Abstract:A study was conducted to investigate the effects of the dietary protein level on the growth, ammonia-nitrogen excretion, and amino acid accumulation in the muscle of juvenile turbot (Scophthatmus maximus L.) in a recirculating aquaculture system. One hundred and forty-four healthy juveniles with initial body weight of (145.08±0.56) g were randomly divided into four groups each of which had three replicates of 12 fish. Groups Ⅰ–Ⅳ were fed with diets containing 41%, 46%, 50%, and 55% protein respectively. The fishmeal content was the only variable in the diet. The results were shown as follows: (1) As the protein level went up, the weight gain rate (WGR) increased first rapidly and then slowly, while feed conversion rate (FCR) showed a reversed trend. The WGRs of Groups Ⅲ and Ⅳ were significantly higher than that of Groups Ⅰ and Ⅱ by 18.48%–65.95%, and compared to Group Ⅰ the FCRs of Groups Ⅲ and Ⅳ were lower by 25.64% and 28.21% respectively (P<0.01). (2) Ammonia excretion rate (AER) of juveniles cyclically varied between day and night, and there were two peaks appearing 6–8 h post feeding. There was a positive correlation between AER and the dietary protein level. The AER of Group Ⅳ was significantly higher than that of other groups by 17.95%–35.07% (6 h post feeding in the morning, P<0.01) and 14.25%–31.27% (8 h post feeding in the evening, P<0.05). The AER of juveniles increased sharply when the dietary protein level was above 50%. (3) The content of 17 amino acids in muscle was higher than 65 mg/100mg in each group, and it rose along with the increase in the dietary protein level (P>0.05). The contents of total amino acids, essential amino acids, and flavor amino acids in the muscle of Groups Ⅲ, Ⅳ were not significantly higher than those of other groups (P>0.05). There was no difference in the amino acid composition between groups (P>0.05). The results indicated that the growth performance of juveniles might not be improved when the dietary protein level was above 50% at which level the AER was markedly increased, and that the content and composition of amino acids in the muscle might not be altered by the dietary protein level. Therefore, the optimal protein level of juvenile turbots may be 45%–50%.
