Koi. Feeding for growth.

I remember at school, when asked to write a story as part of an English exercise, immediately asking the teacher ‘How long should it be, Sir?’ I would be surprised if a large number of us have not been in that position, and received the predictable reply ‘It’s the quality, not the quantity of writing that matters’ In comparison, in areas of everyday life, we are met with the opposing view, where big is best. Jumbo Jets, Big Mac’s, Largest Prize Draw and ‘Biggest Ever’ chocolate bars.

Koi keepers can also arguably be divided into two similar camps.

Firstly those who seek quality in pattern, skin and colour, offering excellent potential for the future. Then there are those who are drawn to a large fish in preference to its grade. It is the potential that koi have of growing into jumbo fish that often draws people into keeping koi for the first time. It is usually the really large koi in a collection that leave a lasting impression, even when shown alongside other smaller higher-grade koi.

Koi have the potential of growing to an enormous size. Interestingly the size of koi is usually measured in terms of their length whereas anglers measure their prize carp in terms of their weight. Jumbo koi are the product of years of selective husbandry, putting large areas of water in the form of mud ponds to one side for just a handful of prized specimens. In fact, probably the greatest single factor affecting the growth of koi is stocking density, with the largest and fastest growing fish coming form large waters.

How big can koi grow?

Firstly, this depends on the precise definition of koi. The largest koi to be found in this country to date is a huge chagoi over 1 metre in length. However, there has recently been news from Australia of some truly massive coloured carp (Unselected offspring of ‘wild’ koi). In Australia carp have become a real nuisance and are regarded as vermin. They have over run vast areas of water and if a carp is caught then it must not be returned but killed! It is reported that some of these coloured carp have reached sizes up to 90lb in weight (13 times the weight of a new-born baby!) and arrangements can be made to have them imported into the UK. They have reached such massive sizes on account of a very favourable climate and the vast areas of productive water available for growth.

Not all koi will have the potential to grow to such large sizes as they will not possess the genetic make up to achieve a jumbo size, irrespective of how ideal the growing conditions. In fact there is a general rule when considering the ultimate size of koi. Higher grade, complex variety koi (Go Sanke) will not have the potential to reach the same size as the less appealing monocolour koi.

The exceptions are the jumbo show winners that show good grade, pattern, volume and size and are consequently worth considerable sums of money. These are the exception rather than the rule as Japanese breeders are still trying to achieve the 1m Go Sanke. This has already been achieved with Chagoi.

This relationship between pattern and size is due to genetic interactions which more often than not make high-grade koi and jumbo size mutually exclusive characteristics. It is for this reason that the largest koi in a collection is usually the drabbest, and is more often than not a chagoi.

How to Optimise Growth

There are three major factors that affect koi growth. These are

a. Genetic make up b. Environment (stocking density, temperature, water quality etc) c. Diet

We cannot influence the genetic make up of the koi in our collection as this is already fixed and determined, so we can only enhance growth by managing either their environment or their diet.

1. Stocking density / Pond area

Fish grow faster in lower stocking densities where they can enjoy large expanses of water. Water area is far more of an influencing factor on growth than pond volume as fish will grow into the space permitted in a pond. A 10,000 gallon pond with a larger surface area than another of the same volume will promote better growth on account of its larger surface area.

Lower stocking rates also reduce stress through less competition between koi during feeding, which in turn will also improve water quality. Studies have shown that a given pond can support a corresponding total weight or biomass of fish. For example this could be either 2 large fish, each weighing 20kg or 20 fish each weighing 2 kg. The lower the initial stocking rate, the greater the rate of growth towards the final biomass for that pond. If 20 x 2kg fish were stocked into a pond with a capacity of 40kg, then little, if any growth would be achieved.

2. Water Temperature.

Koi are cold-blooded, but are warm water fish. Studies have shown that koi grow optimally at 27 degrees C. It makes sense that the warmer a pond can be, the greater the potential for growth. High temperatures must be accompanied by a suitable daylength (minimum of 9 hours) otherwise hormonal interactions which control fish growth will not be ‘switched on’, resulting in negligible growth.

If, as is more likely to be the case, optimal water temperatures cannot be achieved, it is best to aim for as stable a temperature as possible as fish metabolism responds far better to a stable environment.

3. Water Quality

Healthy, stress-free fish are fast growing fish. Adequate solids removal and biofiltration are essential to maintain good water chemistry. One significant factor that has been shown to improve growth (as well as biofiltration) is vigorous aeration. Tests have shown that fish convert their diet into fish tissue far more efficiently when dissolved oxygen in the pond is highest.

4. Food – Quality.

This is a topic in itself, but suffice to say here that a growth diet should have protein levels of at least 40% and the protein must be of high quality and digestibility. The diet must also be balanced so that fish use the protein for growth instead of wasting it as a source of energy.

5. Food – Quantity.

If you are truly on a quest for growth, then providing your fish are backed up by suitable filtration, a suitable water temperature and a handsome budget, koi should be fed approximately 6% of their bodyweight per day. E.g. a 1kg fish should receive 60g of food each day. This is double the maintenance ration and should be reduced with temperature.

However, a common problem experienced by koi being fed too intensively in the quest for rapid growth is for koi to lose their proportion. If koi are fed too rich a diet, too intensively, they may ‘balloon’ and become pot-bellied. If this happens then the feeding rate should be reduced or switched to a slightly less rich diet.

Problems With Fast Growing Koi

Breeders can spot those fish with the genes and potential for rapid growth at the fry stage. ‘Shooters’ soon outgrow their siblings, even turning cannibalistic by as early as 3 weeks old. Quite surprisingly these stronger fish are usually culled early as experience has shown that such fish are low grade, if showing any pattern at all and if not culled will decimate pond fry stocks for the sake of a handful of large but ‘ugly’ koi.

There are rarely health problems associated with rapid growth but fast growing koi can often exhibit inferior skin quality. This is not as a direct result of rapid growth, but is as a consequence of such fish having genes for growth rather than good skin quality.

Fast growing varieties can often dominate or ‘bully’ a pond, especially at feeding time. This may reduce the growth potential of the koi collection as a whole. However, voracious feeders can also encourage the rest of the stock to become quite tame at feeding time. It is no coincidence that the tamest fish are the largest.

Problems may occur when adding a jumbo fish to an existing collection, as it may not be able to adapt to the relatively ‘crammed’ conditions of your pond. Jumbo fish have more than likely been recently harvested from a lake where a handful of fish were stocked to the acre! Be prepared for stress responses in such occasions. This potential problem is yet more incentive for going for growth in your own pond, besides which, quality jumbo koi are usually unaffordable!

As koi keepers we should become familiar with our own ponds, and know how they work, and when they have reached their stocking limit. There are general stocking rules, but each pond will be different on account of volume, area, water turnover, filtration and other factors and adjustments must be made accordingly. A simple rule when going for maximum growth is that the stocking rate can never be too low.

If growing large koi was as straightforward as following the above guidelines then they would be found more frequently in koi keepers’ collections. It is the challenge of growing koi to such a large size that makes even low grade jumbo koi so appealing. Large koi are a testimony to the breeders’ and growers’ efforts, who are not only presenting generations of work at breeding, but also several years of careful pond and fish management, nurturing the ‘select’ to such jumbo yet well proportioned individuals.

There are no short cuts to achieving these large specimens, and very rarely do we have the means of achieving such growth in the UK, having to purchase, at a price, the finished article.

Kill blanketweed and string algae.