Once the Fishing mini-game began, you can only fish in the Lower Reaches fishing spot. However, you can unlock more fishing spots when get to certain requirements.

Effect - Code

Master Den - Fish up Saboten Crest un the Secret Reaches
Middle Reaches - Get 5 perfect fishing in the Lower Reaches
Secret Reaches - Fish up Saboten Bond in the Upper Reaches with Matamune
Taikou Chest (treasure) - Get 9 perfect fishing in the Master Den
Upper Reaches - Get 5 perfect fishing in the Middle Reaches