Effects of dissolved oxygen and fish size on Nile tilapia, Oreochromis niloticus (L.): growth performance, whole-body composition, and innate immunity

The present study was carried out to investigate effects of dissolved oxygen (DO) and fish size (FS) on growth, feed utilization, whole-body composition, and innate immunity of Nile tilapia, Oreochromis niloticus (L.). The experiment was based on a bifactorial design with three levels of DO (low: 0.1-1.5, medium: 2.5-3.0, and normal: 6.0-6.5 mg/L) and two sizes (3.7 and 12.9 g) within quadruplicates. Fish density was kept at 2.1 g/L, i.e., 50 or 15 fish per 90-L aquarium for small or large fish, respectively. Fish were fed up to satiation twice a day for 12 weeks on a 35 % crude protein diet. After the feeding trial, fish were artificially infected by pathogenic bacteria Aeromonas hydrophila for 10 days. It was noticed that DO and FS significantly affected fish growth, feed utilization, whole-body composition, and innate immunity. However, fish growth and feed intake were adversely affected by low DO. Additionally, smaller fish consumed less feed and exhibited better growth than the larger ones. Feed conversion ratio in case of small fish was better than that in case of larger one. Regarding fish body composition, moisture content was affected by FS only, while crude protein, lipid content, and total ash were significantly affected by DO level, FS, and their interaction. It is also noticed that larger fish tolerated low DO better than the small ones where values of nitro blue tetrazolium and lysozyme activity of large fish were better than small one. Additionally, innate immunity increased as DO levels increased. The total fish mortality after 10 days post-challenge was adversely affected by DO, and the highest mortality was observed at low DO in smaller fish, whereas no mortality was observed at normal DO in larger fish. These results indicate that fish growth, feed utilization, and innate immunity were adversely affected by low DO; meanwhile, smaller fish showed better performance than larger ones at normal DO.