Is my pond overstocked?

I have a 6 x 4 x 3ft pond (approx. 500 gallons) and it holds about 19 fish including koi, rudd, orfe, tench, goldfish and one sturgeon. Do you think I have overstocked and if so what size pond would you recommend? Can you also advise me on any precautions I should take with my pond and filter when the weather warms up?

The point that you raise in your letter is quite unique in that you’re seeking advice for a problem that you suspect may occur in the future. It is more usual for a reader to ask for advice on a given set of symptoms or observations. All that I can guess is that you may have had a visit from another pond keeper who, upon looking at your pond’s stocking density may have commented as to it’s apparent overstocked state.

From your description of the pond’s volume, and the number and diversity of fish kept within it, your pond does sound to be heavily stocked. You have not given the sizes of your fish, but I can assume that they will be small and have been in your pond for some time. Furthermore, as you have not described any specific health or water quality issues, I also assume that your pond and filter are coping with your current stocking rate.

What is overstocked? What do we mean by the term overstocked? It is a comparative term used to describe the density of fish in a pond relative to a naturally balanced pond. But compared to the naturally low and self-sustainable stocking density chosen by Mother Nature in lakes and ponds, even two fish in a 500 gallon pond could be argued as being overstocked. At the other extreme, you cannot help but be amazed by some of the pictures from Niigata of small koi that appear to be quite literally crammed into holding ponds, sardine style, prior to sale. Anyone who has seen (and heard!) these stocking rates can soon appreciate that if Japanese koi professionals, whose livelihoods literally depend on the health of their captive, harvested koi, soon realises that there is more to overstocking than meets the eye.

As soon as stocking rates exceed what is natural for a water body, we have to intervene and support fish through the addition of food, and filtration. As long as neither of these two factors become limiting, to the detriment of our fish or their environment, then it could be argued that the pond is not overstocked. This is certainly the philosophy adopted by it our hobby’s professional koi farmers in Japan. They are able (and willing) to keep their stock at incredibly high ‘koi pizza’ stocking rates, by keeping their environment balance through a high turnover and filtration rate. Perhaps when we experience these awesome stocking rates, we may overlook the belt-and-braces filtration that is working behind the scenes to maintain a stable and balanced environment. After all, the farmer can’t expect to sell stressed or unhealthy stock, and in some way, uses this crammed method of presentation as a selling aid to demonstrate his stock’s health and vitality.

So if the Japanese koi farmers can keep thousands of fish in every 500 gallons of their pond water, why might we as pond keepers in the UK regard 19 fish in 500 gallons a as overstocked?

Well, there’s more to this than may be obvious at first. Firstly, the Koi farmer who stocks is koi so intensively is doing so only on a relatively short term basis. He knows from experience, that his stock will remain healthy in this set-up for a short term, while he overwinters his stock indoors and exhibits them for sale. He knows that once the winter is over, he will either have sold out or stocked the better fish out into mud ponds at a far lower, near-natural stocking rate.

I regard the term ‘overstocking’ to describe a stocking rate that puts the fish’s health or quality of environment at risk. It looks at the long-term implications that a specific stocking rate may have on growth rates and the risks or consequences of cross-infection with parasites and other pathogenic organisms. Should any external factors in an overstocked pond change for the worse, then by definition, in an overstocked pond this is far more likely to result in catastrophe.

Technically, as every filtered koi pond has an unnaturally high stocking rate, it is vulnerable to a pump or filter malfunction. The heavier the stocking rate, the less margin for error and the greater the risk of experiencing a fish health problem. Going back to your pond’s stocking rate, should your pump fail then you will have less time to cushion your response than if you had say only nine fish in your pond (and yet each situation is still technically overstocked).

The higher up the overstocking spectrum your pond is, the more likely you are to experience difficulties if things do go wrong within your pond. Rightly so, you are concerned about your pond’s abilities to sustain your stocking rate in warm weather. Every pond has a carrying capacity, which is its ability to support the demands put upon it by the living organisms that thrive within it. One of the limitations that we cannot affect is our pond water’s ability to hold oxygen at different temperatures. We can aerate more, but ultimately we are constrained by how much oxygen our water will retain. So as the water temperature rises, and its ability to hold oxygen decreases, the oxygen demands of all living organisms in your pond (fish, bacteria, plants etc) also increase, putting your whole pond’s stability at risk especially at higher stocking rates.

Add to the above scenario the fact that these higher water temperatures will cause your fish to consume more food, and your filter will as a consequence have increased loads of ammonia, nitrite and associated organic waste to contend with, putting even further pressures on your small yet heavily stocked pond.

It is encouraging to see that you are obviously contemplating a larger pond; but what size should you go for? One of the most common comments made by first time pond keepers is that they wish they’d bought a bigger pond first time round. So when reply to your question, I say go for as large a pond as you can afford or find space for in your garden.

Pond size – why big is best.

A larger body of water is more stable (and forgiving) especially in the first months of a pond’s life. Larger ponds offer you (and your fish) the added benefits of increased water stability (the solution to pollution is dilution). Their large volume also smoothes out extremities in changes in water temperature on both a daily and seasonal basis. This also has a knock-on benefit for us as the pond keeper because as a result, larger ponds are less likely to suffer from nuisance algae problems.

Larger ponds also tend to be easier to maintain, as the pond environment as a whole is more stable, requiring our restorative action far less. Finally, larger ponds mean either more fish or larger fish. Take another leaf out of the Japanese koi farmer’s book – if you want large fish, you have to give them the water quality and volume to grow into; something that is best achieved with a larger pond.

Pond Polyculture.

Did you know that you are practising pond polyculture in your 500 gallon garden pond? This may well have contributed to your success with such a heavily stocked pond. Polyculture is used by fish farmers across the world, and exploits the fact that different fish species inhabit different niches within the same pond. For instance, your top water layer of the pond will be inhabited by orfe and rudd, mid water by koi and goldfish and bottom water inhabited by tench and sturgeon. As these fish inhabit different levels in a pond, the degree of competitive stress between fish is likely to be less than if they were all the same species, living and competing in the same zone of water. By exploiting polyculture, fish farmers are able to harvest a greater bodyweight of stock from a pond as bottom and surface feeders would rarely interact allowing them to exploit the productivity of different niches within a single pond.

In conclusion, from experience, your pond does sound as though it is very heavily stocked and you have been perhaps unknowingly walking the tightrope between success and failure. So fine are the margins for error with such high stocking rates. However, to a Japanese koi farmer, your pond would be relatively under stocked. Nevertheless, I would encourage you to increase the size of your pond so that you and your fish can benefit from all of the advantages of keeping fish in a large pond. Make sure that if you do move up and size, that you increase your filtration accordingly, and take care when running in a new filter over the first three months. Once mature, your pond will provide your fish with increased space, and hopefully, if they haven’t already been stunted, they may respond by putting on additional growth.