I think potential causes have been presented. 1) In the pond all along. 2) Came in with the stockers. 3) Came in with flood water. 4) Dumped in by trespassers. 5) Dropped from heaven by birds.
Only the trespasser possibility is really one that would make me take action at this point (and it seems most likely based on the posts so far).
If the pond owner does not think bucket-stocking is a reasonable explanation, what other preventative action do you suggest?
