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Aeromonas alley
A reason to have a heater on a pond is to prevent an effect, sometimes called aeromonas alley, which is due to the falling efficiency of the koi immune system when compared with the activity of pathogenic bacteria such as aeromonas at low temperatures. Opinions vary slightly as to the precise temperatures, but in general, aeromonas alley can be described as follows:
The koi immune system is designed to keep it healthy and combat infection with white blood cells playing a key part in the system by attacking or devouring invading bacteria. According to Luther Chien, the first proponent of the aeromonas alley theory that I recall reading, the koi immune system is at its most efficient at around 25°C and he also states that no white blood cells are found in the koi bloodstream below about 12.8°C.
Aeromonas bacteria are not active below 4.4°C but their activity and ability to infect rises rapidly as temperatures increase above this point until they reach their peak activity at 15.6°C.
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An advanced but typical
aeromonas infection
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This means that at 4°C koi are almost inactive but, since the aeromonas bug is also inactive, they are safe from infection. If the water temperature rises slowly, the aeromonas bug becomes active and can infect koi because its immune system is weak until the temperature rises above 12.8°C so it is defenceless against this infection until then.
Although this temperature range is called aeromonas alley, aeromonas isn’t the only threat, there are other potentially harmful bacteria that could exploit the weakened koi immune system below about 13°C. A graph showing the relative activity of the koi metabolism compared to the activity of common bacteria is shown below.
Of course, if a pond is kept scrupulously clean and there aren't any bugs ready to infect the fish then it won't matter how efficient or inefficient their immune system is. Even so, this temperature range will always be a potentially risky area where even small numbers of pathogenic bacteria can infect koi and cause infections they are unable to resist. On the other hand, if the pond has a high loading of pathogens then the whole range of temperatures from about 4°C right up to where the koi immune system is at it's peak performance at about 25°C will be a risky area.
That's why different koi keepers can report that they are successful at maintaining different winter temperatures. The fact that someone has always maintained, say, 7°C and has never had a problem doesn't disprove that there is a risky area which we call aeromonas alley, it's just a reflection that their pond has few, if any, pathogenic bugs.
Although the koi specific immune system, which relies on the ability of white blood cells to combat and digest invading pathogens, weakens with reducing temperatures the non specific immune system never switches off and will always provide a measure of protection. If the pathogenic bug count is low then this may well prove to be a sufficient defence against them especially as their activity will also be weakened at lower temperatures.
There are two strategies to avoid aeromonas alley. The first is to heat throughout the entire winter ensuring that the temperature stays above 13°C. This can be expensive and many koi-keepers prefer their koi to have a winter period to “reset their biological clocks”.
A second, cheaper way is to set a heater to prevent the temperature falling below 13°C in autumn and maintain this temperature for as long as possible. Then, as the coldest part of winter approaches, readjust the heater to allow the temperature to fall reasonably quickly to 4°C. Maintain this temperature until spring when the heater can again be used to raise the temperature to 13°C until ambient temperatures are sufficient to keep the water above that temperature without the aid of the heater.
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