The aim of this study was to investigate the effects of a long-term photoperiod (15L:9D) on both the somatic growth and husbandry parameters of sea bass Dicentrarchus labrax. Experiments were conducted in triplicate in high density polyethylene (HDPE) floating cages with net depths of 14m and diameters of 20m. Experiments were performed with approximately 170000 sea bass with initial average weights of 110.63 +/- 3.12g and 110.26 +/- 29.70g for the experimental and control groups, respectively. Beginning in April 2008, extended artificial lighting was used in the experimental cages for 13months; the control group received no additional illumination. At the end of the experiments, the average weights of the experimental and control groups were 362.13 +/- 7.72 and 310.34 +/- 18.15g, respectively. Specific growth rate (SGR) was relatively high in the experimental group compared to the control fish. Feed conversion ratio (FCR) was 2.02 and 2.21 for the experimental and control groups, respectively, with significant differences also found between these groups (P<0.05). Moreover, gonadal fish weight in the control group was significantly higher than in the experimental group, with the gonadosomatic index fluctuating within a 1.71-fold increase.