Those big river cats live practically forever ... for all I know, some of them are survivors of THE FLOOD. Once they're big enough that the herons can't get them, their only predator is homo sapiens. I'm reluctant to eat big fish out of the Shenandoah, Potomac, and James for the reason you cite: too much metal and PCB contamination. The smaller fish, especially from the uppermost parts of the watershed, are a different matter. Just have to make sure you're fishing above the old industrial areas.
Most of the mercury in fish comes from coal burning, it’s everywhere, regardless of industrial plants. There was just a series of articles out about how utterly contaminated the fresh waterways of America are after 100’s of years of coal burning ...