djufo,
Without coming off as patronizing (sorry if I do). I highly doubt if mercury could be used as a preservative. If ANY company got caught "adding" any amount of mercury into their product they would be in Deep, deep, deep trouble with the FDA (usa), CFIA (canada).
Adding unhealthy things is one thing, mercury is another. Mercury is not unhealthy, it is DEADLY. Mercury is very bad Mojo for any living creature because it doesn't go away...like...ever.
Further mercury IS naturally occuring in aquatic environments and is often increased in it's levels by cumulative effects of man's activities. One perfect example is freshwater, man-made lakes from dams. Flooding an area will increase mercury levels in the aquatic environment for years, and years, afterwards from all the rotting and submerged terrestrial vegetation (I thinks it's the veg..) That's why so many fishing regulations have mercury warnings about the fish.
Anyway, blah, blah, blah...