Mercury is a heavy metal pollutant found throughout the environment, and its presence in food, particularly seafood, is a public health consideration. The primary concern is methylmercury, a highly toxic organic compound created when microorganisms chemically transform inorganic mercury. This substance is readily absorbed by aquatic life and accumulates in their tissues because it is excreted very slowly. Bioaccumulation means seafood consumption is the most common source of human exposure.
Seafood with the Highest Mercury Levels
The concentration of methylmercury in aquatic species is governed by biomagnification, which causes levels to increase dramatically. This explains why larger, longer-lived predatory fish consistently exhibit the highest mercury levels, as they consume smaller, contaminated fish over their lifespan. The U.S. Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) advise against consuming species in the “Choices to Avoid” category.
The four primary high-mercury offenders are Shark, Swordfish, King Mackerel, and Tilefish (Gulf of Mexico). These species contain mercury concentrations that exceed the safety limit for regular consumption. Other predatory fish, such as Marlin and Bigeye Tuna, also accumulate significant amounts of methylmercury. Bigeye Tuna contains a substantially higher concentration compared to other tuna varieties.
Health Effects of Mercury Exposure
Methylmercury is a potent neurotoxin that targets the central nervous system. In adults, high-level exposure can lead to neurological symptoms, including loss of peripheral vision, muscle weakness, and impaired coordination. Individuals may also experience sensory disturbances, such as a “pins and needles” feeling in the hands and feet, and difficulties in speech and hearing.
The most vulnerable populations are pregnant women, nursing mothers, and young children, as methylmercury can cross the placenta and the blood-brain barrier. Prenatal exposure is particularly damaging because the fetal brain is rapidly developing and highly susceptible to disruption. This exposure can lead to irreversible developmental impacts, affecting a child’s cognitive thinking, memory, fine motor skills, and language development.
Safe Consumption Guidelines and Low-Mercury Alternatives
Federal health bodies provide guidance to help consumers enjoy the nutritional benefits of fish while minimizing mercury exposure, categorizing fish into “Best Choices,” “Good Choices,” and “Choices to Avoid.” For pregnant women, nursing mothers, and young children, the advice is to eat two to three servings per week from the “Best Choices” list. A standard adult serving size is four ounces, measured before cooking.
The “Best Choices” list includes commonly consumed fish and shellfish that are low in mercury:
- Salmon
- Shrimp
- Catfish
- Tilapia
- Pollock
This category also includes Canned Light Tuna, which is typically made from smaller skipjack tuna and has much lower mercury levels than Albacore or “white” tuna.
Albacore tuna is listed as a “Good Choice,” and consumption should be limited to only one six-ounce serving per week for vulnerable groups. The “Good Choices” category contains species like Grouper, Mahi-Mahi, and Snapper, which can be eaten once per week. By choosing fish from the “Best Choices” category, individuals can confidently maintain a healthy diet that includes seafood.