Effects of green wavelength light on antioxidant and non-specific immune responses of the olive flounder Paralichthys olivaceus maintained at different stocking densities

In the present study, we investigated the effects of green light illumination on antioxidant systems and immunity in the olive flounder Paralichthys olivaceus. Fish at three different stocking densities (900, 1350, and 2250 fish per 8-ton tank) were compared. The effects of green light illumination were assessed by measuring survival rates, mRNA expression activity of antioxidant enzymes (superoxide dismutase and catalase), an oxidative stress-related parameter (hydrogen peroxide, H2O2), and immune-related parameters (lysozyme and melatonin). Overall, fish survival rates decreased over the 30-day period of the experiment, but survival rates were significantly higher among the groups of fish exposed to green light. In high stocking densities groups, mRNA levels and activities of antioxidant enzymes and H2O2 concentrations had increased at 30 days; however, in fish under green light conditions, significantly lower levels of antioxidant enzyme expression were observed. By contrast, parameters indicating immune responses decreased in high stocking densities groups, although in fish under green light treatment, significantly higher levels of immune response were observed. A comet assay revealed that a high stocking density increased the rate of nuclear DNA damage; however, treatment with green wavelength light reduced the frequency of damage. These results indicate that although high density induces oxidative stress and reduces immune system responses in olive flounder, green wavelength light prevents oxidative stress and boosts the immune system.