Which fish contains the most mercury and other heavy metals?

Large size and long lifespan. The longer a fish lives and the larger its mass, the more mercury it accumulates.

Mercury, especially in the form of methylmercury, is not efficiently excreted from the body. With each passing year, the concentration increases, a process known as bioaccumulation. For example, tuna can live up to 20 years, and sharks can live over 30, making them true "reservoirs" of toxic substances. 

Predatory lifestyle and biomagnification. All three species of fish are predators that sit high in the food chain. This means they feed on other fish that already contain mercury. With each "step" up the food chain, the concentration of mercury increases exponentially — this process is called biomagnification. For example, a small fish eats plankton containing mercury, then it is eaten by a medium fish, and in turn, that fish is eaten by a tuna. As a result, the concentration of mercury in the tuna's body is dozens or hundreds of times higher than in the seawater.

High activity and metabolism. Fish like tuna and mackerel are very active. They have a fast metabolism, high levels of oxygen and food consumption. This also means greater accumulation of toxins, as they filter more water, breathe more intensely (mercury is also absorbed through the gills), and eat more frequently. Thus, they accumulate heavy metals, including mercury, more quickly.

Why this is important for humans. Methylmercury is a neurotoxin. Consuming such fish in large quantities, especially among vulnerable groups (pregnant women, children), can disrupt the nervous system of the fetus, reduce cognitive functions, and cause chronic poisoning.

That is why the WHO and FDA recommend limiting the consumption of large predatory fish to 1–2 servings per month, especially for pregnant women.

The larger and more active the marine fish, the higher the risk of high mercury content. This makes sharks, tuna, king mackerel, and swordfish potentially dangerous with regular consumption — unlike smaller, short-lived species like sardines or anchovies.