Winter can be a tough time for fish in outdoor ponds. Although the majority of fish will survive the depths of winter in an unheated pond there are several important steps and factors to be considered to ensure that your fish not only survive winter but flourish the following spring.
Preparation of the Pond
Firstly, assess whether your pond is a suitable environment to overwinter fish by considering the depth of your pond. During the winter months fish will congregate in the deepest areas of the pond where the water is warmest. Ponds need to be at least 1 meter deep to protect fish from the cold surface water. If the pond is too shallow, fish will be exposed to freezing temperatures and suffer from cold shock.
Shallow ponds are also at risk of deep freezing in extreme weather resulting in thick layers of ice at the surface. Ice is a problem for many reasons, most crucially it prevents gas exchange leading to a reduction of oxygen and a build-up in carbon dioxide. Carbon dioxide is acidic in solution so a build up in concentration can drastically drop the pH of the pond.
Deep layers of ice also reduces the amount of space the fish can physically occupy within the pond. Waste products will build up at a faster rate and there will be less oxygen available to the fish. For this reason, it is also important to assess the stocking density of your pond prior to winter.
There are several ways to prevent surface freezing such as the use of bobbing water features like plastic ducks or footballs. Alternatively, de-icers, air stones and continuous water movement will also help reduce the risk of the pond icing over.
Pond covers can be used to insulate a pond during winter and various types are available to purchase, from plastic balls (like those seen in kids play areas and ball pits) to durable thick layers of plastic and foam.
Raised ponds and ponds with windows are particularly susceptible to freezing during the winter, covering exposed windows and sides with insulating material can really help during the coldest months of the year.
You can prevent surface freezing by using bobbing water features like plastic ducks or footballs.
Autumn Cleaning & Water Quality Monitoring
In the winter run up, remove dead or dying vegetation and organic material such as fallen leaves. Consider adding a net over the pond to limit leaf fall. Excess organic material sitting in the pond over winter will contribute to a build-up in ammonia.
Inspect your filters and perform a regular clean of any parts and media as required. Care not to over clean any media and disrupt the biofilter. If necessary and temperatures allow, dose with ‘good filter bacteria’ after cleaning to support the biofilter.
Check your water quality frequently towards winter. If water changes are needed, avoid performing large water changes as this could drop the temperature of the pond quickly. Instead, perform small but frequent water changes until the parameter is improved.
Continue to monitor water quality throughout the winter period.
Filtration
Keep filtration running throughout the winter period. Although the bacteria that make up the ‘biofilter’ are dormant and not processing waste products, running filtration will help keep the water clear and process debris. It will also keep water circulating which can prevent the surface from freezing. It is recommended to turn down the flow rate of the filter and move the outflow (if possible) to shallower areas of the pond to reduce the cooling of the warm zone.
The main reason and benefit in keeping filtration running throughout the winter period is to reduce the risk of water quality issues in the spring. If you turn filtration off during winter and allow the filter media to dry out you will need to start the biofilter from scratch in the spring. This means balancing and managing water quality issues until the biofilter has established which can take 6-8 weeks. At the same time, fish will be waking up and eating more, subsequently producing a lot more waste. The above combination of a new biofilter will quickly result in water quality spikes and disease. The risk of disease is even higher in the spring due to a slow immune system post winter, which furthers the propensity for disease.
Take care with UV sterilisation units during the winter period. Trapped frozen water in UV units can permanently damage the equipment. If the UV units are exposed or at risk of freezing, it is a good idea to disconnect, drain and store sterilisation units somewhere dry and protected until the Spring.
Turn off or bypass any water features that mix cold water from the surface into deeper water, ‘the warm zone’.
Water Features
For unheated ponds it is vitally important to conserve the warmer water in the lower depths of the pond ‘the warm zone’ as this is where all your fish will congregate to overwinter.
There are several ways to do this. Firstly, turn off or bypass any water features that mix cold surface water into the warm zone. It is a good idea to move deep lying air stones to shallow areas or turn them off completely in order to maintain the temperature in the warm zone too.
Assess water features around the pond. Tall water features like waterfalls and fountains will expose falling water to cold ambient air temperatures which will drop the temperature of the pond quickly. Similarly, metallic lips or metallic water features will also cause rapid cooling of pond water. All of the above should either be bypassed or turned off during winter period for maximum protection of the warm zone.
Feeding
Preparation for winter starts as soon as the water temperatures start to decline. Start to wean your fish off higher protein foods onto lower protein diets that a higher in fats and carbohydrates. Wheatgerm based diets are a popular and suitable diet choice for autumn and winter. If in doubt, feed manufactures will give guidance on what water temperatures the food is suitable to feed at, so check the label if you’re unsure.
I advise slowly introducing and eventually swapping onto a wheatgerm based food around a water temperature of 15ºC. Continue to feed fish as winter progresses but reduce the quantity fed to match the appetite of the fish. This will naturally slow down with the cooling temperatures. At 10ºC (water temperature) only feed your fish if they are actively looking for food at the surface, but keep the quantity fed low. Remove any uneaten food after 3 minutes. Do not feed your fish below 7ºC, even if looking for food.
Overfeeding at cold temperatures can cause digestive and buoyancy disturbances as well as a build-up in ammonia.
You can continue to feed sturgeon throughout winter but reduce the amounts fed and try to target feed as much as possible to discourage koi from eating the pellets.
Support the Immune System
Autumn is a great time to boost the immune system of your fish in preparation for winter. Options include dosing the pond water with Vitamin C or a liquid multivitamin called ‘Sakan Ni Pro’.
Specialised, immune boosting diets such as ‘Yugen’ can also be fed during this period.
Health Checks
Pre-winter health checks are very important and will ensure your fish overwinter successfully and flourish again the following spring. Pre-winter health checks will assess water quality, parasite levels as well as the general health of the fish. Depending on the assessment, guidance will be given on relevant treatment plans and tasks for winter preparation.
Heated Ponds
If you are able to heat your pond through winter, it is important to think carefully about the temperature you set the thermostat to. I often I hear clients aiming to maintain their pond around the 12ºC mark. This is risky for a number of reasons. At 12ºC…
1) The immune system of your fish is very sluggish and not working efficiently
2) A lot of the bacteria and parasites species are still active
The result? A risky ‘limbo zone’ where your fish are slow to respond to disease but pathogens are still very much active. Not a good combination!
To avoid this, I recommend either heating your pond to 14ºC or above to keep the immune system ticking along efficiently OR allowing the pond to cool to below 10ºC and setting the thermostat to kick in at 5-7ºC. This will prevent the really cold temperatures and therefore avoid cold shock but also avoid the risky ‘limbo’ zone where bacteria and parasites are more active.
The other key consideration is consistency and stability. If the water temperatures fluctuate repetitively, you will be subjecting your fish to stress and opening the door to disease. Keep water temperatures as stable as possible. Insulating the pond will help you achieve this.
Predators
During the winter period fish enter a hibernation like state, sometimes known as torpor. During this state, the metabolism of the fish drops right down, appetite diminishes and fish become very still and slow. This makes them ideal targets for predators so take every step to make sure you are protecting your fish from local opportunists!
