If you want to target trout in that lake, it sounds like you'll want to find the thermocline layer of the water during most of the year.
For those who don't know, the temperature of deep lakes falls into roughly three layers during the warm part of the year- the warm layer near the surface, the cold layer at the bottom, and the thermocline, which is in between.
The cold layer is most often 39 degrees F/4C, which is the temperature where water is heaviest. (The miracle chemical property that explains why 32 F water floats when it becomes ice, and lakes don't ever freeze solid.) 39 degrees F is too cold for almost any fish species to prefer, which is why you won't catch much, if anything, if you fish the bottom depths of a deep lake.
Meanwhile, the surface layer might be around 75-80+ degrees in the summer, much too warm for trout.
The trout are like Goldilocks, and the thermocline layer is "just right".
So, drop a thermometer into the lake water, and you'll find it after a few soundings. Just make sure that uh the thermometer is attached to a line of some sort.
No doubt a fish finder will help, too, but that's cheating, isn't it?
If the trout in Marburg are like those in Maine trout lakes, there's one especially good time to find them in the shallows- for a few weeks after ice-out, in the spring. Often near the inlet feeder creeks.