Protein, undoubtedly, is the most important and expensive part of the aquaculture diets . Any dietary protein manipulation, without impaired animal growth can be a significant step toward aquaculture sustainability. Protein restriction is a feeding strategy in which aquatic animals can be fed with various protein levels (in range of nearby optimum) during the farm period. This approach can reduce the cost of feed and improve water quality. Protein restriction is a less-investigated nutritional strategy in aquaculture species . Therefore, we ran several trials in different species to test which Protein restriction schedule works better. We tested this in rainbow trout Oncorhynchus mykiss , Siberian sturgeon Acipenser baeri , and whiteleg shrimp Litopenaeus vannamei. The sizes of the animals for trials were 27, 75, and 3 g, respectively and were farmed in different conditions based on their optimum requirements. These species fed diets with proteins in a 30 to 45% range during at least eight weeks of the experiment. Growth, body composition, digestive enzymes, fatty acids, amino acids, immune and antioxidant systems and relative genes were measured.
The output of trial 1 (Siberian sturgeon ) indicated that feeding fish with 40% and 30% protein every other day did not decrease growth, immunity, and flesh quality compared to those fed the whole period with 40% protein. Other tested strategies were planned every other week and in three- five-week schedules (Article 1). In trial 2 in rainbow trout , growth, immune system, and flesh quality declined by feeding fish with a schedule of 45% and 35% protein every other week. Other treatments were similar to trial 1 (Article 2). To test how whiteleg shrimp respond to protein restriction, we designed trials 3, 4, 5, 6, and 7. The question was raised about how fasting and protein restriction interact. We observed that none of the planned protein restrictions, along with one week of fasting, worked well and there was a significant difference between those fed eight weeks of 40% diets compared with other groups (trial 3). Next, we tested different protein restriction schedules and those fed one day of 40% and two days 35% protein in their diets did not differ from the control group (feeding a diet with 40% protein for eight weeks) (trial 4). In trial 5, this schedule interacted with fish meal replacement. The results showed that we could still use the protein restriction scheme even with reducing 33% of fish meal in diets. In the next experiments, we tested how diets with high lipid and carbohydrate levels interact with protein restriction (trials 6 and 7). The results indicated lipid levels in diets could affect the effectiveness of protein restriction.
We understood from these trials that different species react so differently to protein restriction. Further, the immune system, antioxidant capacity, digestive enzymes, and flesh quality should also be considered. Using protein restriction is a promising approach to reduce feed costs by up to 12% without any negative impacts on biological or physiological indices. More studies on various species are required to see which protein restriction plan can potentially be used, especially in marine species.