Also, just because hatchery fish were stocked in watershed, or strays there, does not (necessarily) mean the native genome was changed. Selection weeds out the maladapted. Except in humans, where we elect them.

I and reading about a 40 year study the Grants did on Galapagos Finches on on tiny island. Lots happened over 40 years. New species were evened, hybridization, and so on. One finding, though, was that one species present in year 40 was demonstrably different from what it was in year 0. Natural processes, including evolution, changed them.

I suspect that the only place where you might have the same native fish as you had 200 years ago is in a watershed untouched by humans or natural change in climate.

Life out there is a heck of a lot more complex and varied than we want to believe.