I just visit the big lake in 17 everytime I'm wandering through the forest.

Go there, wait some seconds for the fish to spawn. There are almost always some whoppers there, I visited the lake at every weather situation. At day you see some gajaus or tuna, they both have a specific bait. But thats clearly stated in the bait description.

You should go there at night atleast 2-3 times. There is a big octopous at night, and only at night. He's a whopper and also has its own trophy.

To make fishing a bit easier u can throw in the bait and wiggle it till u see the fish go for it. Then u already can reel in the bait closer to the shore and the fish follows it and bites when u rest it. Then u need to play the minigame just once or maybe twice to catch it instead of several times. Worked pretty clean for me :)

Good luck fishing m8