Well Daver, like you I was under the impression that cats won't reproduce in a pond. I stocked the newly constructed 3 acre pond with 6 cats and forgot about them. A couple of years later, I started catching them on lures and jigs I was throwing for bass and bluegills. 300 have been taken out in the last three years. They are up in the shallows getting ready to spawn right now, so it's time to remove them before they do spawn.
Only thing I can figure is that there is a side hill seep that keeps a little trickle of water going in and that might be the reason they can successfully spawn. Wish my bluegills reproduced as well; part hybrid issue, part cat/bass pressure on the young un's.