Winter and autumn koi foods. Do I really have to offer my koi different foods in the winter?

I am a little confused with the number of different types of food that I can buy for my koi. This is especially true during late summer and early spring when my pond temperature is cooling down warming up either side of winter.

I recently bought a 10 kilogram pail of growth food at a koi show at a very competitive price but have soon realised that if our summer comes to an abrupt end, I may be left with a lot when my dealer recommends I should be feeding a low protein wheatgerm-based food.

Will my koi really suffer if I feed the growth food until it runs out or is it critical that I switch to a wheatgerm diet as soon as the temperature drops?

Could I not simplify things and feed them one diet all-year-round? I’d appreciate some clarification.

Each of the different foods that are available are designed to achieve different things for your koi. In the warmer summer months, when your pond temperatures are at their highest, your koi will have their greatest potential for growth. This is when, if you want your koi to grow, you should offer your koi a higher protein food, recognising that proteins provide your koi with the building blocks for growth. Typically, a summer growth diet may offer your koi anything between 35% and 45% protein, formulated from a mix of animal and vegetable sources of protein (such as fishmeal and cereals).

You might be surprised to learn that this protein content is not as high as the omnivorous diet that carp may consume naturally. Typically this is likely to be made up of a blend of invertebrates such as worms and crustacea mixed in with algae and other aquatic vegetation. So why don’t we feed our koi the same high protein diet that they would consume naturally?

There are two reasons, both of which underpin why we to offer our koi a number of different diets throughout the seasons and why a range of different diets are available for our koi at different times of the year.

Cost. Besides the very costly food additives such as Spirulina and vitamin/mineral premixes, proteins make up the costliest component of the diet. Koi are just like any other fish in that they will use the valuable protein that they digest for either growth or energy. Ideally, we want our koi to use the protein for growth (as it is the only component in the diet that provides growth). There are other components besides protein that koi can use for energy and in order to keep the cost of the diet to a minimum, diets are formulated to ‘spare’ the protein for growth incorporating other cheaper energy yielding components in the diet. Such ingredients would include sources of carbohydrates (cereals, beans and grains) and to a lesser extent lipids such as fish oil.

Water quality. If the cost of the diet was not an obstacle to us, and we could afford to feed our koi a diet that was a very similar to the protein content of a carp’s natural diet, it would still not be a prudent thing for us to do. This is because excessively high protein diets can lead to an increase in nitrogenous compounds being excreted into the water. In the wild, where carp densities are naturally very low, and water volumes much greater than those of a koi pond, this does not pose a threat to the welfare of carp in their natural environment. However, where koi are kept more intensively and in relatively small volumes of water (albeit very well circulated and filtered), levels of ammonia excretion can be significant especially if the koi do not use this protein for growth.

How does it matter whether my koi use the protein in their diet for growth or energy?

When koi use protein for growth, they digest proteins into their soluble building blocks called amino acids which are then transported to the liver where they begin to be recombined into new ‘koi’ proteins. These amino acids are made up of carbon, hydrogen, oxygen and nitrogen (CHON) atoms. However, depending on the koi’s energy demand your fish may also choose to use the same amino acids as a source of energy in stead of for growth. By doing so, they will utilise the carbon, hydrogen and oxygen (CHO) but discard and excrete the nitrogen atoms (N) as ammonia (NH3).

So the more protein we offer our koi at low temperatures, the more likely they are to burn the excess as a source of energy (instead of growth) leading to increased ammonia excretion. This has obvious implications for our cold-blooded koi when our pond temperatures start to cool.

Why it is there a need for a winter diet?

You could feed your koi a higher protein growth food at all year round without your koi experiencing any health problems as a direct result. That is – if they were not kept in an intensively stocked koi pond. However, under these artificial conditions, the ammonia production would be excessive (both direct from the koi and from the bacterial breakdown of the excreted and partially digested diet) as koi are far less able to either digest or utilise the high levels of proteins that are offered to them in the summer. Unfortunately, our cold-blooded koi are slaves to the temperature of their environment and as the water temperature drops in autumn, so too does their ability to grow, and with it goes their protein requirement.

Consequently, a winter diet will typically offer our koi about half the protein they would receive in a summer growth diet. So if you were to offer your koi the growth food that you recently bought up until you have finished the 10Kg pail, even if the water temperature has cooled to below 10 degrees C, you’re not likely to harm your koi directly, but will more likely do so indirectly through the negative effect feeding a growth diet in cool temperatures is likely to have on your water quality.

Winter Filter Implications

In summer, when filter efficiency and bacterial activity is at its highest, even an excessively high protein diet can be accommodated by your pond as your filter bacteria will soon make light work of the ammonia and subsequent nitrite. In cooler autumn and early spring conditions however, your filter bacteria are just as inactive as your koi and are less able to rise to the challenge of an increased ammonia loading. So feeding your koi a lower protein (20%) diet, will prove to be better for your filter and hence better for your fish. In winter when your pond is frozen, bacterial activity will be very low – That’s why we keep perishable items in a fridge or freezer.

By examining the reasoning and science behind the diets that you can offer your koi throughout the year, it is clear why we must feed our koi different diets. Each diet is designed to achieve different things for your koi and their environment when they are fed under different seasonal conditions.

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