I noticed that between skill 75 and 100 I was still catching some junk in the Lushwater Oasis, in The Barrens. The last junk catch was at skill 94. I think the skill required to fish without junk is 100.
It was always hard to confirm these low-level skill requirements, and it's been harder since patch 3.1. I wonder if this was always different, or a recent change? And whether some of the other similar zones (currently listed at 75 skill) are slightly higher?