Fish in Your Pond Over the Winter?

Over-wintering Fish and Koi Ponds.
Winter is a period of transformation in the garden, when plants release their foliage in preparation for the ravages of winter and the growth rate of everything else (including weeds!) seems to take a well earned break. It can also be a time of inactivity for us in the garden, providing us with the opportunity to take stock of how our garden has performed and developed in the preceding months, earmarking changes for when spring time comes around again.
We could also be forgiven for thinking that our pond can be left alone to 'rest' in its apparent state of hibernation during the winter months. In fact, pond fish can be so well hidden in an overwintering pond that we could easily forget about their existence, that is, until they magically reappear in the spring.
To a certain degree, a laissez-faire approach with our pond may be justifiable in the winter, as there is little that fish require from us over this fallow period. However, there are a few simple measures we can take to ensure that, even after the harshest of winters, our fish are there to greet us in the springtime.
What actually goes on in a pond through the winter?
Fish:
The vast majority of pond fish (goldfish, koi, comets, shubunkins and orfe) are related to the carp and have been overwintering in our climate very successfully for hundreds of years.
Fish are cold blooded, which means that they take their body temperature from that of the surrounding environment. When it is warm, they are active, feeding, growing and producing the waste to match. The reverse is true as the water temperature drops. As long as the drop in water temperature is gradual as it usually is between summer and winter, lagging slightly behind the reducing air temperature, then our fish should adapt to what for them is a quite natural phenomenon. They have no choice in the matter and respond to cooling water by a drop in activity as their metabolism slows right down.
A carp's physiology is geared to overwintering and this physiology has been inherited by their ornamental cousins. The shortening daylength as winter approaches causes pond fish to prepare for winter before it arrives, feeding in excess (whenever possible) to store sufficient energy to survive the winter. The onset of cold weather may come as a shock to us, but our pondfish, unbeknown to us, have been preparing for this testing period for months.
As soon as the water temperature drops below 8oC, fish become relatively inactive, drop to the bottom of the pond, not to feed again until the temperature rises above 8oC in the spring. During this lengthy period of inactivity, we can intervene to ensure that any risks to their health and well being are reduced.
The Pond.
Ideally, a pond should be at least 3 feet deep. This provides pond fish with sufficient depth of water in which to be insulated from the freezing air at the pond's surface.
A remarkable phenomenon occurs to water when it cools. Usually, the density of a material increases as it gets colder and this is true for water, but only down to 4 degrees C. Between 4 degrees C and freezing, the water actually gets less dense and floats above the denser 4-degree water, eventually to form ice if the air temperature drops below freezing. Our pondfish take full advantage of this phenomenon by sitting on the pond bottom. In very still and deeper water, quite a stable boundary can form between the less dense 0-4 degrees C water and the denser warmer water.
Water quality (and particularly clarity) is likely to be at its best in a pond during the winter. Reduced feeding and metabolic rates of 'hibernating' pond fish mean that ammonia production is likely to be negligible in a wintering pond. This will not burden a pond filter (which is also relatively inactive at these temperatures) and the fish are not likely to experience ammonia or nitrite problems.
Pond water is also likely to be at its clearest in winter as the cold water retards the growth of suspended single celled algae that can cause green water. The cold water will also hold relatively high levels of dissolved oxygen and as all of the oxygen consuming organisms (bacteria, protozoa and fish) are metabolising at a reduced rate, their need for oxygen is also reduced.

This article is courtesy of one of the many websites out there about Koi. So far our fish are doing fine and many folks do not even go so far. Goldfish are happy campers for years in water troughs for cattle and horses - thru all seasons, under ice and onslaught of huge bovine tongues they seem to do just fine.

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