Once mercury has entered the environment, it is converted by bacteria into organic mercury: methylmercury. Methylmercury accumulates in, for example, fish and crustaceans and shellfish. Predatory fish (e.g. tuna and shark) that eat other fish can contain significant amounts of mercury, much more than fish that eat plankton.

Mercury concentrations in fish vary widely and differ per species and per individual fish. For example, salmon and sardines contain less than 0.1 ppm (part per million) and shark, swordfish, tuna and king mackerel contain more than 1 ppm. This means that a fish meal of about 100 grams of fish can contain a few micrograms to more than 100 micrograms of mercury.