On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the Fishing Lesson side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will meet a girl who claims that her brother has been turned into a fish and is swimming somewhere in the lake. What's more, she wants you to help her fish him out.
The story seems to be quite a stretch, but if you agree to help, you will have to catch 3 fish (you can't kill them with a bow).
As soon as you pull the third fish out of the water, the girl's brother will come out of hiding claiming that they have had enough fish and can go back. Turns out the kids were looking for someone to do the hard work for them. Pass the fish to them to complete this task.