So I fish without a fishing mod, because since patch there hasn't been an updated mod available.
On Friday and some of Saturday I tried fishing in Crystal Lake in Elwynn Forest.
I have caught about 537 fish (I fish for a couple hours here and there rather than long stretches).
Of those 537 fish, I caught:
1 10 Pound Mud Snapper
4 (yes 4) 12 Pound Mud Snapper
The rare fish out of Crystal Lake is the 15 Pound Mud Snapper. Imagine my frustration at catching 5 heavy Mud Snappers, and none of them are the right weight!!!
I also think this achievement is not healthy.....I hope they increase the drop rates.
Really in my opinion the drop rates should be in line with Old Ironjaw and Old Crafty. Those still take time, but they are not nearly as impossible as these other rare fish. Good luck to the rest of you.