SInce you caught fish deep that is the preferred water temp for the fish in your lake. The thermocline is a band of cooler water. You may have been catching fish at the bottom of it. Or the fish may have been feeding on something special that is assocaited with the bottom. As long as there is oxygen avaiable the fish will than seek out the preferred water temp. Get yourself a cheap window therometer and tie a line to it and drop it over the side. Let it sit a couple of minutes at the depth hat you are marking fish. Bring it up fast and check the water temp. You may be surprized at the difference the temp can be in just a few different feet of depth.