and non-predatory species, whether they are on ice or in the freezer.
For more detailed information and up-to-date fish consumption advice, refer to the official FDA guidelines.
