You will get some, not sure what the conditions are where you caught that one. There are many more holdovers than people think, they are significantly harder to catch than the stockies, many fishermen don't catch them even where they are in significant numbers.
You can often tell whether or not it is a stocked fish or not by what kind of shape the fins are in, and the coloration on the fish. A small fish born in the river is very beautiful, stockies less so. This could simply be a dink that was mixed in with the others, but it would be very unusual for that to happen. Also, there is a fingerling stocking program, sounds more likely that this may have been an intentional stocking if there were lots of them.