The hard thing, and this would include dwellers in the immediate area of large cities (radius of 30-50 miles) would see people crawling out of the large cities in a panic. You would see large sections of forests wiped out for wood for fires, lakes would be overfished and polluted by untreated human waste. Home invasion and squatters. Don’t care how many guns a farmer has - urban dwellers will have them too, and there will be too many for the farmer to overcome. It would have to be a cooperative effort like in Jericho, where a town has an volunteer army protecting the city.
If they're dumb enough to poop in their drinking water, they're too stupid to fish. Seriously, you'd be surprised at how many come into the local store and haven't a clue how to put a worm on a hook.