Stocking density effects on growth and stress response of juvenile turbot (Scophthalmus maximus) reared in land-based recirculating aquaculture system

Stocking density is widely recognized as a critical factor in aquaculture and a potential source of long-term stress. The influence of stocking density on growth and stress response of juvenile turbot (Scophthalmus maximus, similar to 3-75 g, initial to final weight) was examined in fish held under low (LD, similar to 0.21-5.31 kg/m(2), initial to final density), medium (MD, similar to 0.42-10.81 kg/m(2)) and high stocking density (HD, similar to 0.63-14.27 kg/m(2)) for 120 days in a recirculating aquaculture system (RAS). In this trial, the growth curve for weight of juvenile turbot in RAS, all fitted by the Schnute model. No significant difference was found in growth performance among the three densities until at the final sampling (Day 120). The final weight and body weight increase (BWI) in the HD group were significantly lower than in other groups (P<0.05, weight: (75.83 +/- 2.49) g, (75.39 +/- 2.08) g, (65.72 +/- 2.86) g and BWI: (2 436.12 +/- 28.10)%, (2 421.29 +/- 4.64)%, (2 097.88 +/- 20.99)% in LD, MD and HD groups, respectively). Similarly, the specific growth rate (SGR), feed conversion ratio (FCR) and coefficient of variation for weight (CVw) were adversely affected by high stocking density (P<0.05). However, there was no difference in survival and Fulton's condition factor (K) of turbot among the different groups. Physiological analyses demonstrated a clear increase in the plasma cortisol level and an obvious decrease in growth hormone (GH) concentration in the HD group on Day 120 (P<0.05). There was no significant effect of stocking density on plasma glucose, Cl- and protein levels. All these findings would provide a reference for selecting the optimal stocking density of juvenile turbot in RAS.