Growing fish require an adequate protein supply in their feed. Among the macronutrients (protein, fat, carbohydrates), protein is of paramount importance in feeding. Besides the absolute amount of protein, the ratio between protein and energy content of the feed is also crucial for optimal fish growth. Approximately 0.4–0.5 MJ of digestible energy is required per percent crude protein [1, 2, 3, 5] . A shortfall in the required energy intake poses risks and leads to increased protein breakdown. This means that the ingested protein is used and metabolized as an energy source and is therefore no longer available for fish growth.
As a serious consequence, this poor feed conversion also leads to increased water pollution due to increased excretion of nitrogen compounds through the gills (more on this under Relationship between feed composition and fish health). A sustained excess of energy intake carries the risk of obesity and puts a strain on the koi. The following section discusses the importance of fat in relation to protein. It is possible to replace a portion of the protein content in a feed with fat. This effect is called the protein-saving effect. Some experiments have shown that, with a constant total energy content, replacing 5–12% of protein with fat is possible without any loss of growth. Therefore, it is conceivable to partially replace proteins with fats [2] . Thus, a feed does not need to have a protein content of 50% to resemble a natural diet.
From a nutritional standpoint, feed with a higher fat content and 40% protein is just as readily digestible as feed with a low fat content and a protein content exceeding 50%. One advantage of this higher fat content is the lower overall protein excretion. This reduces the burden on pond water from nutrients (algae) such as nitrite/nitrate and ammonia/ammonium. Furthermore, the total amount of feed required can be reduced. Fat is therefore of immense importance for energy efficiency. In this context, fat plays the most significant role in the protein-to-energy ratio.
To ensure good fish growth, the protein content of 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 fishmeal have very good digestibility at 95%. Plant protein sources, such as wheat protein, isolated soy protein, or soybean meal, have somewhat lower digestibility at 60–85% [3, 4] . This leads not only to increased water pollution but also to higher feed requirements. More information on protein digestibility can be found in the article Amino Acids.
Scientific sources:
[1] Schäperclaus, W. & Lukowicz, MV (1998): Textbook of Pond Management . 4th revised edition. Parey. Berlin, pp. 51–209.
[2] Otto, D. (2016): Investigations into the composition of commercially available feed for koi and other carp-like fish. Bachelor thesis. Faculty of Agricultural and Environmental Sciences, University of Rostock
[3] Spannhof, L. & Steffens, W. (1995): Introduction to Fish Physiology . Dr. Kovac̆, Hamburg.
