Bullheads, GSF and a few other species can and will swim up VERY small streams... So if there isn't a substantial drop/water fall even during extreme high water events, then you have the chance of those unwanted species entering your pond via up stream migration.
The same is true for down stream migration... If your pond is fed by a small feeder stream or just ground run off and during a very intense heavy rain event, just a couple inches of water laying on the surface can mean that pond polluted with bullheads and GSF two miles away may allow those fish to find their way into your pond.
More times than not, I think these are the most likely culprits for unwanted species in a pond. A close second though is the accidental introduction during the stocking of wanted species. Then the best of them all is the friendly neighbor or friend who does his own stocking without asking you first...
