So, I started on my Fishing levelling today (well, all gathering) and noticed a thing. I have the best gear, fully melded, from ARR. The Mining and Botany nodes are about what would be expected, and the Fishing is mostly okay, but the leves aren't quite right. You can do the Botany and Mining leves no problem, even at +1 difficulty, but the lv50 Fishing leves require fish that can only be caught using a new lv51 bait.

So, yes, you can take the Fishing leves at lv50 but you can't possibly complete them until you hit lv51. This seems to be a pretty big oversight...