Question

why fishes survive in ponds even when the atmospheric temperature is well below zero degree celsius

Answer 1

Below the icy layer on the surface of the pond,its full of water at or below 0 degree.
Water contains latent heat which makes it suitable for fishes to survive.

Answer 2

Water has a maximum density at 4 deg Celsius. When it cools below that the colder layer rises to the surface. If it freezes there, the water below remains in liquid form and to an extent the ice layer prevents the water below it from freezing.
The water in a lake or pond rarely freezes solid. Instead, it freezes from the top down, which leaves plenty of water for the fishes underneath the ice.