Optimum dietary crude protein for culture of genetically improved farmed tilapia (GIFT), Oreochromis niloticus (Linnaeus, 1758) juveniles in low inland saline water: Effects on growth, metabolism and gene expression

Abstract

A 60-day feeding trial was conducted to determine the effects of dietary crude protein (CP) on growth, metabolism and growth-related gene expression in genetically improved farmed tilapia (GIFT), Oreochromis niloticus juveniles reared in low inland saline water (ISW) of 5 g/l salinity. Three hundred and fifteen fish (initial weight 2.68 ± 0.01 g) were distributed (15 fish/tank in triplicates) in seven experimental groups (20–50% CP with 5% increment) following completely randomized design (CRD) and fed with seven isolipidic (6%), isoenergetic (16.74 MJ digestible energy/kg) and hetero-nitrogenous (20–50 % CP) purified diets, respectively. Results showed inverse relation between percent weight gain (WG%) and feed conversion ratio which were significantly (p < 0.05) varied in overall, linearly and quadratically due to varying dietary CP. Overall and linear trend of protein efficiency ratio and overall, linear and quadratic trend of apparent net protein utilisation were significantly (p < 0.05) decreased with increasing dietary CP. Overall, linear and quadratic trend of whole body CP along with overall and quadratic trend of whole body lipid contents were significantly (p < 0.05) increased and decreased, respectively with increasing dietary CP up to 35 % and then decreased further. Whereas, overall, linear and quadratic trend of whole body ash content was increased significantly (p < 0.05) with increasing dietary CP. Overall and quadratic trend of protease activity in 30–45 % CP fed groups was significantly higher (p < 0.05) than 20 and 50 % CP fed groups; but overall and linear trend of amylase activity in 20−25 % CP fed groups was significantly higher (p < 0.05) than 35–50 % fed groups. Overall, linear and quadratic trend of hepatic glutamate pyruvate transaminase activity in 40 % CP fed group was significantly lower (p < 0.05) than that of 20−25 and 45–50 % CP fed groups and similar to 30–35 % CP fed groups. However, these trends of hepatic lactate dehydrogenase activity of 40–45 % CP fed groups was significantly lower (p < 0.05) than 20–30 % fed groups and similar to 35 and 50 % CP fed groups. Whereas, overall, linear and quadratic trend of hepatic malate dehydrogenase in 35 % fed group was significantly lower (p < 0.05) than 20–30 % CP fed groups and similar to other groups. The hepatic insulin like growth factor-1 (IGF-1) and IGF-1 receptor expression showed high correlation (r=0.92 and 0.90, respectively) with WG%. The optimum dietary protein requirement of GIFT juveniles at 5 g/l salinity in ISW was found to be 34.53–38.10% based on both broken-line linear and second-order polynomial regression with respect to WG% and hepatic IGF-1 expression.