Joe is just returning the favor because of the flatheads I airmailed into his pond.
Mikel,
We have crappie in 2 ponds. We never intentionally stocked them in either pond. I'm guessing some crappie made it in disguised at fatheads. Also, during times of flooding, a creek can back up to our ponds. We've had carp, gizzard shad, alligator gar, crappie, and bullheads come in from floods.
We just try to keep a close eye on what is going on in the ponds. Bullhead fry, for example, are very noticeable because they form black "clouds" near the surface. An electrofishing survey could be in order if you think there are more than just a few crappie.