Koi Metabolism and Temperature

If you have ever visited a koi retailer in winter, you will have noticed how seasonal koi keeping is. Unless a koi outlet has recently taken delivery of a shipment from the Niigata autumn harvest, a koi outlet out of season can leave the same impression you get when visiting a seaside resort in winter. The atmosphere takes on a different personality from the summer activity, and you can almost detect the shelves, vats and cabinets yearning for the onset of spring and summer.

Our hobby is so seasonal because our koi are seasonal animals. Being a relatively primitive animal, koi are unable to choose a degree of independence from the temperature of their environment. If their pond water is cold, they have no choice other than for their body temperature to follow. Likewise, as the temperature rises, so too does their body temperature and level of activity. In this way, koi are slaves to their environment and are unable to break free from this seasonal cycle of activity and inactivity (unlike us and other mammals).

In recognition of this natural phenomenon, (which many koi keepers perceive as a problem for koi), more and more of us are choosing to heat our pond, to reduce a risk element of leaving koi to the extremes of winter weather.

How and why does heating a pond affect a koi’s activity?

The external temperature determines a koi’s metabolic rate, in the same way that the temperature of an oven determines the speed at which something is cooked. A koi’s metabolism describes all the natural biochemical reactions that occur within a koi (such as digestion and the manufacture of new compounds). As the temperature increases, a koi’s metabolism responds accordingly as the fish responds by secreting enzymes and hormones that control and regulate its own bioprocesses.

Earlier in the series, we looked at the structure of some of the key organs and senses in a koi (gills, kidneys, ears, intestine etc), and identified how each of these structures functioned. It is the underpinning metabolic rate that determines at what rate each of these organs functions, performing a regulatory function (even though it is regulated itself by the external temperature). Consequently the water temperature determines the koi’s demand for energy, its growth rate, immune system function and ultimately its appetite. This in turn has a knock-on effect as to the amount of waste koi excrete.

Fortunately, the metabolic activity of other organisms within the pond (most notably bacteria) is also controlled by the water temperature. They are beautifully in-tune with koi in that when koi are producing the greatest ammonia, the bacteria are also at their most active, breaking down and processing the waste into less toxic by-products.

Koi physiology responds to other environmental stimuli.

We should note that koi pick up their signals from other sources besides the temperature of the pond water – something that those who heat their ponds should be aware of.

For example, if you were to record the water temperature of your pond on a daily basis over 5 years, you would notice that the water temperature would be different on the corresponding day each year. To overcome this unpredictability of temperature throughout the seasons, the natural world also takes its guidance from another cue that is reliable and dependable, being identical between corresponding days – year in, year out – That is daylength (or photoperiod). Koi biology is in tune with the seasonal, predictable changes in daylength each year, using it as a reliable cue for confirming the time of year. Problems can arise in a heated pond when koi experience a mismatch between the natural daylength and the temperature of their environment and in certain instances, resulting in them missing the spawning window. This means that they may well enter the autumn in a heated pond still laden with the spawn that should have been released in the preceding summer.

By understanding the role that water temperature plays in governing the metabolic rate of a koi’s physiology, koi breeders can manipulate the water temperature (in tune with an artificial daylength) to bring on spawning activity earlier in the year than would have otherwise occurred naturally. For example, we can bring mature broodstock inside in January (after a good freeze) and gradually warm up their holding tanks to 15C. This gentle warming must be accompanied by an artificial increase in their photoperiod (perhaps a 16 hour day) to match the inside temperature. By subjecting koi to this regime for several months, it is possible to manipulate their metabolism in such a way that their body matures the eggs by early April, at least 2 months earlier than would have happened naturally. This allows the farmer 2 extra months in the summer in which he can grow on the fry, getting them to a larger, hardier size ready for their first winter.

A koi’s physiology is adapted to changes in temperature, which in turn, cause changes in their activity. Koi are so adaptable to a wide range of temperatures that we can easily overlook the fact that they are technically a warm water fish, growing optimally at 27C. Their physiology is adapted to a short harsh winter – (something that we are happy to exploit), keeping our koi in the UK somewhere in between these two extremities in temperature for the majority of their life.

Yet just because koi can tolerate such a wide range of temperatures does not mean we can afford to be cavalier and carefree with the temperature at which they are kept. Koi prefer a stable environment and if they are subjected to rapid changes in their temperature, their physiology will not be able to cope, causing them stress.

Stress is caused when koi are subjected to a change in their environment (such as a temperature change) which is over and above their natural limits. Koi respond to such extreme changes to their environment as a threat, releasing a number of hormones to enable their metabolism to mobilise energy rapidly, allowing them to flee in response. Unfortunately, a knock-on effect of these hormones is to reduce the effectiveness of their immune system, which can prove to be catastrophic if the stressor persists for a lengthy period.

As koi have inherited their metabolic and physiological make-up from carp, they are not suitably adapted to living restricted in a crystal clear pond. In natural waters, should carp be subjected to an unacceptable stressor (for example a sudden influx of icy water from thawing snow) they have the freedom to retreat into warmer, deeper and less stressful water. But for koi that are confined within in a koi pond, unable to escape their stressor, they suffer the consequences and become susceptible to disease.

In summary, a koi’s metabolism governs the rate of function of the many processes within its body and is itself governed by water temperature. As water temperature changes, so does the metabolic rate of koi, having a knock-on effect for their appetite, activity and spawning behaviour besides many other processes. So profound is the link between temperature and metabolic rate that the koi hobby as a whole slows down in the cooler months. An increasing number of us are heating our ponds in order to lengthen the koi season and reduce the risks of winter, but care must be taken to ensure that an unnaturally constant water temperature through spring and summer does not conflict with a natural lengthening photoperiod, giving koi a mixed message that may well result in preventing them from spawning, entering the autumn in a bloated, spawn bound state.