Everdream Valley

Complete the main quest: Follow the instructions of your grandparents. Sleep more often, quests appear on the calendar.
As a result, having repaired the bridge to the left of the island with the farm, you will meet your grandfather there, he will teach you how to fish.
In statistics you can see how many fish you caught ( -> -> Statistics -> 2nd column).

PS. If you're fishing and your stats aren't growing, try doing it later. My statistics stood firmly at 5 fish until the 30th day of the game.