It may be a result of Smalma description above of segregating the wild and hatchery fish, but I am aware of at least one hatchery that when they have their quota of eggs, simply closes the gates to the trap and let the hatchery fish spawn in the feeder creek. This is also on one the the rivers where you catch a number of cookie cutter but unclipped fish.
_________________________
Don’t attribute irritating behavior to malevolence when mere stupidity will suffice as an explanation.