Protein

Growing fish require a sufficient supply of protein in their feed. Of the macronutrients (protein, fat, carbohydrates), protein is the most important in feeding. In addition to the absolute amount of protein, the ratio between protein and energy content of the feed is also crucial for the optimal growth of a fish. For each percent of crude protein, approximately 0.4 – 0.5 MJ of digestible energy is required ^{[1, 2, 3, 5]} . Failure to meet the required energy intake poses risks and leads to increased protein breakdown. This means that the protein consumed is used as an energy source and metabolized. It is therefore no longer available for fish growth.

As a serious consequence, this poor feed utilization also leads to increased water pollution due to increased excretion of nitrogen compounds through the gills (more under Relationship between feed composition and fish health ) . If the energy intake is consistently exceeded, there is a risk of obesity and this puts the Koi under stress. The following section discusses the importance of fat in connection with protein. It is possible to replace part of the protein content in a feed with fat. This effect is called the protein-saving effect. Some experiments have shown that, while the total energy content remains constant, 5 – 12 % protein can be replaced with fat without any loss of growth. It is therefore conceivable to partially replace proteins with fats ^[2] . This means that a feed does not have to have a protein content of 50 % to be similar to natural food.

From a nutritional point of view, a feed with a high fat content and 40% protein is just as usable as a feed with a low fat content and a protein content of over 50%. One advantage of this higher fat content is the lower total protein excretion. This means that the pond water is less contaminated with nutrients (algae) such as nitrite/nitrate, ammonia/ammonium. In addition, the total amount of feed can be reduced. Fat is therefore extremely important for energy assessment. In this context, fat is the most important energy source for the protein/energy ratio.

To enable good growth of the fish, the protein content of a feed should not be less than 30% ^[4] . The quality of a protein is defined by its digestibility. This is largely determined by its origin. Animal proteins such as fish meal have a very good digestibility of 95%. Plant protein sources such as wheat protein, isolated soy protein or soy extraction meal have a slightly lower digestibility of 60 – 85% ^{[3, 4]} . This not only leads to increased water pollution, but also to a higher feed requirement. More on the subject of protein digestibility can be found in the article Amino acids.

Amino acids

Proteins in sufficient quantity and quality are of particular importance for the animal. The structure of a protein is determined by the composition of its amino acids. In animal nutrition, the amino acid profile is a crucial quality parameter.

Amino acids are divided into essential and non-essential amino acids. Koi cannot synthesize essential amino acids, so they must be ingested through food. In contrast, Koi can produce non-essential amino acids themselves from essential amino acids.

Only the content of essential amino acids can be used to assess protein quality. According to the principle of Liebig’s barrel (Liebig’s law of the minimum), a lack of intake of essential amino acids can lead to a reduction in growth ^[1] . This means that the koi can only use the individual amino acids for growth until the requirement for an essential amino acid is exceeded. It does not matter how high the amount of the other essential amino acids is, growth only takes place up to this one limited essential amino acid.

One of the essential amino acids is the sulfur-containing amino acid methionine. If the feed contains insufficient animal protein, this amino acid limits overall growth ^[1] . One of the most important tasks of methionine in protein metabolism is its involvement in muscle protein synthesis. Many commercially available koi feeds have an unfavorable amino acid profile that does not meet the needs of Koi and leads to poorer feed utilization ^[2] .

Amino acids are closely interrelated. The fish organism is capable to a certain extent of synthesizing non-essential amino acids itself and thereby substituting other amino acids. For example, cysteine ​​can be used to substitute methionine, tyrosine and phenylalanine to a certain extent ^[3] .

The need for essential amino acids in protein is determined by the fish’s own amino acid composition. The more similar the amino acid profile of the feed is, the more usable the protein appears to be for the animal ^[1] . Fish meal has a very similar amino acid profile. For this reason, a high proportion of fish meal in the feed is beneficial for koi and serves as a sign of high-quality koi feed ^[2]

Carbohydrates

Short-chain carbohydrates are rarely found in the natural diet of a carp, as the carbohydrate content in natural food is low. If animal food sources are available to the carp, crustaceans, insect larvae or mollusks serve as the main food source. Otherwise, the carp also uses available plant food sources such as algae and other plant components.

The proportion of carbohydrates in the plant and animal food sources mentioned is comparatively low. Based on this knowledge, carp are not dependent on carbohydrates and get their energy mainly from fat and proteins. This statement was proven in a study in which the carp were kept exclusively on a carbohydrate-free diet without showing any signs of deficiency ^[1] . Carbohydrates can be metabolized in the koi organism in two ways during digestion. Firstly, carbohydrates are used primarily for energy because they are broken down quickly. Secondly, if there is an excess supply of short-chain sugar-containing compounds, the energy can be stored. This happens in the form of glycogen in the liver or in the muscle tissue or in the adipocytes as fatty tissue.

If there is an excess of carbohydrates combined with too little protein, this fat storage occurs more frequently. The result is poor growth and reduced feed utilization ^[1] . For example, a low-protein but high-carbohydrate feed leads to fattening of the Koi, which cannot grow due to a lack of protein.

The carbohydrate content in feedstuffs comes mostly from grain starch (especially wheat). Grain starch is highly digestible and can be digested by carp up to 85 – 90% ^[2] . The carbohydrate content in a high-quality Koi feed should be less than 20%. From a production point of view, it is not possible to completely avoid carbohydrates in an extruded Koi feed.

Fat

In the natural occurrences of natural food, the dry matter, in addition to protein, is almost exclusively fat. In contrast to carbohydrates and proteins, fat contains a significantly higher energy density.

As a result, it plays a crucial role in the metabolism of a koi. Fats not only serve as an energy supplier in the organism, they are also involved in the construction of cell walls and serve as a carrier substance for fat-soluble vitamins. Since the koi gets most of its energy from fat molecules, an increased fat supply can lead to better protein utilization. Proteins are thus protected and are not broken down to produce energy ^[1] .

Some experiments have shown that, with a constant total energy content, it is possible to replace 5 – 12% protein with fat without loss of growth. It is therefore conceivable to partially replace proteins with fats ^[2] . This effect is called the protein-saving effect. This means that a feed does not have to contain more than 50% protein to be similar to natural food. In nutritional terms, a feed with a high fat content and 40% protein is similarly usable as a feed with a low fat content and a high protein content of more than 50%. One advantage of this higher fat content is the lower total protein excretion. In addition, the total amount of feed can be reduced.

Consequently, fat is given immense importance in energy assessment. In this context, fat is the most important energy source for the protein/energy ratio. High-quality feeds use polyunsaturated fatty acids, mainly from marine animals (fish oil), and only small amounts from plant sources.

Feedstuffs that have a high content of unsaturated fatty acids (such as fish oil) become rancid after a certain time (oxidative processes). To prevent this reaction, antioxidants (e.g. BHT) are added to the feedstuff to prevent the progressive oxidation process. At 88 – 95%, fish and vegetable oils are highly digestible and represent a good source of energy for feeding ^[2] .

Crude fiber

Crude fiber is not very important in feeding Koi. From an animal nutrition perspective, crude fiber components take on the function of dietary fiber. In general, crude fiber is the structural substance of plants and insects that gives them structure and shape. Crude fiber is mainly made up of cellulose, hemicellulose, lignin, pectin and chitin.

The use of components rich in crude fiber is kept to a minimum, as Koi carp are unable to digest these structures. Crude fiber only promotes the passage of chyme (the movement of food pulp in the intestinal tract), but can hardly be used for energy supply. The percentage of crude fiber required for this is too low. The bacterial breakdown in the digestive tract is also considered to be marginal and does not show any significant improvement in digestibility ^[1] . A high crude fiber content is also disadvantageous because the crude fiber content significantly reduces the digestibility of nutrients.

Some studies have shown that nutrient digestibility decreases with increasing cellulose content. An increase in the crude fiber content significantly reduces the percentage of protein and fat utilization. Based on this knowledge, the crude fiber content of a mixed feed should be less than 5% ^[2] . Studies have shown that the crude fiber content in many commercially available koi feeds is higher than stated ^[3] .

Minerals and vitamins

Vitamins include organic substances that are necessary for the metabolism of organisms. Fish can synthesize some substances themselves, but others must be taken in externally. The feed industry adds vitamin and mineral mixtures to complete feed that are based on the needs of carp and are added by most manufacturers in almost identical concentrations.

Fish are able to absorb a wide variety of minerals through their skin and gills. Minerals such as sodium chloride (NaCl), potassium (K), iron (Fe), zinc (Zn), magnesium (Mg), calcium (Ca), copper (Cu) and iodine (I) are macro and trace elements.

A substance that is often used by feed manufacturers is montmorillonite. Montmorillonite is a clay mineral that is supposed to imitate the minerals found in Japanese mud ponds. It is hoped that this clay mineral will produce healthier fish with more expressive colors. Little is known about how it works.

The two most important theories speak of an improved supply of minerals and the elimination of toxins, which relieves the strain on the organism. This allows the fish to invest more energy in developing its color. There are currently several studies on this, but they do not yet show any reliable results.