There may be some clear IG logic to this but I cannot work it out.
I would have thought the underwater sections of the River Tear should be unwithered, much like the reef areas and such, whereas they are generally lightly withered.
Its the only "Withered" wild area that I know of where nothing Withered ever spawns.
I would suggest it should be non-withered but not Misted, like say the Underdark/Coral Cove/ Old Port etc.
What your really asking for is that we need some withered fish spawns.
That explains it SC. Fair enough.