The mercury in Great Lakes salmon and trout can be traced directly to Chinese coal. Coals from every part of the world have a locale signature in their ash and thus can be traced to their source.
And I guess it is environmentally unsafe to mine low mercury coal here in the US. Maybe that is the Environmental Wacko connection here -- we could probably produce safer coal here if we geared up the coal industry instead of importing coal since it cannot be mined here in this country anymore.