Effects of dietary fermented sugar kelp (Saccharina latissima) on intestinal histology, antioxidant capacity, and immune-related biomarkers of post-smolt Atlantic salmon (Salmo salar)

Brown seaweeds such as sugar kelp (Saccharina latissima) are rich in bioactive compounds with potential health benefits for fish. This study evaluated the effects of dietary fermented sugar kelp (FSK) on mid-intestinal histology, antioxidant biomarkers, and immune response in post-smolts Atlantic salmon (Salmo salar). A total of 975 fish (204 ± 37 g) were allocated across 15 tanks and fed one of five isonitrogenous (≈45% crude protein) diets containing 0-4% FSK for 10 weeks. Plasma, liver, muscle, and mid-intestine samples were analyzed for histological and biochemical parameters at the end of the trial. FSK inclusion up to 4% had no adverse effects on mid-intestinal morphology or signs of inflammation. Liver antioxidant capacity was modulated, with significantly higher glutathione (GSH) concentrations and lower malondialdehyde (MDA) levels in FSK 4%-fed groups compared to the control. However, no changes were observed in muscle or intestinal antioxidant levels. Immune-related parameters were also modulated: plasma lysozyme activity doubled at 1% and 3% FSK, and plasma bactericidal activity more than doubled at 3% FSK compared to the control group. Antiprotease activity increased dose-dependently, although plasma peroxidase activity and IgM levels declined slightly. Overall, dietary FSK up to 4% does not alter mid-intestine morphology or induce inflammation. Additionally, it shows potential functional effects in fish, including the modulation of hepatic antioxidant capacity and innate-immune mechanisms, particularly antibacterial effects. These findings support further investigation into the functional use of seaweed-derived ingredients in aquafeeds. The study design and main findings are provided in a graphical abstract.