1. Look for jumping fish close to shore. That is where you want to fish.
2. If you are fishing for troll fish make sure that there are troll fish present. The easiest way to do this is to fish from a boat and look into the water.
3. Perch and Pike can spawn in the Black Forest. If that is all that is present they won't bite on troll fish bait.

If you are interested in just getting fish the easiest thing to do is build a fish trap and wait for the water to flow over and recede.
https://steamcommunity.com/sharedfiles/filedetails/?id=2976178601
https://steamcommunity.com/sharedfiles/filedetails/?id=2976178702