Studies on the utilization of dietary protein and energy by gilthead sea bream (Sparus aurata L.)

Abstract

A series of nutritional experiments were carried out to evaluate the utilization of dietary protein and energy by gilthead sea bream (Sparus aurata L.). Optimum dietary protein requirements and the sparing effect of dietary lipid upon protein were investigated for different fish sizes. Optimum dietary protein requirements were 55% and 42% for 0.8g fry and 60g juveniles, respectively. In 5g fingerlings, dietary protein levels could be reduced from 52% to 45% when lipids were increased from 9% to 15%, best protein to energy ratio (P;E) being 21.9 g protein/MJ of gross energy. These results suggest that dietary protein level could also be reduced from 55% to 50% in fry diets containing 15% dietary lipids. The optimum proportions of dietary protein and lipid level found for 90g growers were 54% and 11%, respectively, the high requirements for protein could be due to an increased protein demand during sexual maturation for gonad development.

The increase of dietary lipids produced an increment in carcass lipid deposition, both in visceral and non-visceral tissue, but these levels were in all cases well below reported carcass lipid contents in wild S.aurata in the Mediterranean. When different dietary carbohydrate sources were evaluated with 42g S.aurata juveniles, the ability of fish to digest carbohydrate was limited in general. Apparent Digestibility Coefficient (ADC) values being lower than 85% regardless of carbohydrate source. Increased amounts of fibre in diets produced lower protein and lipid digestibility, this effect being even more pronounced on carbohydrate digestibility. Corn starch was the most effective carbohydrate source in terms of "energy-yielding", although wheat bran appeared to be a suitable fedstuff for practical diets.

Increased dietary lipid levels supported increased metabolizable energy (ME), recovered energy (RE), energy retention efficiency (ERE) and better protein conversion in 46g juveniles, indicating improved utilization of protein and energy.