Before we consider what we should do to look after our Koi in winter we should perhaps understand their needs. Koi are Poikilothermic. A long word for saying they are cold blooded and as such they utilise the ambient warmth in the water for every function of life. This includes amongst other things growth, resistance to disease, reproduction and repair of damage.

Temperature is the water parameter that exerts the greatest influence on the welfare of our Koi. The obvious effect is upon metabolic rate and activity and therefore the need for food. Temperature influences such things as outbreaks of disease, the immune system and the parasitic lifecycle. Koi Herpes Virus for instance seems to kick in at about 65oF. Water temperature also influences the immune system as below about 50oF the immune system is reduced or even suspended. The parasite life cycle increases with temperature but unfortunately seems to kick in below the threshold temperature of the Koi's immune system, thereby giving the parasite a head start in the spring. The biological life cycle is affected by temperature but also needs the right level of light hours per day. Growth and tissue repair are both dependant on water temperature. Damage repair will only occur above about 63oF.

If you heat your pond the only decision to be made is whether or not to give them a winter (down to 52o F). If you keep the pond at a higher temperature you are likely to reduce the life expectancy of the Koi and apparently also reduce the condition of the Koi, as they will not reabsorb eggs. (See: December Koi Carp "The Sakai seminar")

If you don't heat your pond you should already have started putting your pond and fish into winter mode in September/October.

Firstly you should make sure the fish are in good condition as any marks, ulcers, split fins need repairing before the temperature drops below 63oF. Any damage left is unlikely to repair over winter, leaving a problem for the spring when it arrives. You should also ensure they are fed the right diet. The protein content of the food should be gradually reduced when temperatures begin to fall and once 55oF is reached Wheatgerm should be used, as Koi cannot assimilate protein at low temperatures and only need carbohydrates in cooler water. Once temperatures have dropped to about 45oF you should consider stopping feeding altogether.  If a cold spell is forecast stop feeding before it arrives and then don't feed again until the water warms up in the spring.

One of the worst ways to cool water is by leaving your waterfall running. Turn it off as soon as the cooler weather arrives. You can raise air stones off the bottom or better still turn them off altogether. If the pump is housed indoors pumping warm air into the pond may help. You do not really need air pumps in winter. Water will keep 9.2mg/l of Oxygen @ 68oF and 14.6mg/l of oxygen @ 32oF.   Koi need 5mg/l to be relatively happy and with the rest of the pond reducing its oxygen demand in winter there is ample for the Koi.

Do not turn your filters off in winter. Although bacteria levels in the filter reduce, Koi still excrete ammonia during winter even if not being fed. The low levels of filter bacteria are another good reason to reduce the feed amount, or even to stop feeding in winter. The filter will cope with the ammonia produced in winter, unless you increase feed with impromptu feedings in a cold period.

If leaving the waterfall running is bad for the pond temperature then wind chill is probably an even worse way to cool your pond very quickly. The best way to avoid this is to build a cover. There are several ways of doing this ranging from a simple floating cover, or polycarbonate sheets held in a rigid frame, to a conservatory. If you cover the pond with a floating cover such as bubble wrap then you shouldn't cover the entire pond. At least 25% should be left open to allow the gaseous exchange between the water and the air. If you have a raised cover of polycarbonate sheets the air gap should be 150mm. You should try to avoid condensation dripping back into pond as it is acidic and will lower the pH. This is particularly important if you heat your pond.

An average winter is probably the safest time for your Koi. The water holds plenty of oxygen and the water quality improves. There is less pollution being put into the water and there is less food being introduced. The lower bacteria and parasite levels means the Koi are not under so much attack and there are no unquarantined fish going into pond.

So what can go wrong?

Well for a start very cold water shuts down the fishes metabolism. Both respiration and heart rate slows and can eventually diminish until it stops altogether in some winter extremes. Before they die it may be possible to bring them back by SLOWLY warming them up in a quarantine tank, but don't put them back in the pond until the pond has reached the same temperature as the quarantine tank other wise they may succumb anyway. Shallow ponds may freeze along with the Koi in a particularly cold snap. Again caught early you may bring them back by slowly warming them in the quarantine system.

Carp pox - A herpes type virus can emerge in cold water. Warm water usually sends it on its way and as Koi age they become less susceptible to the virus. The white candle wax symptoms may not always show white as they may be coloured if the roots are near a blood vessel.

Pond sores can arise because Koi will often sit motionless on the bottom of the pond. The sores can become infected, particularly in the spring. The Koi will need to brought into the quarantine tank and slowly warmed in salt backed up with acriflavine or similar.

Swim bladder problems can also occur. They manifest themselves by the Koi lying on their side, usually in late winter or early spring when temperatures fluctuate. The Koi will probably be all right if it moves occasionally, otherwise warm them in shallow water containing salt. You will need to keep them away from the pond until the water in the pond warms up.

And just when you thought it was safe to go out to the pond again spring arrives with all that entails! If you haven't looked after your fish during autumn and winter, spring is the time that you will find out.

Good Koi keeping.