Understanding Mercury and Bioaccumulation
Mercury is a naturally occurring element, but industrial pollution, particularly from coal-fired power plants, has increased its presence in the environment. In water, it is converted into methylmercury, a highly toxic organic compound that is easily absorbed by aquatic life. The primary reason some fish have higher mercury levels than others is a process called biomagnification.
Biomagnification is the increasing concentration of a substance, such as mercury, in organisms as it moves up the food chain. Small organisms like algae absorb methylmercury from the water. These are then eaten by small fish, which are in turn consumed by larger predatory fish. With each step up the food chain, the concentration of mercury increases significantly in the predator's body tissue. Therefore, larger, longer-lived fish that are high on the food chain tend to accumulate the most mercury.
Which Types of Fish Have the Most Mercury?
Based on guidelines from health organizations like the FDA and EPA, specific species are identified as having very high or high levels of mercury and should be limited or avoided entirely, especially by vulnerable populations.
Very High Mercury Fish (Choices to Avoid)
These species are the most concerning due to their consistently high mercury concentrations:
- Tilefish (Gulf of Mexico): This species is known to have some of the highest mercury levels. Note that Atlantic tilefish have lower levels.
- Shark: As apex predators, sharks contain very high amounts of mercury.
- Swordfish: Large and predatory, swordfish accumulate high levels of mercury throughout their long lives.
- King Mackerel: This large, predatory mackerel is on the list of fish to avoid.
- Marlin: Similar to swordfish, marlin is a large predator with high mercury content.
- Orange Roughy: A long-lived, deep-sea fish, orange roughy also has high mercury levels.
- Bigeye Tuna (Ahi): This specific type of tuna contains significantly more mercury than other varieties.
High Mercury Fish
These fish generally have high mercury levels but may be less consistently high than those on the "avoid" list. Consumption should still be limited:
- Bluefish
- Grouper
- Chilean Sea Bass
- Spanish Mackerel
- Some fresh/frozen Tuna (such as Yellowfin)
Mercury Comparison: High, Moderate, and Low Levels
For a comparison of mercury levels in some common fish species based on mean parts per million (PPM), please refer to {Link: FDA/EPA 2004 Advice on What You Need to Know About Fish https://www.fda.gov/food/environmental-contaminants-food/fdaepa-2004-advice-what-you-need-know-about-mercury-fish-and-shellfish}. The table there provides details on relative mercury levels, consumption advice, and the main reasons for these levels across various species including Shark, Swordfish, Bigeye Tuna, Canned Albacore Tuna, Tuna Steak, Halibut, Canned Light Tuna, Salmon, and Shrimp.
Safer Seafood Choices (Low Mercury)
For those concerned about mercury exposure, especially women who are pregnant or nursing, and young children, focusing on fish lower on the food chain is the best approach. The FDA identifies a wide variety of fish and shellfish as low in mercury and safe for regular consumption. This includes:
- Salmon (canned or fresh)
- Sardines
- Catfish
- Tilapia
- Cod
- Pollock
- Shrimp
- Scallops
- Anchovies
- Crab (Domestic)
- Squid
For a full list of choices, the FDA provides a useful reference on its website. It is also a good practice to eat a variety of fish to balance nutrients and minimize potential exposure to any single contaminant. For more information, see the {Link: FDA FDA Advice about Eating Fish https://www.fda.gov/food/consumers/advice-about-eating-fish}.
Conclusion
The type of fish with the most mercury is consistently the large, long-lived predator at the top of the food chain, such as shark, swordfish, and specific types of tilefish and tuna. Conversely, smaller, shorter-lived species like salmon, sardines, and shrimp have the lowest levels. For consumers, especially sensitive groups, the key takeaway is to choose seafood wisely and in moderation. By diversifying your fish intake and prioritizing smaller, low-mercury options, you can enjoy the significant health benefits of seafood while effectively managing your mercury exposure.
