Dietary Protein-to-Lipid Ratio Influences Feed Intake and Utilization, Body Composition, Fatty Acid Metabolism, and Testicular Growth in Atlantic Salmon Postsmolt

Slow growth, adiposity, and early maturation are frequently observed in postsmolt Atlantic salmon reared in land-based facilities. High-energy commercial feeds, traditionally designed to accelerate growth, may contribute to these issues by promoting excess fat deposition, which can lead to reduced feed intake, poor growth after seawater transfer, and enable maturation initiation by the increased available energy. In this experiment, smoltified Atlantic salmon weighing approximately 100 g were reared in triplicate tanks in a flow-through system with brackish water for 18 weeks, growing to ~1000 g. Two diet series with different protein-to-lipid (P/L) ratios were tested: the low-fat (LF) group received a 4 mm pellet (P/L = 50.8/19.5%) for 18 weeks, while the control (CTRL) group received a diet with increasing lipid content and pellet size, starting with a 4 mm pellet (P/L = 46.5/24.7%) for 14 weeks, followed by a 6 mm pellet (P/L = 46.5/28.9%) from weeks 15 to 18. Afterwards, 100 fish from each replicate were pooled into a seawater tank for 11 weeks, where they were fed the same diet (P/L = 44.9/30.2%) using a 9 mm pellet until ~2400 g. During the initial 18 weeks, the LF diet significantly influenced body composition, reduced adiposity, and feed utilization, with the CTRL group showing improved feed utilization and growth after switching to the 6 mm pellet. After seawater exposure, short-term growth compensation was observed in the LF group, leading to equal body weight, condition factor (CF), and slaughter quality between the groups by the end of the experiment. Both groups exhibited low maturation rates; however, LF diet during the initial phase seemed to reduce the advancement of gonadal development transcriptome analysis of visceral fat revealed that high dietary lipid intake and/or increased visceral fat content enhanced tissue growth and differentiation pathways, promoted adipogenesis, and activated immune functions—particularly those associated with adaptive immunity.