In Oregon, unlike much of the remainder of the US, most of the mercury in fish is from volcanic and geothermal mercury minerals rather than from manmade sources. Whether mercury is natural or manmade, it undergoes the same methylation and bioaccumulation processes in the environment. Mineral forms of mercury pose very little direct exposure hazard to people or to the environment, but as soil and rock particles wash into streams and accumulate in stream bottoms, the mercury is transformed into methylmercury by bacterial activity in the sediments. Once this transformation occurs, mercury is readily absorbed by plants and small animal life. As small organisms are eaten by larger organisms, such as predatory fish, the mercury bioaccumulates in the tissues of these organisms.
Fish also absorb methylmercury directly from the water through their skin and gills.
Bioaccumulation processes produce much higher levels of mercury in organisms than in their environment. Since fish are high on the food chain, and humans eat fish, there can be a serious potential for exposure to dangerous levels of mercury. Mercury bioaccumulates in the body of fish and is eliminated from them very slowly. Because of this slow elimination, the older a fish is, the larger the concentration of methylmercury is likely to be in that fish. Since the size of fish generally corresponds to age, it is also true that smaller fish are likely to have lower mercury loads than larger fish of the same kind.