Feeding graded levels of protein to Genetically Improved Farmed Tilapia (GIFT) juveniles reared in inland saline water: Effects on growth and gene expression of IGF[sbnd]I, IGF-IR and IGF-BPI

A 60-day feeding trial was conducted on Genetically Improved Farmed Tilapia (GIFT) juveniles in inland saline water (ISW) of 10 ppt salinity. Seven isoenergetic (400 kcal DE/100 g), isolipidic (6%) and hetero‑nitrogenous (20, 25, 30, 35, 40, 45 and 50% CP) experimental diets were formulated and prepared using purified ingredients. Three hundred and fifteen GIFT juveniles (average body weight 2.68 ± 0.01 g) were distributed in seven experimental groups viz., T20 (20% CP), T25 (25% CP), T30 (30% CP), T35 (35% CP), T40 (40% CP), T45 (45% CP) and T50 (50% CP) in triplicates following a completely randomized design (CRD) with the stocking density of 15 fish/tank (92 cm diameter X 45 cm height) with 200 l water volume. Fishes of different experimental groups were fed with respective experimental diets thrice daily at satiation level. Results indicated that dietary crude protein has significant (p < .05) overall, linear and also quadratic effect on final body weight, percent weight gain (WG%) and feed conversion ratio (FCR) with higher and lower value in T35, T40, T45 and T50 groups, respectively. The protein efficiency ratio (PER) and apparent net protein utilisation (ANPU%) values of fish were decreased with the increasing dietary CP level. The whole body crude protein content was affected linearly as well as quadratically (p < .05) with higher value in T30, T35 and T40 groups, whereas whole body ether extract content of the fish were significantly (p < .05) decreased with the increasing the dietary CP level. Among digestive enzymes, protease and amylase activities were significantly (p < .05) affected due to dietary crude protein linearly as well as quadratically. Though, the expression of insulin like growth factor binding protein I (IGF-BPI) did not show any defined trend, the expression of insulin like growth factor I (IGFI) and Insulin like growth factor I receptor (IGFIR) were highly correlated with WG% (r = 0.92 and r = 0.97, respectively). The expression of IGFIR was also correlated with IGF-I expression (r = 0.93). According to broken-line and second-order polynomial regression analysis in relation to WG% and expression of hepatic IGF-I gene, the optimum dietary crude protein levels of GIFT juveniles reared in inland saline water of 10 ppt salinity were found to be 37.37, 37.72, 40.92 and 38% crude protein with isoenergetic of 400 kcal DE/100 g diet, respectively.