Methylmercury, an organic form of the element, is the primary contaminant of concern in seafood, posing a health risk, particularly to the developing nervous system. Nearly all fish and shellfish contain trace amounts of this compound, which originates from environmental sources and accumulates in aquatic life. This article identifies the tuna species with the lowest levels of methylmercury, helping consumers maximize nutritional benefits while minimizing exposure.
Understanding Mercury Bioaccumulation in Tuna
Mercury enters the aquatic environment from both natural sources and human activities, such as coal combustion, where it is transformed into methylmercury by bacteria in the water. This potent neurotoxin is readily absorbed by marine organisms at the base of the food web. The process of bioaccumulation causes the concentration of the toxin to increase as it moves up the trophic levels, or the food chain.
Tuna are top-level predators, meaning they consume smaller fish that have already accumulated mercury in their tissues. Because the fish eliminate methylmercury at a much slower rate than they take it in, the toxin builds up over the animal’s lifetime. Consequently, the final mercury load in a tuna is directly proportional to its size, age, and position within the marine food web.
Comparing Mercury Levels Across Tuna Species
Methylmercury concentration varies widely across the four main species of commercially available tuna. The lowest mercury content is consistently found in Skipjack tuna, which is the species most commonly sold as “canned light tuna.” Skipjack are relatively small tuna with a short lifespan, which limits the amount of time they have to accumulate the contaminant. Mean mercury concentrations for Skipjack generally fall in the range of 0.126 to 0.144 parts per million (PPM).
The mercury content rises significantly in Albacore tuna, which is typically marketed as “white tuna” in canned form. Albacore are larger and live longer than Skipjack, resulting in a mean mercury concentration approximately three times higher, measuring around 0.350 to 0.358 PPM.
Similarly, Yellowfin tuna, often sold as steaks or in sushi, shows a mean mercury concentration comparable to Albacore, averaging about 0.354 PPM. While Yellowfin are large, their mercury levels are generally slightly lower or equal to Albacore, depending on the specific catch location and size of the fish. These intermediate levels place both Albacore and Yellowfin in a category requiring moderate consumption limits for the general population.
The highest mercury levels among the commonly consumed species are found in Bigeye tuna, a large, deep-diving, and long-lived predator often used for sushi and steaks. Bigeye tuna has a mean mercury concentration of approximately 0.689 PPM, which is the highest of the four types and nearly five times that of Skipjack. This high level is a function of both their large size and their tendency to inhabit deeper waters where natural methylmercury production can be higher.
Safe Consumption Guidelines and Recommendations
Translating the differences in mercury concentrations into a healthy diet requires following official consumption guidelines. The most vulnerable groups, including pregnant women, breastfeeding women, and young children, must be especially careful due to the risk of neurological damage to the developing brain. For these groups, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) advise eating a minimum of eight ounces and up to twelve ounces (two to three servings) per week of fish that is low in mercury.
For the general population, the guidance for tuna involves distinguishing between the species. Canned light tuna, which is primarily Skipjack, falls into the “Best Choices” category and can be safely consumed two to three times per week. This classification is due to its low methylmercury content and high nutritional value.
However, the consumption of higher-mercury options like Albacore (white tuna) must be limited for vulnerable groups to no more than six ounces per week. Bigeye tuna, due to its very high mercury content, should be avoided entirely by pregnant women, breastfeeding mothers, and young children. The official advice also recommends avoiding other very high-mercury fish, such as shark, swordfish, king mackerel, and tilefish.
To minimize risk while still enjoying the nutritional benefits of seafood, consumers can choose other low-mercury alternatives. Fish like salmon, sardines, shrimp, pollock, and tilapia contain minimal levels of methylmercury and are rich sources of omega-3 fatty acids and protein.