||Sea bass (Dicentrarchus tabrax) (135 +/- 4 g) were reared under tank-based recirculating aquaculture system for a 63-day period at four densities: 10, 40, 70, 100 kg m(-3). Fish performance, stress indicators (plasma cortisol, proteonemia plus other blood parameters-Na+, K+, glucose, pH, total CO2- and water quality were monitored. At the end of the 63-day period, resistance to infection was also studied by a nodavirus challenge. A 25-day test was performed on fish from two extreme densities (10 and 100 kg m(3)) and one intermediate density (40 kg m(3)). With regards to the different density treatments, there was no significant difference between the daily feed intake (DFI) and the specific growth rate (SGR) up to a density of 70 kg m(-3). No significant difference was found between treatments concerning the feed conversion ratio (FCR) and the mortality rate. No density effect was observed on the fish stress level (plasma cortisol) or on sensitivity to the nodavirus challenge. Under these experimental rearing conditions, the density above 70 kg m(-3) has an impact on growth performance (DFI and SGR) indicators and also some blood parameters (CO2) at the highest density tested (100 kg m(-3)). This study suggests that a density up to 70 kg m(-3) has no influence on sea bass performance and welfare. At 100 kg m(-3), average specific growth rate was decreased by 14% without welfare deterioration according to the welfare indicators monitored. (C) 2008 Elsevier B.V. All rights reserved.