Mercury occurs in all dark water rivers naturally and I would think that it also occurs in the swamp. The Satilla is suppose to be 10,000 years old so the Mercury has been there all that time. As far as how many makes a meal, I can't answer that. It would depend on the person doing the eating, I guess.
I have been fishing in the Satilla for over 50 years and I make a meal out of ever how many I want to eat at a time. I have never paid any attention as to how often I eat them per week, or month.