This 62-d research aimed to evaluate the effects of dietary
lysine levels (DLL) and salinity on growth performance and nutrition metabolism of genetically improved farmed tilapia (GIFT) juveniles (Oreochromis niloticus). Six diets with
lysine supplementation (1·34, 1·70, 2·03, 2·41, 2·72 and 3·04 % of DM) were formulated under different cultured salinities in a two-factorial design. The results indicated that supplemental
lysine improved the specific growth rate (SGR) and
weight gain (WG) and decreased the feed conversion ratio (FCR). Meanwhile, the fish had higher SGR and WG and lower FCR at 8 ‰ salinity. Except for moisture, the whole-body
protein,
lipid and ash content of GIFT were increased by 8 ‰ salinity, which showed that DLL (1·34 %) increased the whole-body fat content and DLL (2·41 %) increased whole-body
protein content. Appropriate DLL up-regulated
mRNA levels of
protein metabolism-related genes such as target of
rapamycin, 4EBP-1 and
S6 kinase 1. However, 0 ‰ salinity reduced these
protein metabolism-related genes
mRNA levels, while proper DLL could improve glycolysis and gluconeogenesis
mRNA levels but decrease lipogenesis-related genes
mRNA levels in liver. 0 ‰ salinity improved GLUT2,
glucokinase and G6 Pase
mRNA levels; however,
sterol regulatory element-binding protein 1 and
fatty acid synthase mRNA levels were higher at 8 ‰ salinity. Moreover, 8 ‰ salinity also increased plasma total
protein and
cholesterol levels and decreased
glucose levels. These results indicated that the recommended range of
lysine requirement under different salinity was 2·03-2·20 % (0 ‰) and 2·20-2·41 % (8 ‰) and 8 ‰ salinity resulted in higher
lysine requirements due to changes in the related nutrient metabolism, which might provide useful information for designing more effective feed formulations for GIFT cultured in different salinity environment.