Fish that live in a pond are able to survive the winter when the pond freezes because of how they are able to adapt to the drastic changes. When a pond freezes, it never freezes solid. Even when the temperatures drop to well below zero, only so much ice will form on a pond, leaving water beneath it that is about 34 to 40 degrees Fahrenheit in temperature. While people cannot survive in such cold conditions for more than minutes because their body temperature would fall rapidly, fish are able to live in water this cold since they are what is known as ectothermic. This means that fish do not control the temperatures of their body; it will be very close to the temperature of the surrounding water. The common term used to describe this is that fish are "cold-blooded."
The metabolic rate of a fish swimming beneath a layer of thick ice in a pond drops well below what it would be in the summer months. As the water grows colder the core temperature of the fish drops as well. Fish slow down in their activity and are not as quick as they are when the water is much warmer. The energy the fish needs to survive is far less in the cold, which is a good thing since there is not the abundance of food available to a fish in the winter under the ice that there is in the summer. Some species that would be found in a typical pond such as catfish, carp, and bass will store as much fat in their body as possible, eating heavily in the summer and autumn so that they have enough reserves to survive the winter. Catfish and eels have been known to bury themselves in the mud at the bottom of a pond and await the return of spring.
Some species of fish will still actively feed on whatever they can find. Perch, crappies, bluegills, trout and pickerel have been able to evolve so that they can raise the temperatures of their body even in cold water, but to maintain this feat they have to eat. Luckily they still do not have to eat as much as they do in the summer, because the amount of baitfish such as minnows has dwindled due to their being eaten at a great rate throughout the summer and fall. Larger fish will eat smaller fish and be able to successfully capture what baitfish are left in the pond, but the smaller species depend on other sources of food. Crappies, pumpkinseeds, perch and bluegills will feast on the immature larvae of the huge variety of aquatic insects that inhabit a pond. These are found in the mud and muck on the bottom. Tiny zooplanktons are also consumed in the winter as they remain active in the cold pond also. When the weather warms up, the tops of weeds that may be trapped in the ice are pulled on as the water makes its way back through the ice and causes it to rise and float. This can pull the weeds out of the mud at their roots and stir up aquatic insects which wind up being easy pickings for hungry fish.