We have a pond with several fish in it (koi, sturgeon, goldfish). We live in a temperate area where the pond is covered by a layer of ice for several weeks during winter.
How can I ensure the fish survive this period?
Do I need to keep an area ice-free, and if so, how should I do this?
The fish should all be okay as long as the pond isn't going to freeze all the way over. I would say you don't want the pond to freeze over for more than a day or two, because carbon dioxide can build up under the ice, and new oxygen won't be able to get in. Also, since I'm pretty sure the sturgeon are still active during the winter, they will still need a steady supply of oxygen.
The koi (and I think the goldfish too, but I'm not sure about them) will go into what is called "Torper", which isn't quite hibernation it's more like a state of stupor. They become lethargic, sitting in warmest area of the pond they can find. Their metabolisms do slow down while they're in this state, but they will still need a good supply of oxygen.
It's pretty important to try not to disturb them while they're like this (it stresses them out) so I can understand why people don't want to put noisy pumps in their ponds during the winter. If you need to break the ice, I would try pouring boiling water on the ice, or using a hand drill, to make as little noise as possible. I've heard of some people adding salt to lower the freezing point of the pond, but I don't suggest that myself because it changes the chemicals of the water while most people don't have their pumps running, and it's not a good idea to let the water get much colder than 32 degrees Fahrenheit (0 degrees Celsius) in the first place.
Depending on how you feel about your electricity bill, you might be able to get by with dropping a large aquarium heater to keep a part of the pond just warm enough so that it doesn't freeze.
I don't have a fish pond and never have, but I can give you an answer I've derived from watching some pond building shows as well as from articles where people have ponds their livestock drink from.
In the pond building show, they had the water suck up from the bottom/far end of the pond to go up to the filter and return on the top/opposite side. This created a "current" that help kick up the debris and get them properly filtered. However, during the winter, they didn't want to disturb the fish resting on the bottom, so they moved the intake either to the top, or the top/same side as the return.
The idea on the livestock pond that might work for you, is that the person put a small water pump in the pond. You're filter return might do this as well, but the pump pushed the water straight up. It wasn't a geyser, but broke the water surface so that there was constantly moving water and even when the rest of the pond froze, a fairly large area around the pump was ice free. This would keep it open enough to promote oxygen exchange for your fish.
This is an old question, but people needs to understand how this is done properly.
You need to keep an open, ice-free area in your pond but this does not need to be large. One way to do this is to use an aquarium heater and a sheet of styrofoam, you simply make a small hole in the styrofoam and put the heater here. (Note: the styrofoam is to keep the heater floating in the surface of your pond).
If you want to keep the pump and filter running during the winter, you need to move the pump away from the deepest part of the pond. This is to keep the water at the bottom of the pond undisturbed - in other words, this is to avoid your fish having to swim and use energy constantly during the winter as they do not eat at this time of year.
Some information about overwintering pond fish could be found in this article (source: plantsgalore.com).
Before the winter starts, you will need to clean your pond and filter. This is to remove the plant matter and waste from your pond because when waste decomposes, it produces toxic gases.
The temparature in the water needs to be above 2 °C for your fish to survive, but this is normaly not a problem if the pond is deeper than 1-2 meters. But if you live in an area where it is very cold, the pond needs to be deeper.
And last, the oxygen content in cold water is higher than it is in water of a temperature of 20 °C, so in winter the only concern is to get rid of the CO2 (this is why you need to keep an ice-free area in your pond). It is the concentration of CO2 that is dangerous to fish and if the concentration of CO2 is low, the fish need less oxygen. On the other hand, if CO2 concentration is too high it will block the fish' oxygen uptake by binding to the red blood cells). A helpful article about importance of dissolved oxygen in the water could be found here (source: fondriest.com).
Air circulation:
This is critical. Air, oxygen should be added to the pond amid the winter basically on the grounds that the fish are as yet breathing and, along these lines, spending the accessible supply. Additionally moving water takes more time to solidify than still water
Deicers:
Deicers are regularly called "pond warmers" and this is a misinterpretation. They are not intended to warm the whole pond, but rather just to keep a gap in the ice sufficiently open for the carbon monoxide to escape and to give fresh air access.
Smelling salts:
Presently we have to address the smelling salts issue in the event that we need to over winter fish. Around 80% of the smelling salts in the pond originates from the fish. The vast majority of it from the gill capacity and the rest from their waste. Alkali additionally is delivered from rotting natural matter, dead stuff... leaves, plants, bugs, dead fish, and so forth.
Break the ice whenever.
Sustain the fish after the water temp stays beneath 50 degrees.
Leave the pond pumps and Filters running if there is a chance the framework will solidify and the water will be lost.
Include fresh water without utilizing a dechlorinator.
Salt the ice on the pond with the expectation that it will liquefy.
Give garbage a chance to develop on the ice over the winter.
Give anybody a chance to ice skate or play hockey on the pond.
