I stopped watching that video as soon as he called the area below 12' the thermocline. I think in spring, I'm sure someone may differ, the lake just slowly stratifies. That is, the cooler, more dense water settles to the bottom. Water is at 4degC in its most dense state. Water in the lake at ice out is fairly uniform in temperature and can be a crapshoot trying to find fish as they'll be in various places in the watercolumn at any given time. As the surface water warms and pushes the cold water deeper, a rich layer of oxygen will set up in the lake. This is the thermocline. The deeper the lake, the deeper the thermocline. For example, the thermocline on a typical Kawartha lake would probably set up in the 20-30ft range whereas in Haliburton it may set-up at 30-40ft. That being said, fish will congregate in fairly predictable spots to find their preferred temperature range. Wood scattered on the shoreline can warm the water in the immediate area faster than rock will, and theres always a steady diet of baitfish near inflows and creeks. Hope this helps!