Protein evaluation in fish: African catfish as a case study

Résumé

Protein is an expensive component of fish feed and also an important macronutrient for growth and development. Therefore, it is important to accurately determine the protein requirements for each species and life stage cultured. Historically, the supply of optimum dietary protein requires knowledge on 1) the protein requirements of the fish 2) the protein content of feedstuffs. In recent times, this process has shifted to quantifying the amino acid (AA) requirement of fish and the AA content of feedstuffs, respectively. This is because fish, like other animals, do not have a true protein requirement but require a well-balanced mixture of essential and non-essential amino acids. Consequently, the quality of protein depends on its AA content and digestibility. Currently, the protein evaluation in fish feeds is mostly based on the concentration and digestibility of protein in the selected ingredients. This may not be the accurate representation of fish needs due to the variability in the digestibility of crude protein and that of individual amino acids. Dietary protein quality may vary when different ingredients are used in the feed formulation as both the AA composition and digestibility may vary among different ingredients used for feed formulation. Compared to other farm animals, fish diets are rarely evaluated using digestible AA data. The latter method of evaluation has been adopted in pigs and poultry for decades but is yet to be fully explored in fish. Therefore, the aim of this thesis is to assess the digestibility of the amino acids in ingredients in order to improve protein evaluation of fish feeds. Furthermore, the digestible AA requirement and the factors that affect optimum AA utilization in fish is explored. However, the optimization of feed formulation does only depend on the accurate characterisation of feed but also on its utilization efficiency. This implies that despite supplying a balanced AA profile, which meets the daily requirement of fish, the release of nutrients and absorption might be affected by some intrinsic factors. For instance, crystalline AA that is usually supplemented to the diet in order to overcome the AA deficiency problem caused by low quality (e.g., plant) ingredients are said to be less utilised in fish, compared to AA in intact protein. Apparently, the former, which are already in the free form are quickly catabolised and lost rather than used for protein synthesis leading to asynchronous absorption of dietary AA. The concept of ‘nutrient synchronization’ has been applied in other farms animals (e.g., pigs and poultry) to improve protein utilization efficiency but yet to be explored in the field of fish nutrition. Therefore, this thesis further explored some factors that are affecting the optimal utilization of available AA to check whether nutrient asynchrony does occur in fish. African catfish was used as a case study. The main conclusions are; Within ingredients, the digestibility coefficients are not equal for the different amino acids. This variability in digestibility between amino acids is larger in low-quality ingredients compared to high-quality ingredients when using crude protein digestibility as a quality indicator. Based on the linear plateau model, the digestible methionine requirement of juvenile African catfish ranges between 18.7 and 21.4 g/kg per unit of digestible protein, depending on the response criterion. The choice of the mathematical model used in studies to estimate nutrient requirement can impact requirement estimates, for instance, quadratic regression can lead to an overestimation of nutrient requirement. In African catfish, the protein utilization efficiency of digested protein is not influenced by feeding frequency. Ingredient macronutrient composition can alter the kinetics of nutrient digestion. In this thesis, no proof was found for the existence of nutrient asynchrony in African catfish.