Behavioral and physiological responses to stocking density in sea bream (Sparus aurata): Do coping styles matter?

•Sea breams were screened for their stress coping styles (SCS) and reared at low and high densities (LD and HD).•Sea breams showed better growth performances at LD than at HD but no effect of SCS was observed.•Bold individuals displayed higher swimming activity than shy ones at HD, while the opposite pattern was observed at LD.•Effects of SCS were observed on the physiological indicators of stress and welfare mostly at the beginning of the experiment.•At the end of experiment, density had effects of the physiological indicators while the SCS had no more. Stocking density is considered a stress factor for fish and is therefore one of the numerous concerns about fish welfare in an aquaculture context. Stress coping styles (SCS) are defined as a coherent set of individual physiological and behavioral differences in stress responses that are consistent across time and context and appear to be promising for improving fish welfare in aquaculture. The aim of the present study was to describe the physiological and zootechnical performances of gilthead sea bream (Sparus aurata) at different stocking densities (low density, LD: 15 kg/m3 and high density, HD: 30 kg/m3), depending on individual SCS. To do so, the fish SCS were first screened by measuring boldness (prior to the experiment). Three consecutive samplings were performed over the experiment to measure several blood parameters, including hematocrit (Hct), red blood cell count (RBCC), hemoglobin (Hb), cortisol, adrenalin, noradrenalin, glucose, lactate, and lysozyme, to infer the consequence of the SCS profile on the welfare condition in response to stocking density. Finally, swimming activity was recorded in a subsample of individuals (9 BOLD and 9 SHY individuals per density), and BOLD individuals displayed higher swimming activity than SHY ones at HD, while the opposite pattern was observed at LD. According to principal component analysis, physiological parameters are linked to the SCS profile, mostly at the beginning of the experiment, while density effects on physiology remain during the entire experiment duration. In conclusion, regarding all the variables observed, fish SCS appeared to be promising criteria to select the most adaptive individuals relating to rearing conditions and therefore improve welfare.