I have been pondering this question myself for years. I have looked it up on the internets and discussed with people and still do not have a suitable answer.
How do fish end up in water that was not stocked?
Example, the mouse flies I used in July for bass were in a spring fed pond in a field. The owners cleared some woods and dug a hole and it filled up with water (for horses). No fish were stocked. Yet the body of water was full bass and sunfish.
Some say the eggs come in on bird feathers.
Dr. Trout? Anyone? I will be out with the fish biologists with Va. tomorrow and will ask them. We is going electro snakeheading !