Copper, Stone, Crabs, Fish, Cattle, Bananas, Rice all of these have historic trade value. They were all subject to trade in Civ IV what changed? In Civ V it was removed, except for a brief bug which allowed you to trade Stone when it was introduced, commodities like Copper, and Stone is simple enough. For the perishable like Fish, Crab etc. Simply subtract what is fed into your population, which is automatically done when assigning where your populace works. This could have diplomatic affects to Civs who are starving, even in terms of Emergency Aid projects when drought, Floods hit etc.