Question

in cold regions how do aquatic animals remain alive even after the rivers or lakes freeze in winter?

Answer 1

For the most part in fresh water, the lake etc. does not freeze throughout and the ice is floating on liquid water. Fish under the ice are in liquid water and may be active, depending on circumstances. For example, walleye pike in the northern midwest actively feed - thus the popularity of ice fishing. I don’t think they do very well in bodies of water that freeze clear through. In the Antarctic, where water temperatures are below 32 F. (Freezing point of seawater is lower because of dissolved minerals) at least some marine animals have substances in their body fluids that act as ‘antifreeze’.

Answer 2

Underneath the frozen upper layers, the water remains in its liquid form and does not freeze. Oxygen is trapped under the frozen layers. The animals breathe through that oxygen and thus survives under the frozen lakes and rivers.