Mercury naturally exists in the environment, but industrial activities have increased its presence in the oceans. When mercury enters aquatic ecosystems, bacteria convert it into methylmercury, a highly toxic organic form. This compound accumulates in the tissues of marine life, making it the primary contaminant in seafood. Understanding this process helps consumers choose fish options that provide nutritional benefits while minimizing exposure.
Understanding How Mercury Enters the Food Chain
The initial source of mercury is the burning of coal, industrial waste, and mining operations, which release elemental mercury into the atmosphere. This atmospheric mercury settles into water bodies, such as oceans, rivers, and lakes. Once in the water, anaerobic bacteria in the sediment transform the inorganic mercury into methylmercury, the form that readily enters the food web.
The first step in contamination is bioaccumulation, where microscopic organisms like phytoplankton absorb methylmercury from the water. Since these organisms cannot rapidly eliminate the compound, the concentration builds up in their tissues over time. This process then transitions into biomagnification as the compound moves up the food chain.
Biomagnification explains why larger, predatory fish have higher mercury concentrations than smaller ones. When a small fish eats contaminated plankton, and a larger fish eats those small fish, the toxin concentration increases exponentially at each successive trophic level. Consequently, long-lived species that occupy the top of the marine food web, such as sharks and swordfish, accumulate the highest levels of methylmercury.
A Guide to Low-Mercury Fish Species
Fish consistently low in mercury generally have a shorter lifespan, a smaller body size, and a diet lower on the aquatic food chain, often consisting of plants or smaller invertebrates. These species do not live long enough to accumulate high concentrations of the compound. Shellfish, for instance, are excellent low-mercury choices because they are filter feeders and occupy a low position on the food web.
The U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) classify many species as “Best Choices” due to their low methylmercury levels. This category includes favorites like salmon, prized for its omega-3 fatty acids and low mercury content. Farmed salmon often show lower levels than wild varieties, though both are generally safe.
Other popular “Best Choices” include small, fast-growing white fish such as tilapia, cod, and flounder. These species are widely available and are safe to incorporate into the diet multiple times a week. Catfish and pollock are routinely tested and found to have minimal mercury levels, making them reliable choices for regular consumption.
Smaller schooling fish, which are low on the food chain, offer another safe and nutritious option. These include anchovies, sardines, and herring, often consumed whole or canned. Canned light tuna (usually skipjack tuna) has much lower mercury levels than canned albacore tuna, making it a preferable choice for frequent consumption.
Shellfish are among the safest choices regarding mercury content. These invertebrates filter their food supply and do not biomagnify mercury to the degree that larger finfish do. They maintain a consistently low mercury profile.
- Shrimp
- Oysters
- Scallops
- Clams
- Crab
Who Needs to Monitor Intake and Consumption Guidelines
Exposure to high levels of methylmercury is a concern because the compound acts as a neurotoxin, primarily affecting the nervous system. The greatest risk is to the developing brain and nervous system of a fetus or young child. Because of this vulnerability, the following populations need to monitor their fish intake most carefully:
- Pregnant women
- Women who may become pregnant
- Breastfeeding mothers
- Young children
The FDA and EPA issue consumption guidelines to help these vulnerable groups maximize the nutritional benefits of fish while minimizing mercury risk. For women who are pregnant or breastfeeding, the recommendation is to consume 8 to 12 ounces of low-mercury fish per week (two to three servings). This advice ensures adequate intake of nutrients like omega-3 fats, which are important for fetal brain development.
Children should also be served fish, with portion sizes scaled down based on age and calorie needs. Children should eat two servings per week from the “Best Choices” list to support healthy growth. These populations must strictly avoid the four species designated as having the highest mercury levels:
- Shark
- Swordfish
- King mackerel
- Gulf of Mexico tilefish
Albacore tuna, often labeled as “white” tuna, has higher methylmercury levels than canned light tuna and is listed as a “Good Choice.” This classification requires a more restricted intake. Vulnerable groups are advised to limit albacore consumption to one serving per week to stay within safe exposure limits. By focusing on the “Best Choices” list, consumers can incorporate the health benefits of seafood into their diet.