This study aimed to evaluate the efficacy of green and red macroalgae extracts and β-glucans on the disease resistance, gills integrity, and growth performance of rainbow trout (Oncorhynchus mykiss) in long-term use. An algae-based marine sulphated polysaccharides (MSP) extracts, from Ulva sp. and Solieria chordalis, and 60% purified yeast-based β-glucans available in the market were used in the feeding trial. Fish weighting 27.6 g were reared at 14-17 °C, and average dissolved oxygen above 6 mg/L. Fish were divided into four different dietary treatments and 6 replicates (tanks of 70 L): Control, Algae MSP 0.3%; β-Glucan A 0.1% and β-Glucan B 0.1% produced as pelleted feed of 3mm. Fish were stocked at a density of 12 fish per tank (72 fish per treatment) and fed twice a day for 100 days. At the end of the feeding trial, 8 fish per treatment were sampled for blood-based innate immune response analysis. Another 6 fish per treatment were exposed to a bacterial solution containing Aeromonas hydrophila and Pseudomonas anguilliseptica via immersion for 24 hours, and gills were sampled after 72 hours for histopathological evaluation by a specialized laboratory using a diagnostic scoring system. Additionally, a third group of 30 fish per treatment was intramuscularly challenged with the same bacterial solution, and survival was monitored for 7 days. Data were analyzed using one-way ANOVA followed by SNK post hoc test (p<0.05), and survival was analyzed using the Kaplan-Meier method (SIGMA PLOT 11.0, Systat Software, San Jose, CA, USA). Trout fed the Algae MSP exhibited the best overall performance, with a 13% higher weight gain (45.8 g vs. 40.5 g; p=0.017), increased lysozyme activity (117 vs. 79 U/mL; p=0.016), and enhanced complement activity (40.5% vs. 2.4%; p <0.001) and were the only group which differed from control for survival 40% and 10%, respectively (+400%). Likewise, higher complement hemolytic activity was also observed in fish fed β-Glucan B (45.7%). Fish fed Algae MSP showed the lowest gills histopathological score among the challenged groups, indicating a 38% reduction in gill damage, 0.85, 0.53, 0.65, and 0.71, respectively. In conclusion, Algae MSP demonstrated the most consistent overall performance, excelling in growth, innate immunity, survival and gills integrity. The results highlighed the algae-based solution as a tool for strengthening natural defenses in aquaculture due to its broader benefits in long-term use.
Keywords: Health, Survival, Marine sulfated polysaccharides.