Everywhere has mercury, some of it naturally occurring (volcano's erupting, rain dropping it, etc). So there is natural mercury but also alot of mercury from the "non clean" coal that was used years ago. I'm just basing this on what I read, but mercury breaks down and the smallest levels and gets in insects and larva, and tiny fish eat them, and bigger fish eat them, etc. Thats why they say never eat shark or swordfish because they are really high in mercury. Most fish they say are fine if eaten once or twice a month, but really large carnivore fish can contain more.