So I just found this out from some patch news
"Players can no longer catch new high-level fish when fishing near level 80-85 zones unless the account is upgraded to Cataclysm." Source: Patch 4.0.3a Hotfixes - World of Warcraft
That means that if you already have Cataclysm you can actually start farming fish for when it comes, this will give you a good advantage over others when leveling cooking or selling fish on the AH.
I'm still in the search of a spot where you can fish this and will update as soon as I find it. As for know I tried Twilight Highlands but you get ported every time you get close to it.