Temperature is one of the critical factors directly influencing every aspect of our Koi and pond life. Temperature has a direct bearing on metabolic rates of living organisms in the pond. It has a direct bearing on the health and growth of Koi, oxygen levels in the water as well as the oxygen consumption.
There are however, upper and lower limits to what can be considered good or ideal water temperatures. As temperatures rise so does the rate of growth of our Koi. Up to a certain point this is true, but beyond a certain point as temperatures rise, the oxygen saturation levels of water declines. At about 28C there is no further point in trying to achieve higher water temperature as the amount of oxygen that can be dissolved in the water becomes the limiting factor in conversion of food to growth.
On the other end of the scale below 10C metabolism is reduced to the point where the body is hardly functioning. Below 5C koi go into hibernation. And at around 2C for any length of time the fish are close to death.
Unfortunately, oxygen levels in the water and temperature are not synonymous with each other. High temperatures affect the fish because the D.O. (dissolved oxygen) is LOWERED. At the same time the B.O.D. (biological oxygen demand) of the whole system and fish is RAISED. Higher temperatures increase the toxicity of certain substances in the water as well as the invasiveness, and virulence of bacteria and parasitic pathogens. At higher temperatures the rate of decomposition of organics is much faster than at lower temperatures.
While temperature is critical to koi growth and health there is a trade off between warm water and very warm water. Up to a certain point higher temperatures produce faster growth but often with fading colors.
As water temperatures decline body activity decreases. At or around 12C the koi’s metabolism is reduced to the point where the immune system has virtually shut down. Oxygen and food requirements are greatly reduced. Food should be withdrawn at or around 10C. The blood supply moves slowly around the body. This shut-down is not a switch on or off at a certain temperature, it is a gradual change. Different Koi have different tolerances. Some will be more affected by temperatures than others.
Although pathogenic organisms are also slower in their infection rates at lower temperatures, when introduced into the system, they can be still be deadly as koi are almost incapable of fighting infection at low temperatures. An advantage of low temperatures is that not only the koi have decreased activity so does everything else (such as pathogens).
Note: Hobbyists should also take note of the rapid climb in pond temperature immediately after winter. The few weeks between the beginning of August and the middle of September can be dangerous times. Through winter the whole system has slowed down or in some cases biological activity has shut down (not died back). The koi may have not been fed for some time which indicates the bio-filter has not received food and has become dormant.
The sudden increase in temperature brings about a sudden increase in activity of the fish. Give the bio-filter time to re-establish itself by gradually increasing the feeding rate between August and September and continue with water changes.
Suddenly feeding large volumes of food immediately after winter can bring about a deterioration in water quality and associated gill problems.
Note: oxygen consumption increases with temperature.
In pond circumstances there is a natural few degree temperature change over a 24 hour period. However, moving Koi between ponds with a temperature variance of more than 3C or 4C may cause thermal shock and even death.
Temperatures should be gradually adjusted between bags and ponds before the introduction of Koi. A temperature change of 0.2C/min can usually be tolerated.