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White sea bream Diplodus sargus L. is a ¢sh species that develops dominance hierarchies and aggressive behaviour against its conspeci¢cs. On the other hand, stocking density is known to a¡ect social interactions, while an appropriate density for D. sargus e⁄cient farming remains to be elucidated. For this purpose, juvenile D. sargus (14.3 g) were reared in 88.4 L tanks, for 10 weeks, under six rearing densities (10, 15, 20, 25, 30 and 35 ¢sh tank À1 ). Water quality was not affected by rearing density. Best growth, in terms of ¢nal weight, speci¢c growth rate, weight variation and food utilization, was achieved at 10 ¢sh tank À1 . At this treatment the lowest body water content, hepatosomatic and splenosomatic indices, higher body and liver lipid content were also observed. Brain neurotransmitter results indicated increased dopaminergic activity in populations held at higher rearing densities, while serotonergic activity was not a¡ected. Moreover, no signi¢cant di¡erences were detected for liver fatty acids. Results of the present study, compared with previously reported data, suggest that, within the range tested, the lowest realistic rearing density for juvenile white sea bream would be10 ¢shtank À1 . Higher rearing densities led to decreased growth, which seemed to be related to changes in in-traspeci¢c competition and social behaviour.