Effects of stocking density on stress response, innate immune parameters, and welfare of turbot (Scophthalmus maximus)

This experiment was conducted to investigate the growth performance, stress and immune responses, and welfare of juvenile turbot (Scophthalmus maximus) under three different densities (initial density 9.3, 13.6, and 19.1 kg m−2) for 120 days in a recirculating aquaculture system. Turbot were measured every 20 days to evaluate growth biometrically and sampled every month to measure biochemical parameters and mRNA levels of some stress-related genes. No significant differences were detected in the parameters and gene expression among density groups until the final sampling, except Fulton’s condition factor, lysozyme, immunoglobulin M, and complement C3. At the end of the experiment (final density 26.11, 38.22, and 52.25 kg m−2), turbot reared in the high-density (HD) group had lower body mass increase, specific growth rate, and Fulton’s condition factor, as well as higher feed conversion ratio and coefficient of variation for weight than those reared in the low-density (LD) group (P < 0.05). Fish in the HD group had higher serum cortisol, glucose, lactate, and cholesterol levels than fish in the other groups, whereas they had lower lysozyme, immunoglobulin M, and complement C3 and C4 contents (P < 0.05). Fish reared in the HD group also had higher serum chloride and osmolality levels and higher sodium-potassium adenosine triphosphatase (Na+,K+-ATPase) activity and higher Na+,K+-ATPase gene expression levels in gills compared to the other groups (P < 0.05). The mRNA levels of cytochrome P450 1A (CYP1A) and heat-shock proteins 70 and 90 (HSP70 and HSP90) were significantly upregulated, whereas glutathione S-transferase (GST) mRNA levels were significantly downregulated in the head kidney of fish in the HD group relative to fish in the other groups at the end of this trial (P < 0.05). These results indicated that overly high stocking (~50 kg m−2) density can negatively affect the growth performance, serum biochemical parameters, osmolality levels, stress-related gene expression, and overall welfare of turbot.