Note: This game is also titled Harvest Fishing and Kawa No Nushi Tsuri: Wonderful Journey.
Talk to the lady near the well, and she will ask you to get her a Rainbow Trout. After you give it to her she will give you a treasure map (from the Jiza 30 paces east, 10 paces north, 5 paces west). Go under the tree by your house and dig to find the treasure. You must do this to enter the fishing competition.
You must fish for Ayu in the competition. Go to the billboard, have your basket empty, raise your HP, and use fish eggs that you buy from the store. Go all the way downstream and fish where you see a blue person on the ledge in middle of the stream.
You can get frogs on the other side of the river from the village on the big grassy patch near the bridge. You can get insects near the C section behind the fisherman by the rocks.