There is a creek within 1/2 mile downstream and we have had water flow out of the pond once due to rain. Watershed is bordered by gravel road and nothing within a mile that could get to us.
On my multiple attempts to trap crawdads in the creek north and south of the pond I never saw anything but Green Sunfish and other various sunfish resembling BG. No craws, shiners, or minnows. None of the attached made it to the pond....they were returned safely to the water they were taken from.
Pretty much the options at the beginning of the post are my possible choices unless one of the hatcheries had a mixed stock/stray that got in when we put fish in the pond.
Good questions.
1.5acre LMB, YP, BG, RES, GSH, Seasonal Tilapia I subscribe to Pond Boss Magazine