Evaluation of Energy Utilisation Efficiencies of Digestible Macronutrients in Juvenile Malabar Snapper (Lutjanus malabaricus) Reveals High Protein Requirement for Optimal Growth Using Both Factorial and Multifactorial Approaches

Abstract

Malabar snapper (Lutjanus malabaricus) is an economically important marine fish throughout the Indo-Pacific, with an emerging aquaculture industry. Although generic marine feeds are available for production, these are not optimised for this species. Understanding energy utilisation and balance can provide insight into suitable macronutrient profiles for new species to provide a baseline for future development. This study, therefore, evaluated the effect of dietary macronutrient composition (i.e., protein, fat, and carbohydrate) on the utilisation efficiencies of digestible energy (DE) in juvenile Malabar snapper using two isonitrogenous diets (high fat: HF and low fat: LF) with contrasting fat and carbohydrate content. Each diet was fed at four feeding levels (100%, 75%, 50%, and 25% apparent satiation) for 56 days, creating a 2 by 4 factorial design. The maintenance energy requirement of Malabar snapper was estimated to be 76.7 kJ kg^-0.8 day^-1, while the utilisation efficiencies of digestible protein (DP) and fat were 73.6% and 68.3%, respectively. Fish fed with LF, which has lower dietary fat and higher dietary carbohydrate levels, had significantly reduced energy utilisation efficiency for growth and significantly higher partial energy utilisation efficiency of digestible fat (DF) (p < 0.05). Since body moisture is usually proportional to body fat content in fish, this implies that the energy from carbohydrates preferentially enters lipogenesis rather than being available for somatic growth, and adiposity does not directly result in weight gain. Malabar snapper utilises DF in preference to protein for metabolism, demonstrating a protein-sparing effect from lipids at DE intake levels below the maintenance requirement. Conversely, given the higher efficiency of fat retention than protein retention, protein is likely used before fat when energy intake is above maintenance. These findings suggest that Malabar snapper requires high levels of DP in its diet to support growth and that energy from dietary carbohydrates is diverted towards adiposity, consequently reducing growth.

Keywords: Lutjanus malabaricus; digestibility; energy utilisation; growth; lipogenesis; macronutrients.

Figure 1

Mean ± SEM final body fat content (g kg⁻¹, as is) of Malabar red snapper (L. malabaricus) fed with diet HF and LF at four different feeding levels (25%, 50%, 75%, and 100% of apparent satiation) for 56 days (n = 4). Mean values lacking a common letter superscript differ significantly (p < 0.05). HF, high fat; LF, low fat.

Figure 2

Relationship between retained energy (RE) and digestible energy intake (DE) for Malabar red snapper (L. malabaricus) fed with either diets HF or LF ( HF: RE = −48.95 [SE 2.89] + 0.64 [SE 0.023] DE, R² = 0.98; LF: RE = −37.74 [SE 4.96] + 0.49 [SE 0.042] DE, R² = 0.91) for 56 days. By extrapolating to zero energy retention (RE = 0), the estimated digestible energy requirements for maintenance were 76.5 kJ kg^−0.8 day⁻¹ and 76.9 kJ kg^−0.8 day⁻¹ for HF and LF, respectively. HF, high fat; LF, low fat.

Figure 3

Relationship between retained energy (RE) partitioned as nutrients (either protein or fat) as a function of digestible energy (DE) intake partitioned as nutrients (either protein or fat) of L. malabaricus fed either diet HF ( RE as Prot = −16.078 [SE 1.459] + 0.451 [SE 0.012] DE_DP, R² = 0.969; RE as fat = −32.904 [SE 1.753] + 1.373 [SE 0.049] DE_DF, R² = 0.982) or diet LF ( RE as Prot = −15.014 [SE 2.136] + 0.407 [SE 0.040] DE_DP, R² = 0.929; RE as fat = −22.710 [SE 3.172] + 1.848 [SE 0.202] DE_DF, R² = 0.856) for 56 days. HF, high fat; LF, low fat.

Figure 4

Relationship between net energy (NE) and digestible energy (DE) partitioned as digestible protein (DE_DP), digestible fat (DE_DF), and digestible carbohydrate (DE_DC) in L. malabaricus. The NE values corresponding to DE_DP are corrected for variation in DE_DF and DE_DC, the NE values corresponding to DE_DF are corrected for DE_DP and DE_DC, and the NE values corresponding to DE_DC are corrected for variation in DE_DP and DE_DF. The calculations were conducted using Equation (7) is as follows: the measured retained energy for each data point in the data set was added with the estimated fasting heat production (intercept) to obtain the NE value. The NE values are then corrected towards zero DE_DF and DE_DC (where DE_DC = 0) to visualise the effect of DE_DP on NE, ; zero DE_DP and DE_DC (where DE_DC = 0) to visualise the effect of DE_DF on NE, ; and zero DE_DP and DE_DF to visualise the effect of DE_DC on NE, .

Figure 5

Net energy utilisation efficiencies (k_g,NE) of energy partition of digestible protein (DP_DE), Fat (DF_DE), and carbohydrate (DC_DE) in carp, tilapia, snakehead, rainbow trout, barramundi, African catfish, and Malabar snapper using linear relationship between retained energy and digestible energy intake of protein, fat, and carbohydrate. The coefficient data of barramundi and carp were obtained from Phan et al. [14], trout and tilapia were from Schrama et al. [13], snakehead was from Phan et al. [10], African catfish was from Phan et al. [11], and Malabar Snapper from this study.