Introduction
Sustainable aquaculture requires a balance between what benefits the environment and what benefits the farmed fish. The over- or underuse of macronutrients in fish feeds occurs when the utilization efficiency of fish for the nutrients in question is unknown. In fish nutrition, energy evaluation systems have been established to evaluate the contribution of digested nutrients to the retained energy of fish. One of the energy evaluation systems is the Net Energy (NE) (Blok et al., 2015; Noblet et al., 1994; van Milgen et al., 2001). In the NE approach, feed is split into its composed macronutrients – protein, fat, and carbohydrates. The digested amount of those macronutrients and the retained energy in the fish body are biochemically quantified and subsequently described using mathematical models to characterize their relationship (Groot et al., 2021; Phan, 2022; Schrama et al., 2018). Although previous studies using the NE approach have covered a diverse range of fish species, from carnivores to omnivores, a herbivore remains unstudied. Grass carp (Ctenopharyngodon Idella) is a low-trophic fish that possesses a very herbivorous digestive system with a long intestine and no stomach (Zhao et al., 2020). As grass carp has been the most produced finfish in the world for many years (FAO, 2020, 2022), higher productivity and lower production cost are always of interest. Application of the NE approach can clarify how grass carp utilize energy from digested protein, fat, and carbohydrates, supporting balanced macronutrient use to achieve optimal retained energy that is subsequently used for growth. African catfish (Clarias gariepinus), a carnivorous species with feeding habits different from grass carp, was included in this study to compare how efficiently both species gain energy from each macronutrient when fed the same diets.
Materials and Methods
The fish were fed with diets contrasting in the content of macronutrients at high and low feeding levels. Feces were collected and analyzed to determine the nutrient digestibility, and fish bodies were combusted in the lab to determine their gross energy (ISO 9831, 1998). A multiple linear regression equation was built in which digestible nutrient intakes are predictor variables and retained energy is the response variable.
Results
African catfish and grass carp showed the same rate of digestibility for protein, fat, carbohydrates, starch, and energy, implying that the trophic level did not influence the nutrient digestibility of one species. The Net energy equation of grass carp was estimated as NE = 11.7×dCP + 45.8×dFat - 5.2×dFat2 + 11.5×dCarbs, and that of African catfish was fitted as NE = 16.9×dCP + 40.0×dFat + 5.1×dCarbs. African catfish could utilize 100% the digested fat for the retained energy, while grass carp showed a significant (p = 0.05) quadratic decline in the retained energy at high levels of digested fat. On the other hand, grass carp showed a significantly (p < 0.05) higher energy utilization efficiency for carbohydrates compared to African catfish. Based on these results, cautious inclusion of dietary fat is recommended for grass carp, whereas carbohydrate inclusion should be carefully controlled in the diet of African catfish.
Figure 1. Isolated contribution of each macronutrient to the net energy of grass carp.
Figure 2. Isolated contribution of each macronutrient to the net energy of African catfish.
References
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