How do fish and road building cards apply to bridges?
http://www.catan.com/service/combinations/fishermen-catan-rivers-catan says "If you pay 6 fish, you can build a bridge".

Meanwhile, the rules for the Rivers of Catan scenario specifically state that you cannot use the "Road Building" development card to build a bridge, and that would seem to extend to any combination of expansions/scenarios that replaces the normal development cards (including the Traders &amp; Barbarians scenario, or the Cities &amp; Knights progress cards).