Mutual influence of protein and lipid feed content on European catfish (Silurus glanis) growth

We wished to determine protein and lipid content in pelleted raw fish food, necessary for optimal growth of European catfish (Silurus glanis). Experiments were set up in 20 cages, each holding 30 young catfish. Fishes in each cage received a different food combination over a 98‐day period at favourable physical and chemical water conditions. Food protein content varied between 37.5 and 45%, while lipid content, added in the form of soybean oil, varied between 3 and 11%. The oil contained an adequate ω‐fatty acids concentration, necessary for fish growth. The main growth indicators determined at the end of experiment were total body gain, specific growth rate and feed conversion ratio. There was a high statistical difference among the experimental groups receiving variable food combinations for each growth parameter (P < 0.001). Both protein and lipid food content affected growth parameters, but in a different manner. Further analysis – percentage of change depending on lipid to protein ratio and bivariate surface analysis – allowed us to recognize the most economical combination: 39.5% protein + 9% lipid content. The addition of 9% soybean oil to the fish food reduces the necessary protein concentration by 5.5%, with resulting identical catfish growth effects.