The response of thick-lipped grey mullet, Chelon labrosus (Risso), to diets of varied protein-to-energy ratio

Six experimental diets were designed containing two lipid levels (approximately 12% and 24%) and three protein levels (approximately 38%, 49% and 59%) with protein-to-energy ratios ranging from 19.72 to 29.83 mg protein kJ-¹ gross energy. At both lipid levels, an increase in the protein content of the diet was achieved by decreasing the carbohydrate input. After 84 days of feeding to appetite, the fish (juvenile thick-lipped grey mullet, Chelon labrosus (Risso)) fed the high-oil diets of low and medium protein content were significantly larger than those fed low-lipid diets containing similar amounts of protein. The greatest weight gain and optimum apparent net protein utilization (ANPU) were recorded for the fish fed the diet with a P:E ratio of 19.72 mg protein kJ-¹ gross energy. At both lipid levels, increasing dietary protein supplementation led to a decrease in voluntary feed consumption and ANPU. Whole-body lipid appeared to increase in response to a higher dietary oil component. At low levels of dietary lipid, the carcass protein content increased in response to elevated protein supply. This trend was less obvious at the higher level of lipid supplementation. Hepatic glycogen deposition was significantly lower amongst the fish fed the low-carbohydrate diets at both levels of supplemental oil. A significant increase in hepatosomatic index was also recorded which was not directly correlated with either dietary carbohydrate or protein:energy level. It can be concluded from the present experiment that the optimum protein-to-energy ratio for C. labrosus juveniles is in the order of 19.72 mg protein kJ-¹ gross energy when fed the present diets containing 37.9% crude protein and 22.8% lipid. Additionally, for this species, lipid was seen as a more effective source of non-protein energy than a corn starch/dextrin mixture.