Nutrition Requirement Of Dietary Protein And Optimal Replacement Of Fish Meal Protein By Rapeseed Meal And Corn Gluten Meal In Cobia, Rachycentron Canadum L.

Feeding trials were conducted in seawater floating cages (1.5m×1.5m×2.5m) toinvestigate the effects of protein levels, partially replacing protein from fish meal (FM)with rapeseed meal (CM) and corn gluten meal (CGM), supplemental taurine andcholesterol in diets with high plant protein level on growth, digestion and proteinmetabolism of cobia, Rachycentron canadum L. Results of the study are presented asfollows:I. Six isoenergetic (20kJ g-1) and isolipidic (10%crude lipid) diets were formulatedusing fish meal and casein as protein sources to contain graded levels of protein(35.8,41.0,46.1,51.0,55.7and60.4%). Cobia (initial weight16.7g and246.8)were hand fed twice daily to apparent satiation. The results showed that withincreasing dietary protein levels, specific growth rate (SGR) and feed efficiencyratio (FER) increased, but feed intake (FI) decreased. For smaller size cobia, SGRof fish fed the diet containing51.0%protein was significantly higher (P<0.05)than that of fish fed diets containing35.8,41.0and46.1%protein, but was notdifferent from that of fish fed diets containing55.7and60.4%protein. For largersize cobia, SGR of fish fed the diet containing46.1%protein was significantlyhigher (P<0.05) than that of fish fed diets containing35.8and41.0%protein, butwas not different from that of fish fed diet containing51.0,55.7and60.4%protein.For smaller size cobia, protein efficiency ratio (PER) and protein productive value(PPV) of fish fed diets containing41.0,46.1and51.0%protein were significantlyhigher than other groups. However, for larger size cobia, there were no significantdifferences in PER and PPV among dietary treatments. For smaller size cobia,whole body protein increased with increasing dietary protein levels (P<0.05), butwhole body lipid decreased (P<0.05). However, for larger size cobia, with increasing dietary protein levels, whole body protein and lipid all increased(P<0.05). For smaller size cobia, fish fed diets containing51.0and60.4%proteinhad significantly higher relative expression levels of IGF-I in liver. However, forlarger size cobia, there was no significant difference in relative expression of IGF-Iin liver among dietary treatments (P>0.05). For two size cobia, there were nosignificant differences in relative expression levels of TOR in liver and dorsalmuscle at different protein levels (P>0.05). Based on SGR, broken-line regressionanalysis showed that the optimum dietary protein levels were52.5and51.5%, anddigestible protein level were45.2%and44.1%for cobia with initial weight16.7gand246.8g, respectively.II. A60d feeding trial was conducted to investigate the effects of dietary rapeseedmeal (RM) levels on growth, survival, digestion and protein metabolism of cobia(initial average weight:94.6±2.8g). Five isonitrogenous (crude protein:45%)and isoenergetic (gross energy:20kJ g-1) practical diets were formulated byreplacing0(the control),12.5,25,37.5and50%of protein from fish meal withRM. The results showed that with increasing dietary RM levels, feed intake (FI),specific growth rate (SGR), protein productive value (PPV), and feed efficiencyratio (FER) decreased. The FI was significantly lower compared to the controlgroup (P<0.05) with37.5%or more substitution level. Fish fed the diet with25%or more protein from RM had significantly lower SGR, FER and PPV than thecontrol group (P<0.05). Apparent digestibility coefficients (ADCs) of dry matter,protein and energy significantly decreased with increasing dietary RM level. Fishfed the diet with25%or more protein from RM had significantly lower ADCs ofcrude protein and crude lipid, and fish fed the diet with37.5%or more proteinfrom CM had significantly lower ADC of dry matter (P<0.05). Whole bodymoisture content increased, and whole body crude protein and crude lipid contentsdecreased with increasing dietary RM level. Fish fed the diet with50%proteinfrom RM had significantly higher whole body moisture content and significantlylower whole body crude protein and crude lipid contents (P<0.05). Moisture, crudeprotein and crude lipid contents in muscle had similar trends with those in whole body. Fish fed the diet with50%protein from RM had significantly lower activityof AST in liver, total protein in plasma, and relative expression of IGF-I in liver,but significantly higher expression level of IGF-I in muscle than the controlgroup.There were no significant differences in relative expression levels of TOR inliver and dorsal muscle at different RM levels (P>0.05).III. A feeding trial was conducted to investigate the effects of dietary corn glutenmeal (CGM) levels on growth, digestion and protein metabolism of cobia (initialweight:108.2±3.0g). Five isonitrogenous (crude protein:45%) and isoenergetic(gross energy:20kJ g-1) practical diets were formulated by replacing0(thecontrol),17.5,35,52.5and70%of protein from fish meal with CGM. The resultsshowed that with increasing dietary CGM levels, survival, feed intake (FI),specific growth rate (SGR), protein productive value (PPV), and feed efficiencyratio (FER) all decreased. Fish fed the diet with70%protein from CGM hadsignificantly lower survival, FI, SGR, FER and PPV than the control group(P<0.05), while no significant difference was observed among fish fed diets withless than70%CGM protein, although the fish fed the diet with17.5%protein fromCGM had a little higher SGR, FER and PPV. Fish fed the diet with70%proteinfrom CGM had significantly higher whole body moisture and significantly lowerwhole body crude protein and crude lipid contents (P<0.05). Fish fed the diet with70%protein from CGM had significantly lower total protein and cholesterol inplasma, activity of lysozyme, and relative expression of IGF-I in liver than controlgroup. There were no significant differences in relative expression levels of TORin liver and dorsal muscle at different CGM levels (P>0.05).IV.A basal diet (control) with16.8%fish meal and40%CGM was formulated. Theother three isonitrogenous and isolipidic experimental diets were prepared with thesupplementation of1.0%cholesterol,1.0%taurine and a combination of1.0%cholesterol and1.0%taurine to the basal diet. The results showed that fish fed thediet with1.0%taurine had significantly higher survival, feed intake (FI), specificgrowth rate (SGR), protein productive value (PPV), and feed efficiency ratio (FER)than that in fish fed the control. Compared with the control group, growth was not significantly different in fish fed the diet with1.0%cholesterol. Fish fed the dietwith1.0%taurine had significantly higher relative expression of IGF-I in liverthan the control group, while the case was not the same in muscle. Compared withthe control group, there were no significant differences in relative expressionlevels of TOR in liver and dorsal muscle in fish fed the diet with1.0%taurine(P>0.05)...