by Dr David Pool, Founder of FishScience, UK

Wheat germ-rich foods have always been popular amongst koi enthusiasts as the winter months approach. Japanese koi keepers have used these foods for generations and hobbyists across the world have followed suit.

In many cases, the reason hobbyists transition to a ‘winter diet’ goes unquestioned as the results speak for themselves. However, there is a rich science behind the importance of ‘wheat germ’ foods as temperatures begin to cool.

What is wheat germ?

Wheat germ is the name given to a small part of the wheat seed. It is the part of the seed that germinates (hence the name) and allows the wheat plant to start growing. The wheat germ is surrounded by the rest of the seed, which provides the initial food source for the growing seedling. To use this food source, the wheat germ has to be rich in certain vitamins, minerals and essential fatty acids and proteins.

The benefits of wheat germ to koi

The rich and concentrated nutrients contained within wheat germ have many benefits when consumed by koi. For example, wheat germ is rich in:

Vitamin E – This vitamin is important for protecting cell membranes, allowing the effective use of fats and improving the overall health and vigour of Koi. It is also important for the development of the reproductive organs and therefore benefits the fertility of the fish.

Vitamin C – Essential for the efficient use of other nutrients and skeletal formation. In combination with vitamin E, it helps koi overcome problems associated with stress.

Fatty acids – The important fatty acids contained within wheat germ are used to form the cell walls, improve the digestion of other nutrients and as a storable energy source. Wheat germ is particularly rich in linoleic acid, which is essential for tissue repair.

Amino acids – Wheat germ contains Lysine, an important component of the diet to allow tissue repair and formation as well as aiding the immune system.

Wheat germ, as an ingredient in a balanced nutritional food, has important benefits to people as well as koi. Health food stores have recognized these benefits to humans and sell large amounts of wheat germ in a variety of different forms.

Koi foods vary considerably in the amount of wheat germ that they contain. It is an expensive component of the food – after all, only 5 percent of the wheat seed is the germ so the rest must be discarded. As a result, some of the less expensive ‘wheat germ’ foods contain less than 2 or 3 percent wheat germ, whereas high-quality wheat germ foods may contain six times this quantity. European pet food regulations help us to understand the ingredients of a fish food from its description on the label.

Winter feeding for koi

The nutritional requirements of koi vary considerably throughout the year, hence the need for different diets. For example, protein is necessary for growth, but can only be used at warm water temperatures (over 15°C). Therefore, in summer, we feed a high-protein food because the koi can use the protein to grow. Whereas, in winter, a lower protein food is sufficient because the koi are less active and not increasing in size.

Feeding protein-rich foods at low temperatures can even harm koi. Below approximately 10°C few if any of the proteins will be used for growth. Some may be used inefficiently to produce energy for movement etc, but the majority will pass straight through the fish. The resultant protein-rich waste can have adverse effects on the water quality as it decomposes.

In addition, the digestion of proteins into amino acids starts at the front of the intestine where the enzyme Pepsin gets to work. Pepsin works best in acidic conditions where the pH is 1.5 to 2. If a koi consumes a diet rich in protein and then the water temperature drops, the acidic stomach conditions, together with raised levels of Pepsin, can remain in the early intestine for a prolonged period. This can expose the koi to the risk of stomach ulceration and future infection.

To avoid these two problems, many koi keepers simply stop feeding their fish when the water temperature falls below 10°C. The theory is that the fish can obtain what little nutrition they require from within the pond and from their stored food reserves. Whilst this may be a possibility in a natural, planted pond, there is unlikely to be sufficient food in a koi pond. The fish would therefore have to rely on stored reserves in the form of fat and muscle tissue. Not feeding over a 3 – 4 month winter period would lead to the koi being thinner and weaker in the spring, and consequently more susceptible to disease attack later in the year.

Do koi feed at low temperatures?

The behaviour of koi is greatly affected by the temperature of the water they are swimming in. The colder the water, the less active they are and the less inclined they will be to feed. However, they can acclimate to low temperatures. Even at temperatures of 5 – 6°C, koi can be seen to rise to the water surface to feed. However, their activities are greatly influenced by changes in the water temperature. If the water temperature quickly falls from 12°C to 10°C, the koi will stop feeding for a while whilst their bodies get used to the lower temperature. By contrast, if the water temperature rises from 5°C to 7°C the koi may feed quite actively.

In the winter, and particularly in shallower ponds, the temperature changes described can occur every day, with an increase during the day and a fall at night. It makes sense, therefore, not to feed your koi too late in the day as it is likely that the water temperature will start to fall before the fish have had a chance to digest the food.

Good quality wheat germ foods are formed largely from plant ingredients and are characterised by a higher roughage content. The digestible component of the food is quickly absorbed by the fish, but importantly, the remaining ingredients pass more quickly through the intestine. The higher plant content will also reduce the amount of Pepsin produced in the front of the intestine and result in less acidic conditions.

Feeding such a diet to your koi throughout the winter will support the condition of the fish but will have no impact on their growth. The carbohydrates contained within the food are an important energy source for the fish and prevent them from having to use their reserves. As for the wheat germ – it will allow better utilisation of the other ingredients in the food as well as being a valuable food source in itself.

Benefits of winter-feeding on spring health

Providing koi with suitable food during the winter will have a beneficial effect on the health of the fish in the spring. If the fish have not had to use up their stored food reserves, they will be in better physical condition. However, there is also evidence that the amino acids in wheat germ can strengthen the immune system of the fish and ensure they can counter disease organisms which can cause problems as the water temperatures rise in spring.

In extreme cases, not feeding koi through a long winter can lead to vitamin and mineral deficiencies. This can manifest itself in many ways, including lethargy, lack of condition, susceptibility to disease etc. Providing suitable food in small quantities can help combat these issues.

The important aspect of winter feeding is that you should feed very sparingly. As much as the fish will eat in 2 – 3 minutes is sufficient. The keeper must remember that they are not feeding to encourage growth or enhance colouration, simply to maintain the health of the koi.

Floating vs sinking food

There is much debate over the use of sinking or floating wheat germ foods during the winter. Original wheat germ foods were mostly sinking, with the idea being that the fish were lethargic and would not be interested in rising to the water surface to feed. Although feeding a sinking food will get to the bottom of the pond and the fish will feed on it, any uneaten food is very difficult to remove without disturbing the fish. Keepers should avoid any disturbances to their koi during the winter period. Although there are two very plausible arguments, FishScience was developed to provide floating pellets. If the fish are not active enough to rise to the water surface to feed, it is a good indication that they should not be fed. In addition, removing any uneaten food is much easier and less intrusive when it is at the surface.

Information for keepers

Koi should be fed during the winter. It is an important aspect of koi care and can help to keep them healthy and allow them to overcome the dangers of spring. The food should be vegetable based with a good wheat germ content. This will ensure the fish can utilise it as effectively as possible. As with all feeding, keepers mustn’t overfeed – a few sticks or pellets per fish is sufficient.

‘With Wheat germ’ means there is at least 4 percent wheat germ in the food ‘Rich (or High) in Wheat germ’ means there is at least 14 percent wheat germ in the food ‘Wheat Germ food’ means at least 26 percent wheat germ in the food.