I've been told by the guy who runs our local DNR bass, bluegill, shellcracker and catfish hatchery that you can't fish a pond out. He said a certain amount of fish simply won't bite a hook. More likely, several factors come into play, including harvesting EVERYTHING. I believe fisheries biologists are playing the odds when they say what they say.
That said, I practice catch & release because I want to spend the time catching, not cleaning fish.