Abstract
A feeding trial was conducted to assess the feasibility of supplementing taurine in soy-based diets for juvenile starry flounder Platichthys stellatus. The basal diet (Crude protein 66.5%, crude lipid 8.5%) was supplemented with 0 (control), 0.5%, 1.0%, 1.5%, 2.0% and 2.5% taurine to formulate six test diets. Each diet was fed to 40 juvenile fish (22.25 g) in triplicate tanks (120 L) attached to a sea water circulation-system. Fish were fed twice daily by hand to apparent satiation during the 56-d trial. At the end of the trial, fish were counted and weighed for the analyses of growth performance, diet utilization and survival after a 24-h fast. Blood, intestines and muscles were collected for the analyses of serum oxidation resistance, digestive enzymes and body compostion. Livers were collected from the remaining fish at 4 h post-feeding for metabolic enzymes analyses. The results showed that fish fed diets supplemented with 1.0-2.5% taurine grew from 22.25-22.26 g to 47.88-50.40 g with higher average weight gain (25.62-28.12 vs 23.07 g ), specific growth rate (1.37-1.46 vs 1.27%/d ), feed intake (1.04-1.06 vs 1.00%/d), protein efficiency (2.50-2.61 vs 2.44) and lower feed conversion rate (0.84-0.83 vs 0.89) than the control treatment. Diets supplemented with 1.5-2.5% taurine significantly elevated the activities of pepsin (2.47-2.55 vs 2.22, U mg-1 prot), trypsin of distal intestine(14.55-15.24 vs 11.94, U mg-1 prot), hepatic glucokinase (126.62-129.42 vs 105.56, U mg-1 prot) and fatty acid synthetase (125.56-136.89 vs 108.45, U mg-1 prot). All diets supplemented with taurine increased the activities of lipase (32.23-36.67 vs 29.53, U g-1 prot) and trypsin (35.85-37.89 vs 33.54, U mg-1 prot) of proximal intestine, hepatic aspartate transaminase (736.990-832.38 vs 699.24, U mg-1 prot), alanine aminotransferase (477.40-551.86 vs 373.97, U mg-1 prot) and glycogen synthase (2.16-2.59 vs 1.97, U mg-1 prot), as well as serum superoxide dismutase (4.33-4.59 vs 4.07, U mg-1 prot ) and glutathione peroxidase (42.23-50.25 vs 39.17, mol mg-1 prot). Therefore, taurine supplementation benefits juvenile starry flounder growth, digestion, nutrients metabolism and oxidation resistance. The optimal taurine requirement for starry flounder is 1.75%, and the recommended supplementation level is at least 1.6% for maximizing growth of fish fed a low-fishmeal diet (13.6%).