Koi have a fascinating physiology that responds to changes in their environment in a way that not only determines their behaviour, but also ours as their keeper. With a degree of certainty, we are able to predict when koi are likely to ‘hibernate’, grown and even breed, as koi behaviour is controlled by the different seasons.
Even through the winter when koi appear to be lifeless at the bottom of a pond, they are still picking up information from their environment, responding to these environmental stimuli in a way that is beneficial to their survival, and that of their future generations.
By understanding the processes that cause koi to behave in a predictable seasonal routine will help us to provide our koi with the ideal pond conditions and husbandry and enable us to work in harmony with their natural koi calendar.
What time of year is it?
Koi are so well tuned into the seasonal cycles that if they could talk, they’d be able to tell us what time of year it was. Koi pick up a number of different cues from their surroundings that are processed by their nervous system on a continual basis causing a range of physiological changes. The two key environmental factors that set the time in a koi’s calendar are water temperature and daylength.
1. Water temperature. Koi are poikilothermic (their environment sets their body temperature) and are slaves to their environment. When it is cold, they too are cold and inactive. When the water temperature is warm, their metabolism is active and so are they. These seasonal cycles also trigger other less obvious changes within a koi’s physiology, and inform the koi’s endocrine system to release specific hormones that control other body functions. A pond’s water temperature will affect how quickly a koi’s physiology can react in response to those hormones. This is very clear during the koi farming calendar, when sex hormones are injected into brood fish to induce spawning behaviour. These hormones cause males to increase sperm production, while in the females, it causes the eggs to mature and to be released from the ovaries. The time it takes for a female to respond to that injection is a function of the water temperature taking approximately 14 hours at 20 degrees C or a speedier 12 hours at 24 degrees C.
Water Temperature and ‘Degree Days’.
The accumulation of days experienced a certain temperatures also has an effect on the development of eggs and the readying of females through the spring months prior to spawning. Experience has shown that females must experience 1000 degree days before they are likely to spawn. That means once the winter has reset their clocks to zero, approximately 67 days spent at 15 degrees C will amount to 1000 degree days (67 x 15= 1005). Since the koi’s physiology must receive these lengthy temperature cues from its environment for it to prepare itself to spawn. You could say that koi use this temperature cue to show them that it is the correct season to spawn. This is also the reason why most koi spawn naturally in June or July. Koi farmers can manipulate the degree day system to fool females into spawning months earlier than is natural. For example, by increasing the daily temperature to 20 degrees C, you could spawn the fish sooner after only 50 days (20 x 50=1000, allowing your fry a longer growing-on period outside.
2. Daylength. Experience has also shown that koi use daylength (photoperiod) as a trigger for growth and spawning. Daylength is a very reliable way for koi to determine the time of year as sunrise and sunset times are identical on the same day each year. Even if we experience an unseasonally cold June, the daylength that koi experience in June will be consistent to every other June.
Photoperiod and Growth
Photoperiod has also been shown to affect the release of hormones that control growth. There is evidence that koi require a photoperiod of at least nine hours each day for them to release growth hormones that convert food into body tissue. This seasonal cycle of alternating periods of growth is recorded in the growth rings on fish scales. Where growth is fastest (in the summer) the distance between the rings is greatest compared to those laid down in winter.
Similarly, koi use photoperiod as another cue in addition to water temperature for the maturation of eggs in readiness for spawning. A lengthening photoperiod of approximately 16 hours coupled with an accumulation of sufficient degree days of temperature work well together to lead to a spawn. So if koi farmers artificially manipulate the water temperature they must also artificially lengthen the photoperiod (using artificial lights) for the brood fish to spawn predictably.
Where there is a mismatch between the photoperiod and the water temperature (as in an artificially heated outdoor koi pond), there are likely to be some problems with koi growth and spawning behaviour. This is due to the two cues of light and heat giving a koi’s physiology contradictory information, and in effect, confusing its sensory systems, producing unpredictable responses.
The Koi’s seasonal calendar.
Photoperiod and water temperature interact with a koi’s sensory system to stimulate a range of responses throughout the year. Spring: Your koi will have naturally experienced a period of inactivity in the winter as air temperatures will have fallen below freezing. This in effect resets the koi’s seasonal biological clock back to zero ready for accumulating degree days. Over spring the photoperiod increases each day, as will the average water temperature. Both of these cues will cause a koi to become more active, and turned on for growth. Because growth rates will still be quite limited (because of the interactions of a relatively short daylength and cooler water temperature) then koi will only require a low protein food. Furthermore, if too high a protein food was offered at this stage, then it might overburden your filter that is springing back into life after a period of winter inactivity.
Summer: Koi will be at their most active as water temperatures and daylength are at their greatest. Growth rates will be high and koi will demand a high protein diet to fuel that growth. It is also likely that in June and July, mature koi will spawn, having experienced 1000 degree days in conjunction with a lengthening summer photoperiod.
After spawning, female koi must continue to be offered a high protein food for the deposition of eggs, ready for next year’s spawn. Also as the daylength starts to shorten as autumn approaches, and the water temperature also starts to cool off, koi respond to these cues by actively depositing energy reserves ready for the onset of winter. In this way, your koi are preparing themselves for their winter well before you are.
Autumn: Autumnal conditions such as a much shorter photoperiod and cooling water temperatures will soon dampen koi growth and activity. Their growth potential and energy requirements under these conditions only merit a low protein diet. As the photoperiod falls below nine hours each day then the potential for growth is significantly reduced as growth hormone production is now negligible.
Winter: Winter conditions cause koi to enter a natural period of inactivity that they have been anticipating since the daylength started to shorten back in June. No food is required as the cold water temperature prevents koi from using or requiring additional inputs of energy. All energy requirements are sourced from stored tissue.
The undesirable effects that heating a koi pond can have on the koi’s seasonal calendar.
A koi’s physiology and sensory systems predict and work towards a natural period of inactivity in the winter. In fact, it can be argued that koi require a cold period (and that ponds can also benefit from such a period) as this is what is natural for their physiology. If we choose to heat our ponds over winter, we should recognise the affects that our actions might have on our koi and what we can or cannot achieve by doing so.
Temperature / Photoperiod mismatches: When photoperiod and water temperature are in harmony, a koi’s physiology responds in a predictable way (eg by growing, spawning, ‘hibernating’ etc). However, koi that experience a mismatch between these two cues will respond unpredictably. For instance, the number of koi keepers reporting spawn-bound koi in the autumn and winter period has increased significantly over the last few years. This is most likely due to the increase in the number of koi keepers who heat their ponds. This effectively smoothes out and removes some of the seasonal cues, while the koi are still exposed to the natural seasonal photoperiod. This appears to confuse some koi, preventing them from spawning.
Growth rates. Those koi that are kept in warm water ponds over winter and yet experience naturally short winter photoperiods will show a drastically reduced growth rate compared to if they were held at these temperatures in the summer with a summer photoperiod. A lengthy photoperiod (something they do not experience over winter) is required to stimulate koi to produce specific growth hormones. Consequently, growth rates are lower in winter, even in a heated pond. Koi that are held under these conditions over winter will feed, but will not convert this food efficiently into new growth, perhaps storing it and causing a deterioration in body shape and proportion.