Protein-sparing effect of dietary lipid: Changes in growth, nutrient utilization, digestion and IGF-I and IGFBP-I expression of Genetically Improved Farmed Tilapia (GIFT), reared in Inland Ground Saline Water

Protein-sparing by lipid helps in conserving protein exclusively for growth with minimal inevitable protein breakage. It will subsequently reduce feed cost and nitrogenous pollutants in the habitat. In this context, an experiment of 60 days period was conducted to study the proteinsparing effect of lipid in the diet of Genetically Improved Farmed Tilapia (GIFT) (Oreochromis niloticus L) fingerlings reared in inland ground saline water (IGSW) at 10 g L-1 salinity. The experiment followed 4 x 2 factorial design with eight purified diets of four crude protein (CP) levels (30%, 35%, 40% and 45%), each with two lipid levels (6% and 10%). GIFT fingerlings of uniform size (3.00 +/- 0.01 g) were acclimatized and distributed into eight treatment groups in triplicates corresponding to each of the prepared diets and fed to satiation three times daily. The results showed significantly (p < 0.05) higher percent weight gain (WG, 693.26%), apparent net protein utilization (ANPU, 34.03), protein efficiency ratio (PER, 2.35) and improved digestive enzymes (protease, amylase, lipase) activity with lower feed conversion ratio (FCR) for diets containing 35-45% CP and 10% lipid. According to two-way ANOVA, high dietary lipid (10%) resulted highest WG% (678.95), PER (2.02), ANPU (30.67) and lowest FCR (1.31) values. Increased whole body CP (15.54%) and lipid (7.61%) contents were observed in the fish fed high lipid (10%) diets. The viscerosomatic and hepatosomatic index also increased with increasing dietary lipid. The dietary CP levels have not significantly affected the body composition except total ash content. The highest insulin like growth factor-I (IGF-I) gene expression was observed in the liver of fish fed 35% CP with 10% lipid. Results revealed that increasing dietary lipid exhibited enhanced protein-sparing effect in GIFT fingerlings. Thus, the study concludes that a diet containing 35% CP and 10% lipid is optimal for GIFT fingerlings reared in IGSW of 10 g L-1 salinity.