Effects of stocking density on antioxidant status, metabolism and immune response in juvenile turbot (Scophthalmus maximus)

This study was designed to evaluate the physiological and immune responses of juvenile turbot to stocking density. Turbot (average weight 185.4 g) were reared for 120 days in a land based recirculating aquaculture system (RAS) under three stocking densities: low density (LD, ~ 9.3–26.1 kg/m2, initial to final density), medium density (MD, ~ 13.6–38.2 kg/m2) and high density (HD, ~ 19.1–52.3 kg/m2). Fish were sampled at days 0, 40, 80 and 120 to obtain growth parameters and liver tissues. No significant difference was detected in growth, biochemical parameters and gene expression among the three densities until at the final sampling (day 120). At the end of this trial, fish reared in HD group showed lower specific growth rate (SGR) and mean weight than those reared in LD and MD groups. Similarly, oxidative stress and metabolism analyses represented that antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione (GSH)) and metabolic enzymes (glycerol-3-phosphate dehydrogenase (G3PDH) and glucose-6-phosphate dehydrogenase (G6PDH)) clearly reduced in the liver of turbot reared in HD group. The gene expression data showed that glutathione S-transferase (GST), cytochrome P450 1A (CYP1A), heat shock protein 70 (HSP 70) and metallothionein (MT) mRNA levels were significantly up-regulated, and lysozyme (LYS) and hepcidin (HAMP) mRNA levels were significantly down-regulated in HD group on day 120. Overall, our results indicate that overly high stocking density might block the activities of metabolic and antioxidant enzymes, and cause physiological stress and immunosuppression in turbot.