Rapala Pro Bass Fishing

Monster Fishes are some extreme rare species that can be found in every lake. If you look at the Fish-pedia, at the bottom of the description of every lake you can found a list of the Monster Fishes in there and a brief summary of where and when you can aim to catch a rare one.
You can found a Monster Fish in a Tournament or, more like it, in Free Fishing mode.
In this case, select the meteo and season conditions for the Monster Fish you need to appear, then go around the lake with the GPS turned on: when you're quite near a Monster Fish, a blue dot will appear on the screen.

Note: so far I only managed to catch Dottie in Lake Minnetonka and there are 30... Ouch :D