This research aimed to investigate the impact of different levels of light on biological parameters, body
composition and activity of liver and digestive enzymes of Nile tilapia (Oreochromis niloticus) in the biofloc
system. The experiment was performed for 38 days in fiberglass tanks (with 150 l of water) with a density of one
fish per liter. Nile tilapia fingerlings with an average weight of 1.73 ± 0.16 g and length of 4.13 ± 0.1 cm were
selected. Three experimental treatments including 24 h of light (24hL), 12 h of light/12 h of darkness (12hL/
12hD) and 24 h of darkness (24hD) with three replicates were considered. According to the results, the lowest
amount of dissolved oxygen (5.67 mg L 1), pH (7.35), total ammonia nitrogen (2.01 mg L 1) and nitrite (0.24
mg L 1) were obtained in 24hD treatment. Body weight gain, specific growth rate (SGR), survival, feed conversion
ratio (FCR) and condition factor were higher in the presence of light. The lowest amount of body protein
(56.06%) and lipid (25.83%) was obtained in 24hD treatment (P < 0.05). The highest activity of liver enzymes (u
mg 1 protein) including aspartate aminotransferase (AST: 2.98), alanine aminotransferase (ALT: 0.48) and
alkaline phosphatase (ALP: 0.081) was observed in 24hD treatment. In conclusion, our results suggest that the
presence of light improves carcass quality and activity of digestive and hepatic enzymes of Nile tilapia in the
biofloc system.