I'll be honest I never thought of it that way but it's a good point. Food production is the main export of the states I had in mind in my original post. However, food production shouldn't be counted as a "value added" (arguably, since they don't export enough outside US borders to turn a standalone profit) to the American economy. In the same way that Quality Control is not considered "value added" to a factory, but it is still very much a required part of manufacturing.

However, Japan has very few arable farming areas and get something like 70% (I made that up) of their food from the ocean. In the absence of American farmlands, I'd wager that fisheries would be able to sustain urban coastal areas almost indefinitely.

In all the Mad Maxx/Book of Eli style movies, the characters are almost always inland, trying to get to a populated coastal area where survival is more practical.