Appropriate Dietary Levels of Soybean Lecithin and Krill Oil Phospholipids Promote Growth, Antioxidant Capacity, and Lipid Metabolism While Reducing Lipid Deposition in Atlantic Salmon (Salmo salar) Fry

This study evaluated the effects of dietary phospholipid (PL) source and supplementation level on growth performance, antioxidant capacity, and lipid metabolism in Atlantic salmon (Salmo salar) fry. A 56-day feeding trial was conducted using a basal diet containing 1.76% PL and six experimental diets with an additional 1.5%, 3.0%, or 4.5% PL provided by soybean lecithin (SL) or krill oil phospholipids (KOP). Dietary supplementation with 3.0–4.5% SL and 1.5–4.5% KOP significantly improved growth performance, whereas feed conversion ratio was significantly reduced in the 3.0–4.5% SL and 3.0% KOP groups (p < 0.05). At equivalent inclusion levels, no significant differences were observed between SL and KOP in growth performance parameters (p > 0.05). PL supplementation also reduced whole-body lipid deposition and enhanced visceral lipase activity in all groups except the 1.5% SL group, while antioxidant capacity was improved in all PL-supplemented groups (p < 0.05). SL had no significant effect on whole-body fatty acid composition (p > 0.05), whereas moderate to high levels of KOP significantly altered the fatty acid profile, characterized by reduced monounsaturated fatty acids and n-6 polyunsaturated fatty acids, along with increased eicosapentaenoic acid (EPA) levels (p < 0.05). Transcriptomic analysis indicated that PL supplementation affected hepatic lipid metabolism, with both PL sources downregulating apoa2-like, while KOP induced stronger hepatic transcriptional responses related to lipid utilization and innate immune signaling than SL (padj < 0.05). However, gut microbiota analysis revealed no significant differences in the relative abundances of the dominant phyla or in α- and β-diversity among the control, 3.0% KOP, and 4.5% SL groups (p > 0.05). Overall, dietary PL supplementation promoted growth, improved antioxidant capacity, enhanced lipid metabolism, and reduced lipid deposition in Atlantic salmon fry, with KOP exerting stronger effects than SL on fatty acid composition and hepatic gene expression.