Effect of dietary carrageenan supplementation on growth performance, digestive enzyme and antioxidant enzyme activity, immune gene expression, and capacity against S. agalactiae of Nile tilapia (Oreochromis niloticus).

The current study aimed to ascertain the growth, antioxidant capacity, intestinal architecture, digestive enzyme activity, and disease resistance of S. agalactiae of Nile tilapia (Oreochromis niloticus) fed a diet enriched with carrageenan. Using an alkali method, the carrageenan was extracted from the seaweed Kappaphycus alvarezii. Its antiviral and antibacterial properties against Tilapia Lake virus and aquatic pathogens like Aeromonas veronii, Edwardsiella tarda, Streptococcus agalactiae, and Streptococcus iniae were assessed. A total of 450 healthy juvenile Nile tilapia (4.43 ± 0.71 g) were randomly divided into five experimental groups, 30 fish per group, with three replicates. The water quality parameters such as dissolved oxygen (5.65 ± 0.2 mg/L), pH (7.75 ± 0.51), temperature (27.75 ± 1.5 °C), and ammonia (0.2 ± 0.08 mg/L) were monitored. Group 1 (control, normal feed), group 2 (10 g/kg), group 3 (20 g/kg), group 4 (40 g/kg), and group 5 (80 g/kg) all had varying carrageenan concentrations throughout the 60-day study. Then, 30 fish from each group were intraperitoneally challenged with S. agalactiae, and mortality was recorded for the next 15 days. The results showed substantial improvements in initial weight, weight gain rate, survival rate, condition factor, and specific growth rate in the G4 groups compared to the G1 group. The carrageenan-based dietary supplements at the concentration of 20 g/kg enhance digestive enzyme activity (amylase, protease, and lipase) in fish, improving nutrient absorption and overall growth performance. The antioxidant activities, such as a significant increase in SOD, CAT, and GSH capacity and a remarkable decrease in MDA content, were observed in the liver of fish fed carrageenan diets. IgM antibody values, measured by ELISA, peaked G3-treated fishes when compared to other groups. The gene expression analysis using qRT-PCR showed the significant increase immune genes like IL-1β, IL-8, and IgM in the G3-treated fishes when compared to the others. The challenge test with S. agalactiae resulted in lower mortalities by carrageenan administration in a dose-dependent manner, and the highest mortality was observed in fish fed the G1 diet. The study concludes that the inclusion of carrageenan at the rate of G3 is recommended to improve growth performance, antioxidant capacity, and immunity, as well as to regulate intestinal morphology and promote resistance against S. agalactiae in Nile tilapia.