A study on the effects of culture salinity and dietary protein levels on growth, nutrient metabolism, and immune response of juvenile grass carp

An 8-week experiment was designed to study the effects of culture salinity and dietary protein levels (DPLs) on the growth, nutrient metabolism, and immune response of juvenile grass carp ( Ctenopharyngodon idella ). Three diets were formulated with different DPLs (31.19 %, 36.03 %, and 40.18 %) and two salinity levels (0 ‰ and 7 ‰) in a 3 × 2 factorial design. 360 grass carp juveniles (initial weight 3.72 g) were distributed into 18 tanks (180 L). The results presented that the growth performance was increased significantly by 36.03 % DPLs at 0 ‰ salinity, and 40.18 % DPLs at 7 ‰ salinity, respectively. Regarding nutrient metabolism, at 0 ‰ salinity, 36.03 % DPL up-regulated the mRNA expression levels of igf-1 , s6k , and fas , and 40.18 % DPL up-regulated the mRNA expression levels of gk ; at 7 ‰ salinity, compared to the 31.19 % DPL group, 36.03 % and 40.18 % DPLs significantly up-regulated the mRNA expression levels of s6k1 . Furthermore, compared to 0 ‰ salinity, the gk , g6pase , fas , and cpt1 mRNA expression levels were significantly up-regulated at 7 ‰ salinity. Regarding antioxidant capacity, at 0 ‰ salinity, 36.03 % DPL obtained the maximum CAT and SOD activities, and the minimum MDA content; at 7 ‰ salinity, 36.03 % and 40.18 % DPLs obtained the maximum CAT activities. Furthermore, compared to 0 ‰ salinity, the CAT activities and GSH contents were increased, and the MDA contents were decreased significantly at 7 ‰ salinity. Regarding immune response, at 0 ‰ salinity, the stat3 mRNA expression level was significantly decreased in the 36.03 % DPL group, and the nfκb and stat3 mRNA expression levels were significantly decreased in the 36.03 % and 40.18 % DPLs at 0 ‰ salinity. Compared to 0 ‰ salinity, the stat3 mRNA expression levels were significantly down-regulated at 7 ‰ salinity. Furthermore, appropriate protein levels (36.03 % and 40.18 %) and 7 ‰ salinity also significantly improve the immune response. • At 0 ‰ salinity, 36.03 % DPL could improve the growth performance of juvenile grass carp and reduce the FCR. • At 7 ‰ salinity, a higher DPL (40.18 %) is more suitable for the growth of juvenile grass carp. • At 7 ‰ salinity, juvenile grass carp had a more vigorous glycolipid metabolism and showed better antioxidant capacity and immune response.