Study On The Main Nutrient Elements Requirement Of The Red Spotted Grouper(Epinephelus Akaara)

The red spotted grouper(Epinephelus akaara) is an important commercial grouper species, which is regarded as an emerging aquaculture species in China. Our study were conducted to investigate the dietary protein requirement of juvenile red spotted grouper, effects of dietary fish oil replacement by soybean oil on growth performance, body composition and body fatty acid composition of red spotted grouper and effects of different dietary carbohydrate levels on growth, feed utilization and body composition of juvenile grouper. The results are summarized as follows:1. Dietary protein requirement of juvenile red spotted grouperAn 8-week feeding trial was conducted to assess the dietary protein requirement of juvenile red spotted grouper. Six semi-purified diets were formulated with varying crude protein(CP) level(ranging from 32.20% to 61.85%). Each diet was given to triplicate groups(20 fish per group, 7.88 ± 0.04 g per fish). The growth of juvenile E. akaara were significantly affected by the different dietary CP levels(P<0.05). The results of feeding trial showed that fish fed diet with 50.45% CP exhibited the highest weight gain(WG) value. Both quadratic and linear responses were observed on the feed efficiency(FE) and protein efficiency ratio(PER) as the level of dietary CP increased(P<0.05). The lowest feed efficiency(FE) was noted in diet containing 32.20% CP treatment. Moreover, daily feed intake(DFI) was highest in fish fed 32.20% CP diet. There were quadratic and linear responses in viscerosomatic indexes(VSI) and hepatosomatic index(HSI) values with dietary CP increased(P<0.05). Fish fed 32.20% CP diet obtained the highest plasma aspartate transaminase(AST) and alanine transaminase(ALT) activities. Fish fed a diet with the lowest CP obtained relatively lower energy retention(ER), lipid retention(LR) and nitrogen retention(NR). The protein content in the dorsal muscle and liver were significantly affected by the different CP levels(P<0.05). Based on the second polynomial regression analysis of WG, the dietary protein requirement for E. akaara was estimated to be 50.83%.2. Effects of dietary fish oil replacement by soybean oil on growth performance, body composition and body fatty acid composition of red spotted grouperA 6-week feeding experiment was conducted to investigate the effects of dietary fish oil replacement by soybean oil on growth performance, body composition and body fatty acid composition of red spotted grouper(E. akaara). Four isonitrogenous(crude protein 50.26%-51.03%) and isoenergetic(20.82 k J/g-21.01 k J/g) experimental diets were formulated with soybean oil replacing 25%, 50%, 75% and 100% fish oil, respectively. Grouper were randomly distributed into 4 groups with 3 replicates per group and 20 fish per replicate. The results showed that fish fed diet containing 100% soybean oil had relatively low growth performance(P>0.05). With the increasing of soybean oil content, FCR declined and then increased, and the highest value(1.96±0.42) was observed in the total replacement of fish oil by soybean oil treatment. While the PER value showed an inverse tendency with FCR. The total replacement of fish oil by soybean oil treatment showed a relatively high HSI value, but no significant different was observed between each treatments(P>0.05). Whole body moisture and protein were not significant affected by soybean oil replacement level(P>0.05), while grouper fed with the 25% soybean oil group showed the highest whole body lipid content(P<0.05). Body fatty acid composition showed appositive correlation with dietary fatty acid composition. With the increasing of dietary soybean oil level, the content of C18:2n6 significant increased(P<0.05). And the lowest C20:4n6, C20:5n3 and 22C:6n3 content was observed in 100% soybean oil group. These results suggest that the level of soybean oil to replace fish oil should be lower than 75%.3. Effects of different dietary carbohydrate levels on growth, feed utilization and body composition of juvenile grouper.Six isoproteic and isolipidic diets were designated to investigate the effects of dietary carbohydrate level(from 0% to 30%) on growth performance, feed utilization and body composition of juvenile grouper E. akaara(initially weighing, 7.79±0.01 g /fish). Each diet was given to triplicate groups(20 juvenile grouper per cage) for 8-weeks. The WG and FBW were quadratically and linearly related to the increase of dietary carbohydrate level(P<0.05). The WG values increased with increasing dietary carbohydrate from 0% to 6% while at higher levels WG values decreased. FCR, PER and DFI were found to be not significantly related to the dietary carbohydrate(P > 0.05). As the level of dietary carbohydrate increased both quadratic and linear responses were observed on the intraperitoneal fat ratio(IPF). The whole body and muscle lipid content were relatively lower in fish fed the 30% dietary carbohydrate diet compared with the other treatments. Liver glycogen content generally increased with increasing dietary carbohydrate levels. ER and LR decreased significantly with increasing dietary carbohydrate levels. Based on second-order polynomial regression analysis of WG, the optimal dietary carbohydrate level for juvenile E. akaara is estimated to be 7.40 %.