Black Bass infestation
You don't have a time limit for this errand so there's no rush to complete it. The fish can be caught in many different areas, but mainly along the shores of the giant lake. A good place to cast would be along the eastern banks of the lake.
After you collect the 6 Black Bass (it may take several hours), return to Sachiko. She will reward you with the Boiled Prawn recipe. If you had agreed to participate in the bass festival then you will have to complete this errand before any others will activate. Sachiko also does not take the Black Bass from your basket so you can go sell them at the Fish Shop once this errand has been cleared.
Troublesome Great Brown Trout
The fish you have to catch must be 31 inches or bigger. You can find the large Great Brown Trout along the southern coastline of the lake. They are very large and you can't miss them. You can use a Carbon Rod and Small Fish as bait. Once you have caught the fish, talk to Naoko on the south bank of the lake. Akira will reward you with a Deluxe Lunch.
He wants to catch his 100th fish and since he can't go anywhere, he requests that you fetch him an Albino Rainbow Trout, one of the rare white fish. You can find the Albino just south of the bridge but north of the sandy island you can swim to. Since Benkei won't move his big butt it will be tough to cast off the bridge toward the right direction, so it may be easier just to swim to the island. The fish will be a medium-sized shape in the water.
Return with the Albino Rainbow Trout to Benkei and he will move off of the bridge. Continue walking to the island and explore until you find a little shrine. Use the Examine feature to find a Washtub you can use as a boat.
To use the boat stand near water, open your Main Menu, and select the Washtub from your Items menu.
The Straw Millionair, part 2
The seed he is referring to is the ??? Seed that you received from the Harvest Sprite in the Stream map. Kaoru would like to make an exchange. If you give him the ??? Seed he will give you a Vegetable Set.
Feed the Nusshie
You just need to catch 3 fish that are 20 inches or larger. Surrounding the area where Daisuke and Nusshie are you'll find Pink Salmon, which are the perfect size for the big guy. Bring back 3 fish and Daisuke will give you a Treasure Map that Nusshie found in the lake. (Daisuke will take the first 3 fish out of your Basket once you bring the 3 20-inch fish, even if they aren't the required size.)
The Map clue says that someone is waiting on an island with no bridge. The island you're looking for is the one north of Benkei's bridge. In the middle of the island is a tree. Use the Examine ability to find a Piece of Something.
Black Bass Fishing Tournament
Black Bass can be found in the darker water just south of the tournament, that way you don't have to travel so far in the 5 minute time limit. Reach the darker water with your Washtub boat, just south of the boat dock, and use a Casting Rod (if you have one) and Small Fish as bait. After 5 minutes you will be judged on the largest Black Bass you caught. If you win you'll earn 500 G. This is repeatable and it's an easy win!
Kai from Mineral Town
Don't stop talking though, because you'll have to talk to him 50 more times. When he gets his fill of talk he'll finally ask you what kind of food you like to eat. If you tell him you like to eat fish he will say he likes to fish too. Kai then gives you the ultimate Bait Rod, the Mineral Town Special Edition Rod.
Mountain Cooking Contest
The two recipes that are the most convenient to use are Smelt Tempura and Sweet Stewed Honmoroko. The two fish are in the same pond on Benkei's island. Run as fast as you're able to the island and cast your line using Insects and Worms. There are plenty of Smelt, Honmoroko, and Frogs in the pond so it won't take you very long to catch the fish necessary.
If you win the contest you will be invited to participate in the finals located in the Swamp, but you must be a Pro level before you can enter the final contest.