What if the pond temperature suddenly drops in a cold spell in autumn?

The UK weather is a national obsession and its unpredictability has always made it a very popular conversation piece. Indeed, a well known TV weatherman put it just right when he said, ‘The UK doesn’t have a climate, just different days of unpredictable weather’.

This is particularly true of autumn in the UK, when during the months of September and October, Arctic days can be followed by days typically experienced in July and August.

What is the best we can do if we actually do experience a prolonged summer into September and out of the blue have a frosty night or perhaps even the first ice of winter?

Well first of all…don’t panic!

Winter is a natural phenomenon and koi (carp) have been experiencing winters for many hundreds of years. In fact their breeding and development is based around the seasonal cycle which includes a period of cold weather and inactivity. I have yet to see or hear of a total wipe out of koi in an over-wintering pond and at times I feel the over-wintering qualities of koi are often underestimated. However, all ponds and koi are different and there are a number of things that can be done to increase the chances of even the weakest koi’s safe arrival in spring.

Typical behaviour to expect during the first cold snap.

A change in the weather will cause a change in koi behaviour as like all pond fish, koi are poikilothermic in that the external water temperature governs their metabolism/rate of activity. Koi behaviour will contrast that which you have experienced over the preceding summer months.

Decrease in koi activity. Swimming will be reduced and also swimming style will change quite dramatically during a cold spell. Koi will change from an energetic style using fins and the whole body to generate movement to a reserved style using limited body to drift around rather than swim about.

Koi will stop feeding. If the water temperature drops to 10 degrees C or below then as a koi keeper you will have to make what to many koi keepers is one of the toughest decisions on the koi calendar. That is, to feed or not to feed?

One of the problems associated with a cold snap is that it takes us more by surprise than it does our koi. We should have prepared for this day over the last 6 months by feeding our fish a well balanced diet that can be laid down in reserves for the winter. Since mid-June the days would have been getting shorter and the entire living world, including koi have been taking this as their cue that winter is on its way. Winter is a natural phenomenon to koi physiology as it is to ours and just as we have woolly jumpers, insulation and central heating to help us through winter, we too can help koi through this testing but quite natural period.

The problem with a cold snap is that we don’t have the benefit of seeing into the future to determine if this is an isolated cold spell or the full onset of winter. There is a school of thought that maintains that koi feeding immediately prior to a cold spell that turns out to be a prolonged period of cold weather could threaten koi health as the food remains undigested in the gut.

Koi should have been fed a lower protein diet anyway in preparation for winter, making it not too much of an issue if cold weather does take us by surprise. A rule that has not let me down yet is that ‘fish know best’ and it is wise to stop feeding when the koi themselves stop feeding. After all, they’ve been doing this longer than we have!

3. Fish will go to the pond bottom. This is partly as a result of a reduction in activity but also by staying on the bottom, koi are benefiting from the warmer water in the lower layer of the pond. Quite strangely, as water cools, it reaches its maximum density at 4 degrees C. As it cools further, it gets less dense and rises to the surface. Hence in very cold weather, by staying at the bottom, koi are experiencing the warmest water available, reducing the likelihood of experiencing tissue damage. This is why ponds should be built as deep as possible as it is only ponds of at least 4 to 5 feet in depth that this phenomenon of stratification is quite stable and predictable.

For this reason, action should be taken during a cold icy winter spell in a pond to reduce the mixing of warmer deeper water with colder surface water.

You should also respond to a cold snap in the autumn by preparing the pond for the winter in a number of ways and by doing so, will improve the chances of all of your fish reaching the spring successfully. A list of measures to take is shown below. These will be discussed in more detail next month when we explore how to get your koi, pond and filter system safely through the winter.

Prepare to clean out the filter, particularly the mechanical or settlement areas.

Research the most appropriate method of preventing your pond from freezing over.

Look for a fine lightweight net to prevent leaves falling into your pond

Decide on the best pump/filter strategy for your pond over winter

Look at removing any aeration from deeper water.

Jargon Buster

Stratification. When a boundary between two layers of water exists as a result of differences in density or temperature. Stratification impedes the mixing of the two water layers.

Winter Pond Advice.

All you need to know for your pond and pond fish to survive the harshest winter in a garden pond.

Kill blanketweed and string algae.