Fish will often go into a state of torpor
Torpor is a state of decreased physiological activity in an animal, usually by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability.
When the cold months set in. Snow, ice, & shorter days, less outdoor time leads to us slowly losing our ability to see what is happening with our pond fish. These cold conditions can last for many months depending on our location.
What is going on under those layers of snow & ice? What do the fish do?
Fish are in fact cold blooded organisms, as such they rely on the surrounding environment to help regulate their temperature
Fish are ectothermic vertebrates, meaning that their body temperature will fluctuate with the surrounding water temperature, if the water cools down, your fish will, too!
Humans, and most birds and mammals as well, are, ectothermic poikilotherm.
meaning that we produce our own heat (typically by consuming food for energy) and our body temperature does not vary much. In fact, deviate by more than just a degree or two and we become ill; more than that, and we can die.
Because of their environment, fish are very well adapted to be able to withstand some substantial temperature shifts before it causes any serious issues for them, as well as make use of a low-energy state to reduce food requirements over the winter.
If your pond was cleaned and prepped prior to those cold dark winter months, then your koi and pond fish will be just fine. the koi and goldfish in your pond do not do that much during the winter. In no time at all spring will soon be here and your fish will start again with their usual behaviour. Now you have a better idea of what is happening out there under the ice and snow,