Hyrule Bass can be found pretty reliably in the pond next to the tower. If you can’t find *any* though, there’s also a third way to get the doublet... which is by not getting it. Do the mountain with only cold res meals or by carrying torches or any other method that keeps you alive, without speaking to the old man, and he’ll leave it in a chest in his house for you after you complete the Plateau.
I must have run into a bug. Once I completed the Magnesis shrine the fish appeared. Restarted the game and went straight from the tower to that shrine again and the fish were there. It's if you try to get them before activating the tower that seems to be the problem... And thanks, yes I knew about the third way but I'd forgotten about it.