In Fire Emblem Three Houses one of the activities that you can choose to do on your free days at the Monastery is fishing. To fish you’re going to need bait, and this bait comes in various types. There are multiple ways to go about getting bait in Fire Emblem Three Houses and it depends on what part of the game you have reached.
Fishing is introduced pretty early on in Fire Emblem Three Houses, but there really isn’t a reliable source of bait until you’ve reached chapter 6. Once you reach Chapter 6 you will unlock new merchants that will sell you bait which you can use to go fishing at any time . Prior to Chapter 6, your best bet is to do side missions in the Monastery and to look for the glowing blue points of interest that you can pick up. There is plenty of bait lying around, and you should get plenty from doing side missions on your free days.
Once you do reach Chapter 6 you will be able to buy as much bait as you want and will be able to go fishing as often as you like with the chance to catch multiple types of fish, some which are worth a lot of gold. Just head to the new merchants that are unlocked to purchase different types of bait and play the fishing mini-game as often as you want.
Going fishing is one of the ways that you can raise your professor level without spending any activity points.