Effects of dietary protein restriction on growth performance, nutrition, body composition, and activity of some digestive enzymes in red tilapia (Oreochromis mossambicus × Oreochromis niloticus)

Introduction: The primary operational cost in aquaculture is attributed to feed expenses, which plays a critical role in growth enhancement, immune system performance, and disease resistance. To address this, various strategies have been proposed for feed management, including short-term or long-term fasting, as well as cyclic periods of fasting and refeeding to achieve compensatory growth. Another strategy involves restricting the protein content in the diet within a specific timeframe. This method focuses solely on limiting the protein content in the fish diet for a specific duration, after which the protein level is restored to an optimal and balanced level without interrupting the feeding process. Therefore, formulating diets that maximize growth while conserving protein resources, considering economic viability, and minimizing adverse environmental effects, is essential.
Material Methods: A total of 270 red tilapia, with an average initial weight of 5 ± 0.13 g were randomly distributed among 18 aquaria, each with a capacity of 100 liters (15 fish per tank). The fish were subjected to various protein-level dietary treatments over 8 weeks and were fed three times daily at 9:00, 13:00, and 16:00 until satiation. The experimental treatments were as follows; control group: feeding with a diet containing 38% optimal protein, treatment 1: feeding with a diet containing 32% protein restriction, treatment 2: feeding alternately every other day with a diet containing 32% protein restriction and a diet containing 38% protein, treatment 3: feeding alternately every week with a diet containing 32% protein restriction and a diet containing 38% protein, treatment 4: feeding a 32% protein diet for the first 3 weeks, followed by a 38% protein diet for the remaining 5 weeks, and treatment 5: feeding a 32% protein diet for the first 4 weeks, followed by a 38% protein diet for the following 4 weeks. At the end of the experiment, growth, nutritional indices, and body composition were assayed. Enzyme activity was assessed using specific substrates and diagnostic kits. Subsequently, a One-way analysis of variance (ANOVA) was employed to compare the differences among the treatments, and the significance of the results was determined using Duncan's multiple-range test.
Results and discussion: The findings of this study revealed that the control group, which was fed a diet containing 38% optimal protein throughout the experimental period, achieved the highest growth performance and the best feed conversion ratio (FCR). Interestingly, treatment 4, which involved feeding the fish with a 32% protein-restricted diet for the first 3 weeks and a 38% optimal protein diet for the remaining 5 weeks, showed no significant difference in growth performance and FCR compared to the control group (P>0.05). This suggests that when refeeding with an optimal protein diet, moderate protein restriction can induce compensatory growth without negatively impacting growth performance or feed efficiency. Regarding body composition, the control group exhibited the highest protein and lowest lipid content in the carcass. However, treatment 4 showed no significant difference in these indices compared to the control group (P>0.05), indicating that the fish could maintain their body composition despite the initial protein restriction. Similarly, the lack of significant differences in moisture and ash content among the experimental groups further supports the idea that moderate protein restriction does not adversely affect the overall nutritional quality of the fish. The activity of digestive enzymes, including total protease, trypsin, chymotrypsin, and lipase, was the highest in the control group. However, treatment 4 showed no significant difference in digestive enzyme activity compared to the control group (P>0.05), suggesting that the fish were able to maintain their digestive efficiency during the refeeding period. This is a critical finding, as digestive enzyme activity is a key indicator of nutrient absorption and utilization in fish.
Conclusion: In conclusion, the results of this study demonstrate that moderate protein (3 weeks) restriction, when followed by adequate refeeding (5 weeks), can induce compensatory growth in red tilapia without negatively impacting growth performance, body composition, or digestive enzyme activity.