Ok, I think I might have finally figured out what the point of this trait is for. Today I was reading up on some Coerthas fishing spots to round out my log, and I went up to the Weeping Saint location at level 49. When I arrived there, it consistently told me with various high-level baits and lures that I could 'sense' there were fish, but that I couldn't determine the location.

Since i'm legacy and started out higher level I didn't much notice a difference in where/what I fished, but what it seems is that this trait unlocks the 'hard mode' fishing spots. Even though the active help indicates that's generally what it does, it doesn't explain much beyond that, like telling you that specific locations are blocked to you.

I returned to Weeping Saint, at 50, when you obtain Gulleye III, and bam I was able to fish.

If others have figured out, kudos, just posting for those who were curious.