Winter garden pond and cold water temperature swings

As spring approaches I am concerned about what to do if the temperature suddenly rises and then drops again back to near arctic conditions as can happen so easily in this country. How will this sudden rise in temperature affect my Koi? Should I start feeding them again or should I wait until the temperature is more consistent? If the temperature falls again will this make them go back into hibernation? Is there anything I can do to prevent rapid swings in temperature?

Dear Peter,

Thank you for your letter – you are not alone with your concerns about how our freaky and unpredictable weather may affect our koi as they are tempted towards the surface in spring. In many ways our koi are slaves to their environment. They are unable to escape to more favourable conditions should the water quality deteriorate, and their physiology has no choice but to take their temperature from that of their surroundings. So as the pond warms up, their metabolic rate increases as do other body processes. This is clearly evident when we see their increased activity and changes in behaviour, in particular their increased appetite. The reverse is true when the pond water cools.

Fortunately for our koi (and our own peace of mind), the circumstances that you describe are not as desperate for your koi as you fear.

Pond water stability.

One of the benefits that koi (and fish in general) have over their terrestrial counterparts is that pond water acts as a buffer against rapid swings in air temperature. So that when air temperatures may swing rapidly from day to day (and even between day and night), our ponds do not follow the same temperature extremes, but map out a temperature somewhere between the previous pond temperature and the current air temperature. This is because water takes longer to heat up and cool down than the air – something that is evident if you live near the sea. In the spring, the sea will still be much cooler that the warming air temperature, often promoting off-shore breezes. Whereas, in early autumn, the sea is likely to be warmer than the rapidly cooling air, having stored up the preceding summer’s heat – something that will often cause on-shore breezes to develop. The same temperature lag will be evident in a koi pond – particularly in larger volume, deeper ponds that benefit from less of the overall volume being in contact with the cooling air at the surface.

You can see from the chart that records the temperature swings in a pond through a typical year that the temperature changes within a pond are quite smooth and gradual. If the air temperature had been plotted on the same graph you would see a very jagged and erratic line. This again shows how a pond adopts an average temperature, cushioning the koi from the daily extremes of changes in air temperature.

The more prolonged the rise in temperature, the more likely the pond will rise up to those temperatures, but gradually so that your koi will not experience the sudden changes in temperature that we do outside.

I suggest that you buy an accurate pond thermometer so that both now and as autumn approaches you can track the pond’s temperature. You may be pleasantly surprised by the stability of your pond’s temperature.

The thermometer will show you the actual pond temperatures that your koi are experiencing and allow you to make the informed decision about how to respond and how to feed them.

Rather than being overprotective, and trying to second-guess what the weather will do, offer your koi what they will eat sparingly in several feeds. Use the water temperature to determine the type of food you offer. As koi keepers, I think we can be collectively guilty of not giving koi the credit for being able to cope with these ‘natural’ swings in water temperature. One of the main factors that makes koi (and carp) the most widely captive-reared fish in the world is their in-built tolerance of such a wide range of temperatures.

However, one aspect that they cannot tolerate is poor water quality, and this is why your choice of food at this time is crucial. If you choose and feed unwisely at this time then you are likely to cause a negative impact on your koi through the deterioration of their environment.

You should wait until the water temperature has risen to 8-10C and your koi are actively seeking food before offering them food. Spring feeding should be carried out with both filter and fish in mind – and in that order. The reason why we use the term koi food and pond food interchangeably is because the food we offer our koi also affects the pond – and is something you should pay greatest attention to as the pond starts to warm up (however erratically).

If the temperature falls again will this make them go back into hibernation?

If the pond temperature drops sufficiently, fish will show typical overwintering behaviour but only for a short term until the typical spring weather returns. They will stop feeding (having no choice in the matter) and drop down 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 5 feet in depth that the phenomenon of stratification is only remotely possible.

Is there anything I can do to prevent rapid swings in temperature?

You could install a thermostatically-controlled pond heater to help iron-out any erratic falls in temperature. An electric in-line heater (ranging from 1Kw upwards – and not to be confused with a small 100W pool heater for keeping a hole in the ice) can be set to come on at a given temperature (say 10C) so that if the ambient temperature was to drop suddenly below that, then at least the pond water would remain at 10C with your koi and filter being unaffected. This would remove a lot of your worry and concerns you may have for the well being of your koi.

In summary, I think the best advice is not to panic. Carp have been experiencing natural variances in climate for many hundreds of years quite satisfactorily. 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!

Feeding and water temperature

Nature dictates that if it is warm enough for fish to feed then it is also warm enough for them to utilise that food (to a lesser degree at the coolest temperatures). The poorer they do digest and assimilate food, the greater the waste that will be excreted into the pond, burdening your filter and potentially leading to a water quality problem.

Therefore, you should offer your koi a low protein diet at these temperatures because:

1. Your koi’s potential for growth is limited.

Protein is the key component of a diet for producing growth in koi. When water temperatures are at their highest, so is a koi’s potential for growth – so we should offer them a high protein growth diet if we want to capitalise on that potential. But if koi can’t utilise all that protein for growth (such as at lower temperatures) then the majority of the protein will end up in the pond. So we feed them a low protein diet as that is all they require at these low temperatures.

2. For the benefit of the water quality.

If you were to offer a high protein diet at these temperatures, the excess protein (with its inherent high nitrogen content) will be excreted as ammonia (NH3) – loading the pond with a burden for the filter to deal with. Add to this situation the likelihood that the filter will be under performing at these low temperatures and your koi will stand a high risk of suffering from poor water quality.

So as long as you offer your koi a low protein diet, you’ll be safe-guarding your pond against unforeseen eventualities such as a sudden drop in temperature (followed by a drop in your koi’s appetites and performance of your filter)

Temperatures in the UK

Unlike the highly predictable photoperiod (daylength which is dictated by sunrise and sunset), air temperature can and often will be very different for the same day each year. Nevertheless, the relatively large body of water found in a pond ameliorates the random peaks and troughs in daily air temperature to create a gradual rise in water temperatures through spring and summer (as shown on the graph).

However, you will find that the further north a pond is situated in the UK, the longer it will take to reach 10C and the lower the ultimate peak temperature will be in the summer. This natural temperature phenomenon is borne out by the fact that far more successful carp farms are located in the southern half of the UK, where they experience a more favourable climate for growing fish.

Winter Pond Advice.

Winter Pond Information Centre: Overcome ice, frozen ponds and freezing weather safely.

Kill blanketweed and string algae.