You must be level 35 and have a skill of 225.
Visit the fishing trainer in Stormwind (in the canal district).
You get sent to Dustwallow Marsh and then sent around to different places to catch rare fish. I recommend you get coords on those fish instead of just going randomly. It can go quickly if you know where you need to be casting.
And from reading the posts at thott.
The Nat Pagle quest is open to both factions. So, drag your hides over and get the quest to go fishing for rare fish.
Coords are as follows:
Just outside Theramore between the coast of Dustwallow and Theramore Isle on a small island.
Nat Pagle is at -58,59
Nat Pagle map location
Feralas Ahi - Anywhere in the river, even near Camp Mojache.
62,51 -in Feralas
Savage Coast Blue Sailfin , Anywhere along the Savage Coast (STV west coast).
34,31 -Stranglethorn Vale
Sar'theris Striker, Anywhere along Sar'theris Strand (near Shadowprey Village).
26,76 -in Desolace
Misty Reed Mahi Mahi - Anywhere along the beach (extreme west coast).
80,94 -Swamp of Sorrows
Once you have all 4 fish:
Back to Nat Pagle @ 58,59