Where does the Mercury In Ocean Fish come from?

Question of the day: were mercury levels high before people started burning coal and polluting? No one seems to know, but tuna mercury levels haven't changed in 30 years, which could mean one of two things: the mercury we put into the ocean is quickly removed, so yearly mercury discharge doesn't compound or there was a lot of mercury in the ocean before industrialization. Maybe it's really slow to filter out of the ocean, so the sporadic but high levels of mercury from volano eruptions could have put it all there.

ScienceDaily: Mercury In Ocean Fish May Come From Natural Sources, Not Pollution