Immunoprotective Effects of Dietary Fucoidan and Laminarin on Juvenile Blunt Snout Bream (Megalobrama amblycephala)

Bacterial diseases occur frequently in freshwater aquaculture in China, highlighting the need for green immune preparations. Fucoidan and laminarin are commonly used feed additives in livestock and poultry industries; however, their application in freshwater aquaculture remains limited. In this study, juvenile Megalobrama amblycephala were fed diets supplemented with fucoidan or laminarin, each at 0.5% and 2% (w/w), for 8 weeks. After the feeding trial, fish were challenged with Aeromonas hydrophila. Cumulative mortality rate, tissue bacterial loads, innate immune and antioxidant enzyme activities, and immune-related gene and protein expression were evaluated. Dietary supplementation with 2% fucoidan significantly improved the growth performance and feed utilization of juvenile M. amblycephala, while 0.5% fucoidan and 2% laminarin groups only enhanced feed utilization. After A. hydrophila infection, cumulative mortality rates for the control, 0.5% and 2% fucoidan, and 0.5% and 2% laminarin groups were 76.67%, 67.78%, 57.78%, 62.22%, and 58.89%, respectively. Both supplements increased post-infection survival, reduced tissue bacterial loads, and enhanced host bactericidal and antioxidative abilities by elevating innate immune and antioxidant enzyme activities. Innate immunity of juvenile M. amblycephala was improved by regulation of immune-related gene and protein expression levels. Both additives demonstrated significant immunoprotective effects, markedly reducing mortality and tissue bacterial loads following infection. This study provides a theoretical foundation for developing green immunostimulants and antibiotic alternatives for bacterial diseases in fish.