I know I have seen waterfowl and of course Disney planted fauna around the Rivers Of America but I am not sure if there are fish in the said waters!
I wonder if Disneyland stocked the Rivers of American with fish for the added touch of realism. I do recall seeing something swimming around in the river and I don't mean humans!
There definitely are fish in that stagnant pond area as you cross over from Fantasyland into the edge of Frontierland! The water is so dark it is hard to tell what lives in them waters!