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¹ Ege University, Faculty of Fisheries, Aquaculture Department, 35100 Bornova, Izmir, Turkey DOI : 10.4194/1303-2712-v12_2_05 Viewed : 4443 - Downloaded : 4994 In this study, the effects of the long photoperiod regime (15L:9D) on both somatic growth and husbandry parameters were investigated in growing gilthead seabream (Sparus aurata). Experiments were conducted in high density polyethylene (HDPE) floating cages which have net depths of 14 m and diameters of 20 m. They contained approximately 180,000 gilthead seabream with average weights of 78.91±8.54 g and 71.39±4.06 g for experimental and control groups, respectively. Additionally, artificial illumination was applied to experimental cages for 13 months, while it was not performed on the control group. At the end of the experiments, fishes of experimental groups reached to 425.19±5.06 g on average (total n=362,676), while this value was estimated as 305.42±25.01 g (total n=345,829) on average in control groups (P<0.05). In addition, feed conversion ratio (FCR) was calculated as 2.04 and 2.37 for experimental and control groups, respectively, however significant differences were found between these groups (P<0.05). Moreover, gonadal weight of fish in the control group was significantly higher than the experimental group. Gonadosomatic index was recorded as 0.36 % in the control group and 0.15 % in the experimental group (P<0.05). Keywords : Gilthead seabream, S. aurata, cage culture, long-term extended photoperiod, growth parameters, gonadal development