It's no longer a limiting factor. With lakers free to travel, they go where the action is, and in the winter, the action is often just below the ice. Understanding winter habits of lake trout means exploring the ways of their prey. Both live just below the surface of the water. Even in winter, sunlight is the master link in the food chain.
Sunlight penetrates ice and snow of a frozen lake, causing photosynthesis of phytoplankton.
Zooplankton eat phytoplankton. Whitefish, smelt, and herring eat zooplankton. In fact, in lakes lacking baitfish, zooplankton is a staple in a laker's diet. With the drama of the food chain unfolding close to the surface and the lack of a temperature barrier in winter, the presence of high-up feeding lakers is a given. But only top predators thoroughly enjoy the freedom that comes with uniform water temperatures.