Trade

To mention trade income briefly, despite the encyclopaedia saying it improves the income of ports and settlements along the trade route, I believe it is a faction wide activity and not closely related to individual province management. It depends on trade agreements you have to make with other factions through diplomacy; the amount of trade is determined by the quantity and value of the raw material and luxury export goods you have under your faction’s control. (The breakdown is available on the Trade and Finances screen.) So upgrading towns with these resources so that you can export more of them will increase trade income, limited somewhat by the principle of supply and demand. The goods you import from your trade partners can help you if you need a particular resource, e.g. marble, to construct a level V building.

The other faction must have a port to which you have access via sea from a port of your own, or a shared land border. Unlike previous iterations of Total War, there is (thank goodness) no longer any limit on trade agreements imposed by how many ports you have. Remember that obviously once you invade a province you cannot trade with it – any income now comes directly from taxation.

One point on trade relevant to province management is that certain buildings, e.g. the wine merchant line from the city centre (that starts as a generic forum), increase trade revenue (not maritime wealth which is different). They do this as a % bonus. I assume this is a faction-wide modifier, as there are no stats shown for province level trade. This modifier could be very useful if compounded.

Trade may be reduced by an enemy navy on a sea trade route. The navy (fleet or transports) goes into raiding stance and right clicks on the trade route. A proportion of the trade income is lost and goes into the coffers of the raiding faction. Blockading a port (by right clicking an enemy port with a fleet or transports) will disrupt all trade if there is no other port to port connection between your faction and your trade partner; this effectively cancels that trade agreement.

Armies can go into raiding stance on land. This reduces the upkeep cost of the army and incurs a diplomatic penalty (yes, you are not restricted to raiding enemies and can even raid in your own territory for a public order penalty). I don’t think this actually reduces the diplomatic trade income along a land trade route in the same way as a sea trade route, because you don’t actually click on the land trade route, and don’t actually earn any income directly; it might reduce the wealth income of the raided province.

Sea trade route raiding is different from sea region piracy in the same way that diplomacy-based trade is different from maritime commerce. Maritime commerce is a province-based type of wealth derived from port buildings, and piracy is a modifier that reduces it. This is why, unlike other types of wealth modifier in a province, the modifier values for maritime commerce can be different between different settlements in the same province –  the ports may border on different sea regions. Other modifiers, such as culture or agriculture, are the same because they summate across the province and are applied to the base wealth of that type in each settlement.

Piracy is lower if you own all the ports bordering on the sea region, and is worst if you share the sea region with ports of enemy as opposed to neutral factions. You can reduce piracy by having a navy (fleet or transports) in patrol stance in the sea region. Reducing piracy by patrolling depends not on the strength of the fleet but on the number of separate fleets. Patrolling can make a difference of around 15% of total maritime commerce for all settlements in all provinces bordering on that sea region; the navy must have spent no more than half its movement points to go into patrol stance and must remain stationary to stay in this stance.