EFFECTS OF DIETARY PROTEIN LEVEL AND FEEDING FREQUENCY ON THE GROWTH PERFORMANCE,BODY COMPOSITION AND PROTEIN METABOLISM OF JUVENILE LARGE YELLOW CROAKERS,PSEUDOSCIAENA CROCEA R.

An 8-week 3×2 two-factorial experiment was conducted to examine effects of dietary protein level(40%,45% and 50%) and feeding regimen(twice daily and once daily) on the growth,body composition and protein metabolism of juvenile large yellow croakers [initial average weight of(13.64±0.18) g] cultured in floating net cages.Three isolipidic(crude lipid 12%) practical diets were formulated to provide three dietary protein levels of 40%,45% and 50%. Feeding frequencies were twice daily(05:00 and 17:00) and once daily(05:00). Three cages(replicates) were randomly allotted to one treatment with combination of dietary protein level and feeding frequency in a complete randomized design.Each cage was stocked with 50 juvenile large yellow croakers.Fish were carefully fed to apparent satiation by the same person during the feeding trial.The temperature ranged from 26.5 to 30.5°C,salinity from 29‰ to 33‰,and the dissolved oxygen was approximately 7 mg/L during the experiment period.At the termination of the feeding experiment,the total number and mean body weight of the fish in each cage were measured.Five fish were randomly sampled from each cage for individual proximate composition analysis.Activities of alanine aminotransferase(ALT) and aspartate aminotransferase(AST) in serum and liver were determined.The results showed that survival rates,ranging from 90.00% to 94.00%,were not significantly affected by feeding frequency or dietary protein level(P 0.05). Compared with fish fed twice daily,fish fed once daily had significantly lower final weight and specific growth rate(SGR),but significantly higher feed conversion ratio(FCR) at 40% and 45% dietary protein level(P0.05). The specific growth rate of fish fed the diet with 45% protein was significantly higher than that fed 40% dietary protein(P0.05),but had no significant difference with that in group with 50% dietary protein when the feeding regimen was two times daily(P0.05). However,for the regimen of once daily,SGR of fish fed the 50% protein diet was significantly higher than those with 40% and 45% dietary protein(P0.05). Regardless of feeding frequency,higher dietary protein level lead to higher whole body moisture,body protein contents,and significantly lower body lipid contents.Dietary protein level and feeding frequency both had no significant effect on Hepatosomatic index(HSI),Viscerosomatic index(VSI),activities of ALT and AST of serum(P0.05). Higher activity of liver ALT and AST were observed in higher dietary protein level at each feeding frequency.Feeding frequency had no significant effect on liver ALT and AST(P0.05). Regardless of feeding frequency,higher dietary protein level lead to higher liver ALT and AST.Significant interactions between dietary protein level and feeding frequency were found on the growth and FCR of juvenile large yellow croakers(P0.05),but not on the HSI,VSI,condition factor(CF),body composition and the activity of ALT and AST in serum and liver(P0.05). This research indicated that 45% of dietary protein level should be used for juvenile large yellow croakers fed two times daily.However,when the feeding regimen was once daily,dietary protein level should be increased to 50% in order to obtain the maximum growth rate.