Partial Replacement of Fish Meal With Soy Protein Concentrates and Poultry Byproduct Meal for Atlantic Salmon (Salmo Salar) Post-Smolts Reared in a Recirculating Aquaculture System

Abstract

This study evaluated the effects of partially replacing fish meal (FM) with a balanced mixture of poultry byproduct meal (PBM) and soy protein concentrate (SPC) on the growth, physiological responses, and economic performance of Atlantic salmon (Salmo salar) post-smolts reared in a recirculating aquaculture system (RAS). In total, four extruded diets were tested for 12 weeks: two experimental formulations—high-FM control, experimental diet (ED)1; 25% FM replaced with PBM + SPC, ED2—and two commercial reference feeds. Growth performance, feed efficiency (FE), and survival were statistically comparable across treatments, indicating that PBM–SPC inclusion did not compromise nutrient utilization. The proximate composition, amino acid (AA) profile, and lipid deposition in fish stabilized. The PBM–SPC diet maintained a favorable n-3/n-6 fatty acid (FA) ratio and a high retention of eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) comparable to FM-based feeds. Plasma biochemical indices and digestive enzyme activities were unaffected, demonstrating normal hepatic and metabolic function. Among diets, plasma antioxidant enzyme activities and serum lysozyme levels were similar, suggesting a maintenance of oxidative stress and immune balance. Expression of hepatic genes interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), IL-10, FA synthetase (FAS), and peroxisome proliferator-activated receptor α (PPARα) confirmed that the PBM–SPC diet did not trigger any inflammatory or metabolic stress. However, histology revealed normal liver and intestinal morphology without signs of steatosis or enteritis. The PBM–SPC diet reduced feed costs and achieved a lower economic conversion ratio (ECR) while maintaining profitability equivalent to or greater than commercial feeds. Overall, the results demonstrate that partial substitution (25%) of FM with PBM and SPC sustains optimal growth, physiological stability, and economic efficiency of post-smolt S. salar under RAS conditions.