I am glad to see fall here and the cooler weather that comes with it. After working all my life as an underground coal miner I am used to working in around 55 degree temperatures year around so I do not do well in hot, humid weather at all. And for the most part neither do most fish.
The big thing is that in cooler water there is a good oxygen level where fish can be healthy and active and feed and move about. This gives the fish a positive feeding attitude and can create a great opportunity for an angler to catch some nice fish and a good number of them also. Then as the season goes on and turns into summer the temps greatly increase and this makes the water surface and main lake shallow water become very much warmer. And this puts a big drop in the oxygen level content and a fish cannot stay in this type of conditions long.
So, the fish are forced to move to deeper cooler water where the oxygen levels are better. If you have the right type of graph or fish finder unit on your boat, it will show temperatures at different depths. There will be a certain depth where the temperatures will change a lot.
This layer or band of water is called the thermocline. Below this line is where the best oxygen will be and where the baitfish and gamefish will be located and be the most active. Once you establish this correct depth just adjust you fishing patterns and baits to work in that area.
All of this sounds confusing but cooler temperatures will bring fish back shallow soon and great fishing will be back too.