Effects of dietary protein levels and rearing density on growth performance and stress response of Nile tilapia, oreochromis niloticus (L.)

This study was based on a 3 × 2 factorial design with three levels of dietary protein (25%, 35%, or 45%) and two rearing densities (D1=150 and D2 = 300 fish/m3). In this study, Nile tilapia, Oreochromis niloticus (L.), (1.8 to 2.5 g) was distributed into the aquaria at a rate of 150 vs. 300 fish/m3. Fish of each density were fed on a diet containing 25%, 35%, or 45% crude protein (CP). Fish were fed on one of the experimental diets till satiation twice daily, 6 days a week, for 10 weeks. At the end of the feeding trial, blood samples were taken to determine the different physiological variables. The growth parameters were positively affected by protein level and inversely affected by stocking density, but there was no effect of their interaction. Final body weight of tilapia, fed with different levels of protein diets, varied at D1 from 7.1 to 10.1 g and at D2 from 6.4 to 9.1 g. The best feed conversion ratio was obtained with 45% and 35% CP diets at lower density with insignificant difference. The highest values of protein efficiency ratio and protein productive value were obtained with 25% CP at stock densities of 150 and 300 fish/m3. Moisture and CP contents in the whole-fish body were insignificantly affected by both factors, while ash content was significantly affected by protein level and rearing density. Total lipid content was affected by protein level alone. All physiological variables including activities of aspartate aminotransferase, alanine aminotransferase, total protein, total lipids, and glucose in plasma were significantly affected by dietary protein levels and/or rearing density. The overall results presented here indicate that the best growth performance of Nile tilapia was obtained when the fish fed on the 45% CP diet and were reared at a stocking density of 150 fish/m3.