I wish I would have saved it and I've tried to find it again but can't. It was a study of rainbow trout. They stocked a pond with trout and fished for them. The one's they caught were put in a nearby pond leaving the one's they couldn't catch in the first. No matter how you tried catch the remaining fish in the first pond, including live bait, they simply wouldn't bite.
They repeated the stocking several times with the same results. But you could fish the second pond with fish that already been caught and catch them again and again.
Also, there is a book by Leonard Wright called Fly Fishing Heresies where he states that as a kid an old guy would give him flies to use. But he wanted them back if they caught fish, no matter how tore up they got. He would use those tore up flies to modify his "perfect" patterns to imitate the tore up ones.
I've been keeping old flies for years, especially dry flies. You ever notice how the bite would shut off when you lost the fly they were biting on even though you put on a new one?