Combined Schizochytrium and Canola Oil Effectively Replaces Fish Oil in Atlantic Salmon ( Salmo salar ) Under Chronic Thermal Stress

This study evaluated the replacement of fish oil (FO) with Schizochytrium oil (SCZO) and canola oil (CNO) in diets of Atlantic salmon ( Salmo salar ) under chronic high‐temperature conditions. Six diets replaced FO at 0%, 20%, 40%, 60%, 80%, and 100% (S0–S100) with SCZO and CNO at a constant replacement ratio of 27.1:72.9 (w/w), while maintaining comparable dietary EPA + DHA levels. Fish (101.01 ± 0.25 g) were stocked at 20 fish per tank with three replicates per treatment and fed for 6 weeks during which water temperature increased from ~15.5 to 25.0°C. Compared with S0, weight gain (WG) was significantly higher in S20–S60, whereas protein‐efficiency ratio (PER) and survival were higher and feed conversion ratio (FCR) was lower in S20–S100. Plasma glucose, triglyceride, aspartate aminotransferase (AST), alanine aminotransferase (ALT), low‐density lipoprotein (LDL), and cortisol were lower in S20–S100 than in S0, whereas high‐density lipoprotein (HDL) was higher in S60–S100. Antioxidant enzyme activities were higher and hepatic malondialdehyde (MDA) was lower in S20–S100, whereas myeloperoxidase (MPO), lysozyme, and respiratory burst activities were higher in S40–S100. Intestinal villi length was increased and vacuolization was decreased in S20–S100. Genes related to fatty acid synthesis, heat‐shock responses, inflammation, and apoptosis were downregulated in S20–S100 compared with S0, whereas fatty acid catabolism‐related genes were upregulated in S20–S100. Muscle DHA was higher in S20–S100 and crude protein in S40–S100 than in S0. Taxonomic profiling showed lower abundances of Vibrio and Photobacterium in S20–S100 than in S0 and increasing abundances of Lactobacillus and Bacillus with FO replacement, accompanied by increased alpha diversity and distinct beta‐diversity clustering. Quadratic regression estimated optimal FO replacement levels of 58.6% for WG and 81.6% for survival. Overall, SCZO with CNO effectively replaced FO and supported growth, physiological responses and survival of S. salar during chronic high‐temperature exposure.