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Ok I get how anadromous fish like trout end up over a wide range of different watersheds. Actually it's neat how each watershed have unique DNAs too, since they spawn normally in the same waterways once they get there.
But I don't understand how freshwater only fish are over a wide range of watersheds.
Obviously fish can swim but different watersheds only connect if you go through the ocean.
Take pickerel for example, native in new Jersey but also Florida. How the hell did the same species of pickerel end up there?
I'm guessing watersheds change overtime? For example a huge lake could shrink down to a few creeks and those then separate right between watersheds, and the opposite with a lake forming between watersheds
