Effects of DL-methionine-supplemented dietary soy protein concentrate on growth performance and intestinal enzyme activity of gilthead sea bream (Sparus aurata L.)

The present study evaluated the effects of partial fish meal (FM) replacement by a soy protein concentrate (SPC) on growth performance and digestive enzyme activity of gilthead sea bream. Isonitrogenous (45 % crude protein) and isoenergetic (23 kJ g−1 gross energy) diets with 20 % (SPC20), 40 % (SPC40) and 60 % (SPC60) of SPC inclusion were evaluated against a diet containing FM as the sole protein source. Diets were allocated in triplicate groups of 26 g fish (8 kg/m3/tank) and administered for 3 months. Growth, feed utilization, carcass proximate composition, serum cholesterol levels and digestive enzyme activities including <alpha>-amylase, maltase, leucine aminopeptidase and alkaline phosphatase were evaluated. At the end of the trial, final weight (74.5 g) and feed efficiency (FE, 0.78) indices were significantly lower in the SPC60 group compared with the FM (93.2 g and 0.95, respectively). Similarly, body lipid content (9.3 %) and serum cholesterol levels (119 mg/dl) were significantly lower in fish fed the SPC60 diet compared with the FM group (12.3 % and 243 mg/dl, respectively). Concerning enzyme activities, maltase and leucine aminopeptidase were significantly decreased in the pyloric caeca and the anterior section of the intestine in fish from the SPC60 group, while <alpha>-amylase activity in the anterior and posterior sections (P < 0.05) was significantly increased in the SPC40 and SPC60 groups. Overall, increased dietary inclusion of SPC in gilthead sea bream resulted in a gradual reduction in growth performance, but substantial reductions were not observed until the replacement of FM by SPC exceeded 40 %.