Culture salinity alters dietary protein requirement, whole body composition and nutrients metabolism related genes expression in juvenile Genetically Improved Farmed Tilapia (GIFT) (Oreochromis niloticus)

Abstract

An 8-week experimental trial was designed to explore the effects of dietary protein level (DPL) and salinity on the growth performance and physiological parameters of genetically improved farmed tilapia (GIFT) juveniles (Oreochromis niloticus). Six diets were formulated with different dietary protein levels (18.20%, 24.32%, 30.60%, 36.42%, 43.27% and 49.49%) and two salinity levels (0and 8‰) in a 6 × 2 factorial design. 720GIFT juveniles (initial weight 3.25 g) were distributed into 36 plastic tanks (180 L) and fed each of the diets four times daily to satiation. The results suggested that the growth performance was increased significantly with increasing DPLs up to a certain level (36.42%) in freshwater and brackish water. The whole-body lipid content of the fish decreased significantly with increasing salinity, while the protein content increased (P < 0.05). However, the protein content increased significantly and the lipid, moisture and ash contents were not significantly affected by the increase in DPL. With the increasing DPL at two salinities, the expression levels of hepatic insulin-like growth factor-1 increased significantly, reaching a maximum at 36.42% DPL. With the increasing DPL at the two salinity levels, the expression level of oligopeptide transporter 2 mRNA significantly decreased (P < 0.05). The target of rapamycin, 4E binding protein 1, ribosomal protein S6 and oligopeptide transporter 1β mRNA levels were not significantly affected by the DPL, but they increased significantly with an increase in salinity (P < 0.05). Regarding glucolipid metabolism, glycogen synthase, glucokinase and glucose transporter 2 mRNA levels decreased with an increase in DPL (P < 0.05). The fatty acid synthase and glucokinase mRNA levels decreased with an increase in salinity, while glycogen synthase and glucose transporter 2 mRNA levels increased (P < 0.05). The phosphoenolpyruvate carboxykinase mRNA level was not significantly affected by the DPL, but it increased with increasing salinity (P < 0.05). For the feed conversion ratio and weight gain rate, the broken-line model analysis showed that the optimal protein requirements at 8‰ salinity were higher than those at 0‰ salinity and the protein requirement were 37.70%, 37.86% (8‰) and 33.70%, 38.21% (0‰), respectively. GIFT juveniles exhibited improved glycogen synthesis at 8‰ salinity and fat deposition at 0‰ salinity.