No. Once mercury has entered the flesh it never does get out. It is sort of "stuck". If I remember correctly, the mercury actually is stuck in the liver of the fish which is where the fat is also stored. However, as the blood circulates it carries some with it.

One thing to be aware of is putting a contaminated fish into an area not contaminated can actually spread mercury contamitnation to other fish. The spread would happen as the original contaminated fish were to die and it's body get consumed by microbes and the food chain starts all over again. The contamination per fish would not be as bad as for the original fish.

As for finding a body of water that is mercurcy free? I think someone will find a snowball in hell first.