Master Chef Confidence
The first fish you need is a Big Itou that is at least 63 inches or larger. Those are back at the Waterfall in the Stream. Bring the large fish back to the apprentice and next he requests a Silver Buna that is 7 inches or bigger. You can find those in the Swamp, along the green algae water near the eastern bridge. After the Silver Buna you are to hunt down at least a 9 inch Oshorokoma from near the bridge where you met Naoko in the Stream. The last fish you need is a Big Grass Carp from the southern Rapids that's 50 inches or larger.
When finished the master Chef will come by and thank you for letting him cook all the different fish. In exchange he'll give you the recipe for Huge Catfish Miso Soup.
The Bankrupt Restaurant
King Akame can be found in the southern area of the Rapids map, and the Big Snakehead are around the coastline of the island in the southern area of the Swamp. Return the fish to the fishmonger and he will reward you with the Stewed Motsugo recipe.
Pop Star vs. Naoko
Ayu notices you walking up and asks if you would teach her how to fish. Naoko objects but Ayu continues to complain how mean Naoko is because she won't teach her. The two of them argue until Ayu decides the issue will be settled by whoever catches the largest Electric Eel. Since Ayu doesn't know how to fish you get to catch the eel for her. You can catch them along the shores just north of where the two girls are arguing by using Small Fish or a Spinner lure.
Bring the Electric Eel back to Ayu and her Manager and the faceoff will begin. If the eel you brought back is 60 inches or larger you'll win and be rewarded with the Grilled Big Eel recipe.
The Straw Millionaire, part 5
Final Showdown With Naoko
Each time you are given a specific size of fish to catch and you bring it back, Naoko will take her turn at the challenge. Then it will be your turn again. Naoko doesn't take the fish out of your basket and since you can only bring 1 fish back to her, you will have to sell the last round's fish to the Fish Shop.
You will need to catch 4 fish; one that is 1 to 11 inches, 16 to 22 inches, 26 to 35 inches, and finally a fish that's larger than 40 inches. When the two of you are finished with 7Up your rivalry is over. Naoko is impressed with your fishing skill improvements and rewards you with the Boss Fly Rod.
The Last Treasure map
Head back to the Stream and talk with the sleepy old man. Smarty notices that the fossil is shaped like the River King and suggests you talk to the villagers. He also gives you the Master's Pole.
The other villagers talk about feeling an earthquake up north. If you take your Washtub to the Waterfall you'll discover you can go into a secret cave. You have unlocked the Underground Lake where the River King lives!