At the time the achievement was added, you needed to catch every fish, big fish, and legendary big fish in the game, plus a few of the 15 new big fish added that patch. Only 6 legendary big fish have been added since then, so you still basically need to catch everything but a few big fish. The newest big fish require at least 1300 Gathering or so, and around 1150 Perception (for mooching) to catch.