Koi keeping can put many unnatural pressures on koi that these artificially selected fish have not had to contend with in their wild ancestral form. Over thousands of years, their physiology has adapted to a sedentary, slow moving existence in the mid to deeper waters of naturally murky water. Yet in steep sided koi ponds, water is crystal clear and does not provide koi with their natural cover.
They are kept in relatively high stocking densities and are encouraged to feed at the water’s surface when their mouths actually point downwards, perfectly adapted for scavenging off the bottom. Furthermore, their natural daily feeding routine as a scavenger is to feed on an ‘as and when’ basis, as solitary fish, sampling and ingesting any potential food item.
Compare this to the feeding frenzy of a koi pond, where koi may be limited to 2-3 feeds a day and consequently have no option but to devour their food like predators rather than scavengers, competing against other koi for every last pellet. This feeding behaviour is now regarded as the norm in koi keeping, something that can be used as a useful barometer to indicate the health status of koi. Generally, healthy koi will eagerly feed, unhealthy koi will suffer from a loss of appetite.
We can take it quite personally when people don’t eat what we serve in front of them and the same can be said for koi. However, rather than taking such rejection as an insult, or to worry that by not feeding, the koi will starve, we should respond to rejection at feeding time as an indication that there has been a deterioration in the koi’s environment or health. If we were not to intervene, then such symptoms may be the fore-runner of greater koi health problems.
There are several reasons as to why koi may not feed, and it is a matter of discounting potential likely causes through a process of elimination.
1. Sudden changes in temperature.
Koi are poikilothermic (see Nishikoi Encyclopaedia) and their metabolism and appetite is determined by water temperature. We quite naturally don’t expect koi to feed in winter because of the reduced water temperature and a sudden drop in temperature in summer (for which the British climate has a reputation) will have the same effect on their appetite. If you have noticed a recent cold snap, then so will your koi. To verify that a cold snap is the cause of a loss of appetite, take notice of how they feed once summer returns (if it does!).
2. The unwanted attentions of a heron or cat
can cause fish to stop feeding through stress, becoming less tame through a self-preservation mechanism in response to the threat of predation.
If this has been the cause of your koi not feeding, then it is likely to be accompanied by them also being more withdrawn and less willing to come to the surface. Keep an eye out for a heron at first light or at dusk and similarly for a cat to verify whether this is the cause.
Changes in diet can be a real turn-off for some koi.
If koi have become used to a specific food, or have been fed one food on a long term basis, then they may not immediately adapt to a new diet. Differences in smell (which is very important to koi) and palatability can affect a koi’s reaction to a new diet. The fact that koi respond differently to a new diet may not necessarily reflect its quality, but merely that it is different from what they have been accustomed to. Persevere with the change and as long as the new diet is palatable, the koi will adapt to it
The single most influential factor on a fish’s health is water quality. If there have been detrimental changes to any aspect of a koi’s environment, then it is likely to affect its behaviour and appetite. Water parameters to test for are rises in ammonia or nitrite or fluctuations in pH, deviating from the desirable pH range of 7 to 9. If it is possible to detect any changes in water quality then this would suggest a likely cause to their loss of appetite. If no changes have been identified, then water quality can most probably be discounted, but always keep an open mind.
Koi are likely to stop feeding if they are suffering from disease.
It is quite possible for koi to be under attack from a pathogen without there being any visible signs of lesions or damage to koi tissue. Besides losing their appetite, koi suffering from a disease are likely to show ‘classic’ behavioural changes associated with that disease.
A very useful procedure is to take a mucus scrape and examine the sample under a microscope to view any likely parasites. If there are large numbers of parasites such as Trichodina, Gyrodactylus, Chilodinella etc visible then this is the likely cause of loss of appetite, and can be treated accordingly. This, however, is not completely satisfactory as it is best practice to identify the likely cause of the koi becoming more susceptible to disease in the first place, being able to prevent it from happening again having cured it.