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Chris on Koi

Pond Depth

First published in 1996 – updated 2006

This essay was written in response to enquiries that were received after publication of the article recalling my personal experiences on koi nutrition in KOISA # 75.

Society members queried the statements that were made on depth and growth. Thank you for the interest and response. In particular, members asked about the statement - "It may come as a surprise to some collectors, but I have never considered depth to be critical in achieving growth." KOISA readers have asked me to elaborate on this statement.

One of the claims made in koi literature and by some South Africans are that ponds have to be deep to achieve good growth in koi. Further claims were and are regularly still made that koi keepers cannot grow koi to a large size in a shallow pond. Please note the two words that have been highlighted.

The first two questions I began asking myself, as I made notes of my observations over the years, were -
1. do all koi grow to a large size in deep ponds? and 2. do all koi remain small in shallow ponds? The answer to both questions is clearly no. If the answer to both questions were yes, this essay would end here. We could then fall into line with so-called world opinion and live happily ever after. The next two questions I started to ask myself were (no, I do not always talk to myself, but thanks for asking) 3. how many large koi are there in South Africa and 4. how many deep ponds compared to shallow ponds are there?

No matter what we are told or have been led to believe, it is an indisputable fact that many koi keepers are growing koi to a large size in shallow ponds. When I observed this fact I tried to find the other factors that must be present that have a far greater influence on growth than pond depth.

The final straw that broke the camels back and turned me away from common consensus was that koi are not easy to view in deep ponds. It is of concern to me that people can pay a fortune for a fish and hardly ever see it or at the very least see it with difficulty. Rather like an eccentric art collector hiding beautiful works of art in a dark cellar never to be seen again.

Koi are bred specifically to be admired and appreciated. Year's ago I decided I would rather have koi that were 60cm or 70cm long in a "shallow" pond and see them each and every time I walked past, than a deep pond with possibly slightly larger koi that were hardly ever seen.

Everyone has a different perception of what a "large" koi is and what a "deep" pond is. We should therefore, define these two concepts before engaging in debate.

Although we are told koi can grow to a meter in length there very few koi of this size or even 90cm in South Africa. We truly dramatic 90cm + koi winning the Jumbo Prize on the National Koi Show. The number of 80 cm koi is increasing. There are some genuine 70 cm or longer koi, especially of high quality. Even on the SAKKS National Koi Show very few koi above 65cm are seen.

When we reach the 60cm - 65cm size bracket we find that most koi keepers would be happy with koi of this length. We also discover that no matter what we do and irrespective of pond specifications, the vast majority of koi simply cannot grow much bigger than this size. Genetics becomes the great leveller of the playing fields and the ultimate limiting factor.

As a matter of interest a summary of 150 top Japanese ponds and pictures of their favourite koi revealed an average koi length of about 65 cm. Only the very top collectors had koi over 70cm that were of high class and finish.

Pond depth also needs to be defined. A pond 2 meters deep is considered deep. A pond 3 meters deep is considered a tourist attraction. A pond 1 meter deep is considered shallow by many enthusiasts. Ponds below 50cm should not have koi in them. I agree, however, there are many koi keepers who have ponds and water features that are 50cm deep. They have koi living quite happily in them.

We can confidently define a deep pond as a pond that is 2 meters or deeper. Shallow ponds are defined as 80cm - 1.2 meters in depth. Anything in between is a medium depth pond.

One finds that statements on pond depth originate from the UK and from Japan. The main reason for deep ponds in these countries is their extremely cold and more importantly - long winters. In parts of Japan and the UK pond water temperatures frequently drop below 5*C in the long winter months. Some ponds freeze over. Not only this, they remain very cold for much longer periods than in South Africa. It is the length of time that ponds remain cold that is problematic for koi.

What is a problem with temperature, and this has been widely published in the UK, is the length of winters. In order to bring some relief to koi in long, cold winters, deep ponds are made. At 4*C a peculiar thing happens to water, an inversion takes place. The "colder" water rises and the "warmer" water sinks. This is why ice floats. The middle water is then drawn to keep the lower levels stable and undisturbed. The koi can settle in the lower regions and wait out the winter months in a state of semi-hibernation.

In South Africa we do not experience water temperatures as low or for as long as in the UK and Japan. As a matter of fact many overseas visitors have made the comment that South Africa has ideal conditions for growing koi - long hot summers and short mild winters in most parts of the country.

The other information regarding depth and growth is split into camps of different opinions. One claims that the vertical swimming provided by depth is very important for growth and body shape. Another camp claims that it is the increased water pressure of deep ponds that influences body shape during growth. And another opinion is that the air bladder needs depth - i.e. pressure. There is one further theory I have come across about depth - koi need depth because they have to stand on their tails to feed. The exponents of this latter theory claim that because koi cannot stand on their tails in shallow ponds they cannot feed properly and consequently do not grow (??!).

Depth will affect body shape to a certain degree, as the muscles exercise against the pressure. Depth will possibly strengthen the air bladder but can someone please explain the theory of vertical swimming having an influence on body shape and growth? Exactly how does swimming vertically improve body shape? Even in the east there does not seem to be consensus on depth amongst enthusiasts as Takayuki Izeki made a statement in Nichirin - " if water is deeper (2 meters) the water pressure hinders growth and a good body shape cannot be hoped for."

Another factor not mentioned with regard to pond depth is that, in some countries, it is much cheaper to dig down an extra meter to increase water capacity than it is to extend the surface area for the same purpose.

Perhaps we should reach common ground. Is water temperature essential for growth? Yes. We know koi do not grow in low water temperature. It is easy to see that there will be no growth in a deep pond with a water temperature of 5*C. However, there will be growth in a shallow pond with water temperature of 25*C. Common ground has been reached. Temperature is essential for growth.

Is oxygen essential for growth. Yes. The examples in the above paragraph apply here as well. I have come across an interesting claim that it is very difficult to grow jumbo koi on the high veldt in South Africa (1600m above sea level). If I recall correctly, this persons definition of a jumbo koi was 75cm or bigger. It was maintained that we cannot get enough oxygen into the water. About 20% less oxygen can be dissolved into water on the high veldt than at the coast. Couple this with the high water temperatures we get in our ponds and he could very well be right. (both altitude and temperature affect oxygen levels in water).

Is water quality essential for growth. Yes. If the water quality is poor your koi will probably not be growing because they are dead. Jokes aside, water quality is applicable in all circumstances.
Is food, or more specifically nutrition, essential for growth. Yes. A group of koi in a deep pond that never receive any food will not grow. Koi in a shallow pond that do receive food will grow.

Nutrition from birth is possibly the most critical factor in achieving growth other than genetics. Plus the closely interconnecting factors of oxygen, water quality and temperature. I request readers to re-read some of the statements I made in the article on koi nutrition. One of the most important was - people are underfeeding their collections on quality food, which is one of the main reasons why they do not get growth.

It is a fact that koi keepers are growing koi to 70 and 80cm, with very reasonable body shapes, in "shallow" ponds of about one meter. It is a fact that jumbo koi of above 70 - 75 cm are rare, both here and in Japan. Yes, some society members have visited very wealthy koi keepers in the east with ponds full of high quality jumbo koi, but relative to the total numbers of koi in the world and relative to what we can afford in South Africa, jumbo koi are rare. Recent personal discussions with Takeda san produced the following reply to the question - how many good quality jumbo koi are there in Japan? The reply - maybe a thousand.

This brings us to the crux of the matter - my statement - I have never considered depth to be critical in achieving growth (size). Depth may have a minor influence on body shape of some koi. Depth may have a minor influence on the rate of growth in some well fed koi. But a deep pond cannot and will not guarantee you size. In most cases depth can easily be compensated for by exercising the koi against a current to achieve the ideal body shape. While it is wonderful to have the best - i.e. a pond of 2 or 3 meters deep, is this pond depth critical to the growth of the fish? The answer is no. There are other factors that can be classified as critical to growth.

At some point we must ask ourselves - do we want to attract people to the hobby with reason and reality? Not everyone wants or can afford a tourist attraction. Not everyone wants a pond with hundreds of thousands of litres of water. We therefore, have to weigh up the pros and cons of recommendations on pond specifications from a practical point of view that will accommodate the wishes, needs and budgets of the vast majority of koi keepers.

Chris Neaves

Last Updated on Friday, 31 October 2008 11:52