Fish survive when a lake freezes by receding to the lower depths of the lake where the water is not frozen. In the winter, the water still holds a sufficient supply of oxygen and food for active fish.
In the warm months of the year, it is natural to find cooler water at the depths of a lake; the exact opposite is true in winter. Due to the way that molecules arrange themselves in the water, warmer water stays at the bottom of the lake and cooler water stays at the top. Fish tend to swim to the warmer areas when lakes freeze over, and some even go to the very bottom and burrow into the mud there for added warmth.
As winter wears on, levels of oxygen in the water, leaving fish in a vulnerable state. When oxygen is low, fish may reduce their activity level and shut down feeding until conditions are more optimal. Persistent levels of low oxygen can result in freeze out, which is also known as winterkill, causing fish to die off due to lack of oxygen in the water. This is a natural phenomenon that is seen in harsher winters or winters that are particularly long and extreme.