Dietary Sporolactobacillus laevolacticus Improves Growth Performance, Intestinal Health, and Immune-Antioxidant Related Responses in Juvenile Coho Salmon (Oncorhynchus kisutch)

Probiotics are considered promising feed additives for enhancing fish health and production performance in aquaculture. This study evaluated the effects of dietary supplementation with Sporolactobacillus laevolacticus on growth performance, feed utilization, intestinal health, and physiological responses in juvenile coho salmon (Oncorhynchus kisutch). Fish were fed a control diet or diets supplemented with S. laevolacticus at 0.89 × 107, 0.90 × 109, or 0.87 × 1011 CFU/g for 10 weeks. Compared with the control, S. laevolacticus supplementation significantly increased final body weight, weight gain rate, specific growth rate, and protein efficiency ratio, while decreasing the feed conversion ratio (p < 0.05). It also significantly enhanced intestinal protease and α-amylase activities, improved serum biochemical and immune-related parameters, and promoted better intestinal morphology (p < 0.05). Additionally, S. laevolacticus supplementation led to elevated expression of antioxidant-related genes, reduced expression of pro-inflammatory genes, and altered gut microbial composition, characterized by a decrease in Proteobacteria and increases in Firmicutes and Lactobacillales. Among the tested dosages, 0.90 × 109 CFU/g produced the most consistent improvements in growth performance, digestive function, intestinal health, antioxidant and immune responses, and gut microbial composition, and was therefore identified as the optimal supplementation level. Collectively, dietary S. laevolacticus at 0.90 × 109 CFU/g improved growth performance and intestinal health in juvenile coho salmon, highlighting its potential as a probiotic candidate for coho salmon aquaculture.