Effects of dietary phosphorus level on growth, body composition, liver histology and lipid metabolism of spotted seabass (Lateolabrax maculatus) reared in freshwater

The present study was conducted to determine the effects of dietary phosphorus (P) levels on growth performance, body composition, liver histology and enzymatic activity, and expression of lipid metabolism-related genes in spotted seabass (Lateolabrax maculatus). Seven diets were prepared to contain available P levels of 0.48% (the control group), 0.69%, 0.89%, 1.10%, 1.28%, 1.51% and 1.77% and feed fish (4.26 ± 0.03 g) to satiety twice daily for 10 weeks. Significantly higher weight gain and specific growth rate were recorded at P levels of 0.69%–1.51% compared to the control group. Feed conversion ratio decreased with increasing P levels up to 0.89% and increased thereafter. The lowest liver lipid content, viscerosomatic index and lipid content of whole-body were obtained in the 0.89%-P group among dietary treatments. P and calcium (Ca) contents in whole body were increased, while liver triglyceride and cholesterol contents were decreased with increasing dietary P levels from 0.48% to 1.77%. The highest activity of hepatic lipase was recorded in the 1.10%-P group among dietary treatments. Compared to the control group, 1.10% P enhanced the proportion of HUFA and reduced the proportion of SFA and MUFA. The histological observations showed that P deficiency (0.48%) led to the vacuolization of hepatocytes and increased number of lipid droplets. Meanwhile, overall liver tissue structure was improved when P level increased to 1.28%. Compared to the control group, expression of lipid metabolism-related genes such as FAS, ACC-2 and SREBP-1 was decreased at 0.89%–1.10% P group while an opposite trend was observed in the expression of PPARa2 and CPT-1 genes. The current study showed that 0.89% dietary P levels could promote growth performance of spotted seabass and reduce lipid accumulation in the liver. A broken-line regression analysis based on weight gain showed that the optimum dietary P level (available P) for juvenile spotted seabass reared in freshwater was 0.72%.