Fishing - How do you know what bait/lure to use?

[Shadygrove]

.... Is it trial and error where you use every single bait at every single location?

Not really. Some is plainly marked lakes, rivers, coastline, etc. Some even are marked for stuff like bottom or insect feeders. Beyond that its what do you want to catch? Mooching you use the lowest level bait your target fish will bite on. Isolating a particular fish is the T&E part.

Also I find the notes on the fish entries in http://xivdb.com/ QUITE helpful.

[StouterTaru]

Mooching you use the lowest level bait your target fish will bite on.

That's the best method for any type of fishing. Find the level of your target fish, pick the type of bait that is 5 levels or less below it, and pick the location that has the fewest types of fish lower than it.

I find http://ffxiv.gamerescape.com/wiki/Category:Seafood to be very useful.

For example, the razor clams mentioned above.
-Level 20 saltwater fish, rat's tail is level 15 saltwater, there's your bait.
-Crescent Cove has only one other fish that bait can catch, Skull Valley has 3 others but 1 is Rothyl Oyster, The Salt Strand has 3 others all junk. So Crescent Cove if you just want razor clams, Skull Valley if you are fishing up mats for Bouillabaisse or want to diversify on the MB.
-Final step is to go out and fish. Give it at least half an hour before you give the spot a thumbs up or thumbs down.