Caring for your koi in winter

Winter care for koi, as the ambient temperature drops, and the water in our koi pond gets colder, we are once again made aware of the effect of temperature on our fish.

Our koi are cold-blooded creatures and their bodies function at the temperature of the surrounding water. In fact, temperature has a most profound effect on almost all the life forms in the pond. The fish and plant life, micro organisms and even the bacteria in the biofilter.  It affects the rate of chemical activity within their body cells. Consequently, as the water temperature drops koi will tend to eat less, be less active, breathe slower and consume less oxygen. Food will move slower through their gut as their metabolic processes slows down. At 11oC it can take five days for food to move through their intestines. At around 10oC the immune system of the koi effectively becomes inhibited. The biofilter will also be ineffective and the bacteria in a state of rest.

Because seasonal variations in weather may vary greatly from place to place, koi keepers should be guided by the actual water temperature and behaviour of their koi. Do not follow the prescriptions of textbooks blindly.

As the temperature drops algal growth will slow down and the pond water will become crystal clear. Oddly, the colours of the koi will intensify and become brighter in cold water. Feed the fish an easily digestible food such as wheat germ pellets. Do not feed too much. Below 12oC circulate the water less vigorously. If possible only the top water in deep ponds should be circulated, leaving the bottom water still and warmer. Water falls and venturi’s will cause the water temperature to drop more rapidly during cold spells. Flush bottom drains regularly.

Below 10oC koi go into a state of virtual hibernation and will gather motionless in the deepest parts of the pond. They should not be disturbed or offered any food, even when begging.

Written for Animal Talk magazine 

Servaas de Kock & Ronnie Watt