Stress and immune responses in skin of turbot (Scophthalmus maximus) under different stocking densities.

Fish skin and its mucus provide the first line of defense against chemical, physical and biological stressors, but little is known about the role of skin and its mucus in immune response to crowding stress. In the present study, we investigated the stress and immune responses in skin of turbot (Scophthalmus maximus) under different stocking densities. Turbot (average weight 185.4 g) were reared for 120 days under three densities: low density (LD), medium density (MD), and high density (HD). After 120 days, fish were weighed and sampled to obtain blood, mucus and skin tissues which were used for analyses of biochemical parameters and genes expression. The results showed HD treatment significantly suppressed growth and enhanced plasma cortisol and glucose levels (P < 0.05). In mucus, the activities of lysozyme (LZM), alkaline phosphatase (ALP) and esterase in HD treatment were lower than LD and MD treatments (P < 0.05) In skin, HD treatment resulted in up-regulation in malondialdehyde (MDA) formation and heat shock protein 70 (HSP 70) mRNA level, and down-regulation in activity of superoxide dismutase (SOD) and the transcriptions of glutathione-s-transferase (GST), interleukin-1β (IL-1β), tumor necrosis factor -α (TNF-α), insulin-like growth factor- (IGF-) and LZM (P < 0.05). Overall, the data suggested that overly high stocking density was a stressor which caused an immunosuppression in skin of turbot. Moreover, this information would help to understand the skin immunity and their relation with stress and disease in fish.