Soybean and lupin seed meals as protein sources in diets for gilthead seabream ( Sparus aurata): nutritional and histological implications

Abstract

The use of vegetable protein sources in diets for freshwater fish has been studied in more detail than for marine fish species. Two experiments were conducted to compare the effect of the partial substitution of fish meal by two different vegetable protein sources, soybean and lupin seed meals. Mean feed intake and growth were not significantly influenced by type or level of plant protein in the diet. Feed utilization indexes such as feed efficiency, protein efficiency ratio and protein productive values were not significantly affected by the type of plant protein in the diet, although a general reduction of these values was observed with increased inclusion of soybean meal. Histological studies showed an increased deposition of lipid and decreased glycogen deposits in the liver with increased levels of dietary soybean meal. Protein digestibility coefficients for lupin seed meal diets were similar to the control and 10% higher than those for the soybean meal diets. A significant reduction in trypsin activity was observed in fish fed the lupin seed meal diets, and for soybean meal diets when the substitution level reached 30%. Diets including plant protein showed a higher peak of ammonia excretion rate, which appeared 2 h later than that of the fish meal diet. Highest values of dissolved ammonia were registered in fish fed a soybean meal-based diet. These results suggest that properly treated lupin meals could be an important alternative dietary protein source for gilthead seabream.