Protein-sparing capability of dietary lipid in herbivorous and omnivorous freshwater finfish: a comparative case study on grass carp (Ctenopharyngodon idella) and tilapia (Oreochromis niloticus x O-aureus)

Two, 8-week feeding trials were conducted to compare protein-sparing capability of dietary lipid in herbivorous grass carp (Ctenopharyngodon idella) and omnivorous tilapia (Oreochomis niloticus x O. aureus). Utilizing a 2 x 3 factorial design, experimental diets containing two levels of crude protein (380 and 250 g kg-1) and three levels of lipid (0, 40 and 100 g kg-1) were formulated for use in both feeding trials. Growth performances showed better response of both fish fed 380 g kg-1 protein diet than those fed 250 g kg-1 protein diet. Despite the dietary protein level, weight gain (WG), specific growth ratio (SGR), feed conversion ratio (FCR) and protein efficiency ratio were much higher (P < 0.05) for grass carp fed 40 g kg-1 lipid diet than those fed 100 g kg-1 lipid diet; however, there were no significant differences in tilapia fed the two diets. The feed intake of grass carp fed lipid-free diet was the lowest, but it tended to decrease with increase in dietary lipids in tilapia. Lipid retention (LR) was negatively correlated with dietary lipid concentration of both fish. Viscerosomatic index (VSI), hepatosomatic index (HSI), intraperitoneal fat ratio (IPF) and whole-body and liver lipid content positively correlated with dietary lipid concentration of both fish. Plasma parameters and liver enzymes activities were also positively correlated with dietary lipid concentration of both fish. Liver lipid contents were higher and enzymes activities were lower in grass carp when compared with tilapia. These data suggested that there was no evidence of a protein-sparing effect of dietary lipids in grass carp. Tilapia has relatively higher capacity to endure high dietary lipid level compared to grass carp.