Why is there mercury in fish?
Mercury is a naturally occurring element that is released into the environment through volcanic activity and human industrial processes, like the burning of coal. Once in the atmosphere, it settles into bodies of water, where microorganisms convert it into a more toxic organic form called methylmercury.
Fish absorb methylmercury from the water as they swim and, more significantly, through their diet. As larger, predatory fish consume smaller, contaminated fish, the mercury becomes more concentrated, a process known as biomagnification. This explains why larger, longer-lived predators like shark and swordfish have the highest mercury levels, while smaller, shorter-lived species contain the least.
Which fish have the lowest mercury levels?
Because no fish can be certified as having zero mercury, the goal for safe consumption is to choose varieties that have the lowest possible concentrations. The FDA and EPA categorize fish into "Best Choices," "Good Choices," and "Choices to Avoid" based on their mercury content.
Best Choices (Very Low Mercury)
- Anchovy
- Atlantic Mackerel
- Catfish
- Clam
- Cod
- Crab
- Crawfish
- Flounder
- Haddock
- Hake
- Herring
- Oysters
- Pollock
- Salmon (Wild and Farmed)
- Sardines
- Scallops
- Shrimp
- Squid
- Tilapia
- Trout (Freshwater)
- Tuna (Canned Light)
Good Choices (Moderate Mercury)
- Albacore Tuna (Canned and Fresh/Frozen)
- Carp
- Grouper
- Halibut
- Mahi Mahi
- Snapper
- Yellowfin Tuna
Mercury Levels in Wild vs. Farm-Raised Fish
One common question is whether farm-raised fish are safer. Some studies suggest that farm-raised fish may contain slightly lower levels of mercury than their wild counterparts, primarily because their diet is more controlled and less likely to contain high concentrations of methylmercury. However, the overall mercury level is most dependent on the fish species itself, its size, and its position in the food chain, with smaller, shorter-lived fish being the best choice regardless of their origin. For example, wild-caught salmon still has very low mercury levels, while farmed grouper would still carry a moderate risk.
A Comparison of Low and High Mercury Fish
| Feature | Low Mercury Fish (Best Choices) | High Mercury Fish (Choices to Avoid) |
|---|---|---|
| Typical Examples | Salmon, Sardines, Shrimp, Tilapia, Cod | Shark, Swordfish, King Mackerel, Tilefish (Gulf) |
| Size and Lifespan | Generally smaller and shorter-lived | Larger, long-lived predators |
| Diet | Eat plankton and smaller organisms | Consume many smaller fish, leading to biomagnification |
| Best for Sensitive Groups | Yes (pregnant women, children) | No (should be avoided) |
| Omega-3s | Often high (e.g., salmon, sardines) | Can be high, but mercury risk outweighs benefit |
| Consumption Frequency | Can be eaten 2-3 times per week | Should be severely limited or avoided |
Guidance for sensitive populations
Certain groups, including pregnant women, women planning to become pregnant, nursing mothers, and young children, should be particularly cautious about mercury intake. For these individuals, the developing nervous system is more vulnerable to the effects of high mercury exposure. The FDA and EPA recommend that these groups consume 2 to 3 servings per week from the "Best Choices" list to gain the nutritional benefits of fish, especially omega-3 fatty acids, while minimizing risk. It is also recommended to limit albacore tuna to no more than 6 ounces per week due to its higher mercury content compared to canned light tuna.
Making smart seafood choices
Selecting fish with the lowest mercury levels is the best strategy for safe and healthy consumption. By opting for smaller, shorter-lived species from the "Best Choices" category, you can enjoy the many nutritional benefits of seafood, such as protein and omega-3s, with minimal risk. While no fish is truly mercury-free, understanding which varieties carry the lowest risk allows you to make informed decisions for yourself and your family. For more information, you can consult the official FDA guidelines.
Conclusion
While the search for a fish with absolutely no mercury is futile due to environmental factors, a wide variety of fish and shellfish contain extremely low, safe levels for consumption. Following advisories from health organizations and prioritizing smaller, shorter-lived species ensures that you can enjoy the significant health benefits of seafood without concern over mercury exposure. Choosing options like salmon, sardines, and shrimp is an excellent way to maintain a healthy and balanced diet.
