The Bioaccumulation of Mercury in Fish
Mercury is a naturally occurring element that makes its way into our oceans, lakes, and rivers. Human activities, such as industrial pollution and coal burning, also contribute to environmental mercury levels. Once in the water, microorganisms convert mercury into a more toxic form called methylmercury. Fish absorb this compound from the water and through their diet. This process is known as bioaccumulation.
Methylmercury concentrations increase as you move up the aquatic food chain, a process called biomagnification. This means that larger, longer-lived predatory fish accumulate the highest levels of mercury because they eat many smaller, mercury-containing fish over their lifetime. This is the primary reason some fish are a much greater concern for mercury content than others.
High-Mercury Fish: The Choices to Avoid
Health authorities advise avoiding or limiting consumption of certain fish species with high mercury levels, particularly for pregnant women and young children. These typically include large predatory fish.
List of High-Mercury Fish
- King Mackerel
- Marlin
- Orange Roughy
- Shark
- Swordfish
- Tilefish (from the Gulf of Mexico)
- Bigeye Tuna
- Tuna Steak
Lower-Mercury Alternatives
Many fish offer great nutritional benefits with low mercury levels, and these are generally smaller fish lower on the food chain.
List of Low-Mercury Fish
- Salmon
- Sardines
- Shrimp
- Catfish
- Tilapia
- Cod
- Canned Light Tuna
High Mercury vs. Low Mercury Fish: A Comparison
| Feature | High-Mercury Fish | Low-Mercury Fish |
|---|---|---|
| Typical Species | Shark, Swordfish, King Mackerel, Bigeye Tuna, Orange Roughy | Salmon, Sardines, Shrimp, Tilapia, Cod, Catfish |
| Life Span | Long-lived, providing more time for mercury accumulation | Shorter-lived, limiting the time for mercury build-up |
| Position in Food Chain | Apex predators (eat other fish), resulting in biomagnification | Lower on the food chain (eat plankton and small organisms), less accumulation |
| Primary Health Concern | Risk of neurological damage, especially for unborn babies and young children | Generally safe, provides health benefits like omega-3s with minimal mercury risk |
| Consumption Advisory | Avoid or limit consumption significantly, especially for vulnerable groups | Recommended for regular consumption, even for pregnant women and children |
Practical Steps for Reducing Mercury Exposure
Fish provide important nutrients, including omega-3s. The key to safe consumption is making smart choices. For pregnant or breastfeeding women and young children, the FDA and EPA recommend 2 to 3 servings (8 to 12 ounces) per week from the low-mercury list. Children's serving sizes should be smaller. Opt for canned 'light' tuna over 'white' (albacore) due to lower mercury. Be aware of local fishing advisories for mercury levels in specific waters. If no information is available, limit consumption of locally caught fish.
Conclusion
To minimize mercury exposure, be mindful of which fish are high in mercury, typically large predatory species, and prioritize low-mercury options. This is especially important for vulnerable populations like pregnant women and young children to protect neurological development. Choosing low-mercury fish allows you to safely enjoy the nutritional benefits of seafood.
