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Most of the mercury that contaminates fish comes from household and industrial waste that is incinerated or released during the burning of coal and other fossil fuels. Products containing mercury that are improperly thrown in the garbage or washed down drains end up in landfills, incinerators, or sewage treatment facilities. The mercury then leaches into the ground and water. Once mercury enters the water and soil, it is naturally converted to methylymercury by bacteria. In water, the bacteria are eaten by plankton and other small creatures, which in turn are eaten by small fish, then larger fish. Mercury doesn’t easily leave the body of an organism, so the amount of mercury builds up in species as they go up the food chain in a process called bioaccumulation (PDF). Larger, older fish accumulate more contaminants than smaller, younger fish.

SOURCE:

https://doh.wa.gov/community-and-environment/food/fish/contaminants-fish