mikel, I caught 2 crappi in a row last summer in my pond. I killed both of them. I was pretty worried that I would catch more but I never did. I tried minnows and fished for them for a couple hours but never did catch anything but a couple bass and some GSF. I still don't know how they got in the pond but I'm glad they never spawned (hopefully I haven't spoken too soon).
I have a few guesses as to how they got in there. The usual suspects: someone just dumped them in or they came with some other stocked fish.