wow, that is kind of mind blowing, WTH were they thinking, isn't releasing domesticated animals into the wild illegal? anyway, I take rats very seriously, I pretty much always have traps out for them. I would recommend buying 10 snap traps and using them like snares, placing them anywhere you think they might run out in the open, usually along fence lines. look for tell tail signs of where they run. they are creatures of habit and when they get out in the open they run for their dear life to avoid predation and that is where they are most likely to run right across a snap trap and get caught. periodically I put food in the traps when it seems they have been there long enough to blend into the scenery but aren't catching anything. also, eliminate night time sources of open food, take up chicken feed at night and store everything in metal containers. good luck, be persistent, get a handle on them before they breed!