Lipid contents in farmed fish are influenced by dietary DHA/EPA ratio: A study with the marine flatfish, tongue sole (Cynoglossus semilaevis)

The present study was conducted to investigate the effects of dietary DHA/EPA ratio on lipid accumulation in farmed fish, with an 80-day feeding trial on tongue sole Cynoglossus semilaevis, a typical marine flatfish. Six experimental diets differing basically in DHA/EPA ratio, i.e., 0.61, 1.02, 1.46, 1.91, 2.32, and 2.75, were randomly assigned to 18 tanks of juvenile tongue sole. Fish were reared in a flow-through seawater system and fed to apparent satiation twice daily. Compared to very low (0.61) or high (2.32 and 2.75) dietary DHA/EPA levels, moderate dietary DHA/EPA ratios of 1.02 and 1.46 induced higher lipid accumulation in fish flesh and whole fish. Although also inducing increased liver lipid accumulation, the moderate dietary DHA/EPA ratios did not compromise the fish health in terms of growth performances and hematological parameters. The fatty acid profile of fish tissues reflected closely those of the diets, but DHA had more efficient deposition in fish flesh than EPA. Excess EPA or DHA in diets down-regulated the mRNA expression of lipogenic genes in fish tissues, and DHA seemed to be more efficient than EPA in up-regulating gene expression of β-oxidation-related proteins. Both esterification and hydrolyzation of triglycerides in fish tissues seemed to be more active in groups with intermediate DHA/EPA ratios (1.02 and 1.46). The present study provided a new way to modulate the nutritive quality of farmed fish, as well as to make better use of precious LC-PUFA resources.