[c3534l]

Specialize and trade. If other regions are really good at farming wheat, why bother farming it yourself when you can produce fish and grapes?

[fsckboy]

trade is ruled by comparative advantage: If country A is better at producing wheat, and country A is also better at producing fish, country B should focus on producing the one which it is less comparatively bad at producing, and then trade. That will give them the highest GDP/income.