esshup,

To interpret your "transfer" answer - do you believe that the most likely vector (when fish are inexplicably present in a pond) is fish traveling upstream through surface flows? [Obviously, introduction from other bodies of water in the upstream direction should NOT be surprising if water flows from those bodies can reach the subject pond during flood events.]

It appears (in your experience) that fish can navigate surface water flows shallower than their dorsal fin.

Likewise, I assume they could also work their way up an inclined pond outlet if the downstream end was in a pool (rather than air-gapped 3' above the stilling pool). After all, they can navigate fish ladders.