Aquaculture production has risen steadily over the past 40 years, surpassing capture fisheries around 2022 (FAO, 2024). Sustaining this growth requires more aquafeed and improved, sustainable formulations. Due to rising costs and limited supplies of fish meal and fish oil, feeds are increasingly shifting to plant-based and novel ingredients—such as insect meals, single-cell proteins, and industrial by-products—supporting more circular and sustainable production, to keep up with the demand. Additionally, this increases the variability in ingredient and macronutrient composition of new aquafeeds. Aquaculture is characterized by a high species diversity with specific nutritional requirements, thereby highlighting the need for accurate species-specific fish feed evaluation systems. Providing a more accurate energy-based formulation results in a more efficient diet by better predicting energy retention and growth performance. Currently, dietary energy evaluation and feed formulation for fish is established on a digestible energy (DE) basis. This system is based on the DE obtained from the diet using the general efficiency (fraction from 0 to 1) of utilization of the dietary energy for retaining energy (RE), also fulfilling basic metabolism of the animal. This system projects a linear relationship between the RE obtained from a certain DE intake. Estimating an equation (i.e., RE = µ + β DE), where the slope (β or KgDE) is the utilization efficiency of DE for energy gain, and the minimal metabolism (µ) know as the fasting heat production. This system does not make a distinction between the source of energy (protein, fat, and carbohydrate) in the diets, thus assumes one generalized efficiency. Nevertheless, recent studies have demonstrated this to not be true, obtaining different efficiencies of energy use between diets differing in macronutrient composition. To consider this variability in the utilization of the different macronutrients, the KgDE is divided into three types of digestible nutrient intake; protein (dCP), fat (dFat), and carbohydrates (dCarb), estimating how efficiently the animal can obtain energy from each one of them. This is known as the net energy (NE) system, already being used in cows, pigs, and poultry, and currently being developed for fish. In this method RE is a function of the three separate macronutrients (i.e., RE = µ + β1 dCP + β2 dFat + β3 dCarb) where the different slopes (β1, β2, and β3) are coefficients of the utilization (KgNE) of each digestible macronutrient intake individually (KgNE;dCP, KgNE;dFat, and KgNE;dCarb) for energy gain. Those coefficients can be used to calculate the efficiency of utilization by dividing the coefficient (β1, β2, and β3) by the energetic value of each macronutrient; 24.6, 39.5 and 17.2 kJ for protein, fat and carbohydrates, respectively. This approach allows to obtain different recommended NE intakes based on the macronutrient variation of the diets, with species-specific formulas of RE. Recently, NE equations for several fish species have been estimated: rainbow trout, Nile tilapia, barramundi, snakehead, among others (Phan, 2022; Schrama et al., 2018). A relatively low variability in efficiencies was detected in dCP (47% to 64%) and dFat efficiencies (78% to 93%). However, the highest variability occurred in the dCarb fraction, with efficiencies ranging from 5 to 70%, being relatively low in carnivorous (high trophic) fishes. This variations remarks the species-specific nature of the nutrient utilization, supporting the need for species-specific formulations, as is currently being addressed. Recently, new NE equations for relevant carnivorous fish species of production interest such as: Atlantic salmon (Salmo salar), European Seabass (Dicentrarchus labrax), and Yellowtail Kingfish (Seriola lalandi) have been estimated. New estimations of RE have been made using separate coefficients for dCP, dFat and dCarb. In addition, this coefficients also allow to calculate the utilization efficiency of energy in the form of protein, fat, and carbohydrate (those will be presented). This approach enhances the accuracy of estimating retained energy. This enables to make a more accurate prediction in new aquafeeds with high variability of macronutrient composition; hereby supporting the shift to novel ingredients.
References
FAO. (2024). The State of World Fisheries and Aquaculture 2024. FAO. https://doi.org/10.4060/cd0683en
Phan, L. T. T. (2022). Net energy evaluation for fish feed.
Schrama, J. W., Haidar, M. N., Geurden, I., Heinsbroek, L. T. N., & Kaushik, S. J. (2018). Energy efficiency of digestible protein, fat and carbohydrate utilisation for growth in rainbow trout and Nile tilapia. British Journal of Nutrition, 119(7), 782–791. https://doi.org/10.1017/S0007114518000259