Production of Nile tilapia Oreochromis niloticus reared in a limited water exchange system: The effect of different light levels

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.