Bringing fish and pond out of winter into spring

Over the winter I have let my fish hibernate away. I decided not to keep my pond heated but did keep it covered through the coldest months. I stopped feeding them but checked them every day and made sure they were okay.

The weather will start warming up soon and I wanted some advice on things to do to prepare the fish coming out of their winter sleep. Will I need to examine them or administer any sorts of treatments? Or is just best to leave them alone?

I know the warmer weather may bring on parasites or any underlying health problems that have lain dormant in the winter. I wanted to be one step ahead and try to prevent any problems before they occur. What would you suggest?

Thank you for your letter. It is interesting to here that having weighed up the pros and cons of heating your pond, you opted not to heat it. Your choice is actually contrary to the trend of an increasing number of koi keepers who are opting to heat their pond over winter. You also took action to protect your fish over the coldest months by covering you pond, I assume, with a frame cover in bubble-wrap.

Quite naturally, your fish will have stopped feeding as the water temperature dropped below 8oC, and will have quite easily lived off the energy they would have stored in the preceding warmer months. Even though the level of pond activity would have been relatively uneventful during these colder winter months, there is still the likelihood of a number of things deteriorating through the winter, or set to deteriorate as the weather starts to warm this spring. You're quite right to take a cautious view over this coming spring period as there is the potential for a number of koi health issues to ensue. But with a little preparation and foresight as to what could happen, you should be able to prevent any significant problems from occurring.

I am a great believer in adopting a hands-off approach overwinter just as you have, subjecting your koi to a natural (but protected) winter period. Evidence and experience shows that koi benefit from a cold period in their seasonal cycle and their growth rates in subsequent warm periods are likely to be greater as a result. A cold period also resets their biological clock leading to a more predictable and larger spawn, should they be sexually mature.

A fallow period also presents a pond (and it's keeper) with a rest period, where quite naturally, the pace of life decreases as metabolic rates in all aquatic organisms have no option but to tail off from the heights of summer.

Just as our koi feed less (because of their reduced energy requirements), so do the bacteria that break down and process their waste. Likewise, the rates of metabolism and division in pathogenic organisms such as bacteria and parasites will also decline to negligible levels, reducing the disease risk to our pond fish over winter. In the UK, it could be argued that we are keeping koi at their most northern extremity and as a consequence, may in all probability subject them to prolonged exposure to sub-zero temperatures. I agree wholeheartedly with your added precautions during these periods, and know of some koi keepers who turn on their heating either side of a naturally cold winter to shield their koi from having to experience such lengthy periods of harsh cold weather.

Naturally, those that heat their ponds will also in all likelihood have a turnover of water through their filter. In your case, you do not say whether you have maintained a throughput of water through your bio-chamber. If not, this is one of the major pitfalls you need to avoid when spring arrives, as a result of the increased feeding and ammonia/waste production.

If you have kept your filter running over winter: Even though biofilter activity levels cease around 5OC, a continual supply of pond water through your chamber will help retain the maturity that has built up and benefited your pond over the previous season. The density of the bacterial population will decline (due to the drop in no avail food and water temperature) but you will retain a residual population whose richness is in their diversity.

That is, the bio-filter will contain members of the widest range of bacteria that will immediately process a diverse range of inorganic and organic wastes as water temperatures increase in spring. You will not have to wait for different bacteria to colonise naturally (and slowly) as you would in a new pond.

The greatest liability for those who do opt to keep a filter online over winter is the potential chilling effect that the air-chilled water flowing through the filter and pipe work may have on the overall temperature of the pond. The lagging of exposed pipes and the avoidance of any air contact with moving water can keep their chilling effect to a minimum.

If you have not kept your filter running over winter: Even though you will not have experienced the chilling effects (or running costs) of those that may have opted to keep their filters running through winter, you will still be at a great disadvantage come springtime, when you and your fish start to demand a higher degree of filter performance once feeding starts. A filter that has been allowed to dry out over winter will have lost it's diverse bacterial population as well as the associated comprehensive waste processing capabilities. This could well lead to what is widely regarded as the largest obstacle to successful pond keeping - New Filter Syndrome.

This is where the rate at which waste produced by your fish (and in some instances other bacteria) exceeds the capabilities of the biofilter to break it down, leading to an accumulation of toxicity in the pond water with the associated fish health problems. We are limited by the rate at which different and successive populations of bacteria will colonise a new filter until between them, the whole team of bacteria exploiting their own and different niches will cope with the diversity of wastes that is thrown at them.

If you fear you might find yourself in this position this spring, there are several things you can do to avoid subjecting your koi to self-made health problems.

Test for ammonia and nitrite regularly once feeding starts. You will usually find nitrite to be the persistent offender. If it proves too stubborn for your bacteria to break down (the specific nitrite-processing bacteria may not even have taken up residence in your filter) then you can dilute the pollutant by carrying out several partial water changes.
Feed a low protein diet sparingly. As most ammonia originates from the protein element of a diet, by offering a low protein diet, you will limit the amount of ammonia (and nitrite) that will enter your pond. You should not be too concerned for your fish's welfare by feeding them a low protein diet as at these low temperatures, it is all that their metabolism requires. Continue to offer your fish a low protein diet until you are confident that your tests have shown that your filter has cycled and processed the nitrite and will be able to handle increasing levels of waste.

Things to check once your fish have started to feed in spring.

Fish behaviour. One of the first signs that a fish may be showing signs of ill-health or disease is a change in behaviour. Keep a lookout for individual fish that swim separately from the others. This may mean the odd fish staying towards the pond bottom, or hanging near the surface. Look out for those that might hang at a strange angle (either head down or up by a few degrees). Also, make sure that all of your fish move in response to the stimulus of your shadow or silhouette rather than remaining motionless. If there is a fish that shows signs of atypical behaviour then net it and take a closer look.
Close inspection of all fish. Many koi keepers choose to net and bowl all of their fish in turn at springtime to give them a close inspection. One of the classic complaints that can often go overlooked until it has progressed too far are sores and ulcers that have developed unseen on the ventral (underside) surface of a koi's body. Koi may spend the majority of winter resting on a pond bottom, and the constant contact with the bottom in addition to a gentle rocking motion can soon lead to the abrasion of body defences and the onset of a pernicious bacterial infection. I have seen many apparently healthy and intact koi that were even voracious feeders, die without any apparent reason or warning, where upon closer inspection, they had developed a well-hidden but fatal ulcer on their ventral surface (barely).

Other features to look at when bowling and netting your fish is the condition of the skin and fins. Check that they are all intact and splayed/erect. This is also a good opportunity to take a mucus scrape from your fish to assess the level of parasitic infection. Be careful not to over-react with anti-parasite treatments in response to the slightest appearance of a parasite (eg trichodina, gyrodactylus). I would be more inclined to take a parasite count, record it , and reassess in a few days' time. This will give you a good indication as to whether the parasites are increasing or decreasing on the fish and will prevent you from treating unnecessarily. Depending on what you find, treat (or do not treat) accordingly.

In summary, springtime can be a risky period for your koi. The degree to which they are at risk will be a factor of how you have overwintered your pond, and how you choose to bring your koi out of their winter period. The three factors you need to keep a lookout for are:

Changes in behaviour in any of your fish
Hidden sores or ulcers and other tissue damage
Deterioration in water quality as a result of new filter syndrome.

With a little cautious pond keeping over the first month or so of fish activity, you should be able to wean your fish and pond of their winter habits, introducing them to the joys and prospects of spring and summer.

to the joys and prospects of spring and summer.

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