Why is tuna high in mercury?

Tuna is one of the fish that contains the most mercury because the long-term feeding process causes them to accumulate mercury in their bodies.

Tuna is an excellent source of high-quality protein, heart- and brain-healthy fatty acids, and essential vitamins and minerals like vitamin D, B12, and selenium. However, this nutrient-rich fish has a major drawback: it is loaded with mercury, a heavy metal that is harmful to the nervous system, according to IFL Science .

Yellowfin tuna is one of the types of tuna processed for canning. (Photo: Thahaali).

Compared to most other seafood, tuna meat is relatively high in mercury . The U.S. Food and Drug Administration (FDA) estimates that tuna contains an average of 0.126 and 0.689 parts per million of mercury. Some types of tuna have even higher concentrations. For example, albacore tuna, also known as longfin tuna , typically contains about three times more mercury than skipjack or yellowfin tuna.

Most fish with significant concentrations of mercury are apex predators such as sharks, swordfish, marlin, and king mackerel. This is due to the process of bioaccumulation . Inorganic mercury enters the natural environment through industrial pollution, and is then converted to its organic form, methylmercury, by microorganisms in marine ecosystems. Small amounts of methylmercury can also form naturally by bacteria in water and sediments.

This chemical is consumed by small aquatic organisms, which in turn become food for larger predators in the food chain. Because mercury is absorbed faster than it can be metabolized or excreted, large amounts of the metal accumulate at each level of the food chain, with the apex predators consuming the most. Mercury is so abundant in the environment that even tuna living in the middle of the ocean contain some of the chemical.

A 2024 study found that mercury levels in tuna have remained stable despite a decline in global mercury emissions since the 1970s. Researchers speculate that the stable levels of mercury in tuna may be the result of mercury moving upward from deeper ocean layers to surface waters , where tropical tuna feed. It is likely that the mercury was released into the environment years or decades ago.

The amount of mercury in food is very small, but it can build up over time, especially if consumed in large amounts over a long period of time. According to the Environmental Protection Agency (EPA), symptoms of methylmercury poisoning can include problems with peripheral vision, a tingling sensation, usually in the hands, feet, and around the mouth, lack of coordination, impaired speech, hearing, and walking, and muscle weakness.

However, you would need to consume a fair amount of tuna over a long period of time before you need to worry about mercury poisoning. Some scientists estimate that a healthy adult could consume about 95 grams of tuna a week before running a serious risk of mercury poisoning.