Truly Legendary Achievement
Catch your first legendary fish
Each fishing location will have a series of goals. Three are available from the start and the fourth will unlock when you have completed the first three. Once you have completed the fourth one, you will unlock the goal of catching the legendary fish. Accuracy is the key, if you hit the bulls-eye or close to it, the legendary fish will almost definitely spawn. Catching them is pretty simple, just do what you would normally do. This will come quite early.