Random Fish
Three-Spined Stickleback
You will discover Jango has also come to visit the crystal cavern. After you explain that you're looking for a Rope, Jango says he does have one but he requests that you bring him a Great Brown that is 28 inches or larger.
The fish can be found in the previous Upper Waterfall area, but it is easiest to find a Great Brown in the Mountain Stream during the morning hours. Take your fishing gear to the area and fish directly at the beginning of the water way. You will have to cast your line diagonally to get far enough north.
Between the time you talk to Ren and the time you finally hand over the correct sized Great Brown, Jango will have his shop set up in the Underground Lake. He won't move back to the Pond until after you complete his request.
Once you have caught the required size of Great Brown, bring it back to Jango. He will give you the Rope in exchange for the large fish. Jango also tells you that you'll have to use your pet Tenma's flying abilities to get the Rope up to the top so you can climb up.
Head back to the waterfall and go to the spot Ren pointed out earlier. Tenma must be at least on his 3rd stage of growth or higher or else he won't be able to fly to the top of the cliff. The bird will take the Rope, fly up to the top of the falls, and then lowers it down to the ground. Now you can climb up to the next area.