Effect of Sargassum oligocystum Hot-Water Extract on Innate Immune Response and Survival of Summer Flounder Paralichthys dentatus to Vibrio harveyi Challenge

The present study evaluated the effects of hot-water extract of a brown seaweed Sargassum oligocystum on the non-specific immune response and survival of summer flounder Paralichthys dentatus, to Vibrio harveyi bacterial challenge. Fish were either immersed in 100 or 500 mg/L or injected with 0.1 or 0.5 mg per fish of the hot-water extract. The innate humoral (lysozyme, plasma protein, bactericidal activity), cellular (respiratory burst, hematocrit), and disease resistance to Vibrio harveyi infection were determined. Results showed that all the experimental treatments either by injection or immersion, significantly enhanced respiratory burst activity and hematocrit values of the fish. In the experiment, all the treatments of hot-water extract significantly affected the lysozyme, plasma protein, and bactericidal activity of the experimental fish from days 1 to 5 after delivery. Following the bacterial challenge mortality decreased significantly in all treated groups. In summer flounder, administration of hot-water extract of S. oligocystum either by injection or immersion was found to be an immunoprophylactic for finfish aquaculture. The efficacy of using hot-water extract as a feed supplement or feed additive needs further examination.