How much methylmercury is found in fish?

Levels of methylmercury vary greatly, largely based upon the species, size and age of the fish.

According to the FDA, in general, methylmercury levels for most fish range from less than 0.01 parts per million (ppm) to 0.5 ppm. The average concentration for commercially important species is less than 0.3 ppm. In a few species, methylmercury levels can reach 1 ppm, which is the limit allowed by the FDA in fish intended for human consumption. This level is found most often in large predator fish, including shark, swordfish, king mackerel and tilefish. Fresh-water species -- such as pike and walleye (which are also predator fish) -- sometimes have methylmercury levels in the 1 ppm range, if they swim in waters polluted with high mercury levels.