Growing big koi pond fish

Living in a world where biggest is best and quantity dominates quality, it stands to reason that we have the desire to see our koi grow to their full potential, and a sizeable potential at that.

Koi can grow to an enormous size, with several varieties now exceeding the previously mythical 1 metre mark and ‘wild’ coloured carp reputedly reaching 90lbs in weight in Australia. It is human nature for our eyes to be drawn to the largest koi in a pond, irrespective of their grade or quality and having seen large koi at shows or in other ponds, we are keen for our own koi to also achieve their full growth potential.

There are 3 factors that interact to control a koi’s growth rate and ultimate size of our koi. These are:

The koi’s genetic make-up

The koi’s growing environment (water temperature, water quality and stocking density)

The koi’s diet.

We cannot influence the koi’s genetic make-up, but can work with its blueprint for growth. The genetic coding for the particular growth characteristics of each koi will interact with their environment and diet to determine the size and growth rate of each koi.

Although koi are cold-blooded, they are warm water fish, growing optimally in a stable 27degrees C with excellent water quality and a low stocking density. Assuming that koi are provided with an environment that will allow them to grow, then a koi’s diet is the final piece of the growth jigsaw.

Growth Diets

Protein is found at the heart of any diet. It is the only component of a diet that koi can use for growth, but must also be part of a complete and balanced diet also containing adequate oils, carbohydrates, vitamins and minerals. Protein levels in a diet are the first thing a koi keeper will scrutinise, but the absolute protein content of a koi food is not the only factor that will impact on koi growth rates.

Protein Quantity Naturally, carp will feed on a diet that has much higher protein levels than those offered in an artificial koi food. Preferring to feed on worms, insects and other invertebrate life, carp will often consume a diet of 50-60% protein whereas most koi diets will provide protein levels between 30% and 40% – So why the difference?

Research has shown that fish can choose to use the food they consume in different ways, particularly protein.

Fish are just like any other animal, in that they will eat to satisfy their energy requirements. For koi, this will enable them to swim, feed, breath and maintain basic physiological functions. They can obtain this energy through their intake of oils, carbohydrates and proteins, using proteins as their preferred energy source. Consequently, ‘wild’ carp naturally consume a high protein diet that satisfies their energy demand, with any excess being used for growth.

So why not offer koi a similar 50-60% protein in a koi food?

To offer koi an ultra high protein diet has several disadvantages.

Proteins are complex compounds made up from carbon, hydrogen, oxygen and nitrogen molecules. When koi opt to ‘burn’ protein as a source of energy (rather than utilising it for growth), they burn the carbon, hydrogen and oxygen but excrete the nitrogen – in the form of ammonia. Therefore, the more koi utilise protein for energy, the greater their ammonia output. In a ‘natural’ environment this does not pose a problem for carp, but where koi are stocked intensively, this would lead to water quality problems.

As protein, particularly high quality protein, such as fish meal, poultry meal and wheatgerm is in high demand across the world, it commands a high price. Supplying a high protein (50%+) diet for koi would cause a prohibitive increase in the cost of diets and be a needless extravagance as a koi’s ability to utilise protein efficiently tails off as protein levels in the diet increase.

Replacing valuable protein as a source of energy.

Research has shown that by replacing a proportion of the valuable protein in the diet with alternative sources of energy (oils and carbohydrates) that koi will utilise those for energy, reserving the premium protein for growth. This practise is called ‘protein sparing’ and is used in many aquaculture diets to ensure that cost-effective growth is achieved.

The protein-energy balance

Focussing solely on the protein content of a diet does not give us an accurate picture as to how koi are likely to use it. Protein in the diet must be balanced with a diet’s energy content to ensure it is used for growth.

1. Protein and Energy Relationship

If the diet contains too much energy in relation to protein then koi will store excess energy as fat.

2. Energy and Protein Relationship

If the diet contains too much protein in relation to energy then koi will be forced to use protein as an energy source. This will adversely affect growth rates in koi.

3. Correct Protein Balance To Energy

The ideal balance. When protein levels are in balance with the energy content of a diet, koi will not store excess energy and there is sufficient protein for growth.

Protein Quality

Having determined that putting excess protein in the diet should be avoided in koi diets, we must ensure that the protein in the diet is of sufficient quality to allow tissue growth.

We should not simply be satisfied by the protein level in a koi food as the quality of the protein will also affect growth. For example, imagine two different diets (Diet A & Diet B), each offering 40% protein – Apparently there is nothing to choose between them. However, Diet A uses feathermeal as its protein source and Diet B uses white fishmeal as its protein source. The protein levels of either diet are identical, but Diet B will offer your koi the best growth potential as the protein is of a high quality. This is an extreme example, but illustrates the point well, that the sources of protein should be investigated rather than simply the protein level when comparing different koi foods.

When a food is described as containing ‘highly digestible protein’ it relates to the fact that koi can break it down and utilise the amino acids. I.e. Diet B contains highly digestible protein whereas Diet A does not, making the protein in Diet A unavailable for koi to use in growth.

Proteins are made up of soluble building blocks called amino acids. When a protein is digested (not that straightforward for a stomachless koi) it is broken down into amino acids which are absorbed into the blood and taken to areas of tissue growth. Koi require 10 of the 24 amino acids to be provided in their diet (they can manufacture the other 14 themselves).

hese 10 essential amino acids must be provided in the diet or koi health (never mind growth) will be impaired. Animal sources of protein (fishmeal, and poultry meal) offer the complete range of essential amino acids whereas vegetable sources may only provide the majority of the essential amino acids. Therefore, to allow growth, a diet should provide between 30-40% of protein, the majority of which is animal in origin.

How much food?

Having identified that a growth food should contain a good percentage of highly digestible animal and vegetable protein that is balanced with the food’s overall energy content (so that koi use the protein for growth rather than energy) then we need to identify how much food we should feed. This is governed by temperature and food quality but speaking generally, research data shows that a maintenance ration equates to feeding approximately 3% of a koi’s body weight each day. (When did you last weigh your koi?!)

For a 1 Kg fish, 30g of food a day is required to maintain its health and current bodyweight. Data suggests that an optimal ration for growth is 6% BW per day with ratios that exceed 3% BW likely to be sufficient for growth at cool UK temperatures. Beyond 6% BW, the more food offered, the less efficiently it is digested and utilised for growth, leading to greater excretion.

Furthermore, there is evidence that koi utilise food more efficiently if it is offered in small amounts in regular intervals rather than in a single large feed. This relates back to a koi’s digestive physiology which, being without a stomach, handles smaller feeds more effectively.

In summary, the growth potential of every koi is locked up in its genes and we can unlock that potential through feeding a suitable diet. Growth foods focus on the quality and quantity of protein, its digestibility and its relationship with other ingredients in the diet. Further growth improvements can be achieved by feeding the correct quantity of food in a manner that koi can utilise the protein most efficiently for growth.

Kill blanketweed and string algae.