| Three trials were conducted to investigate the effect of the dietary phosphate sources, phosphorus supplemental level, and calcium/phosphorus ratio on growth, body proximate composition, tissue phosphorus content and its biochemical indexes of juvenile cobia (Rachycentron canadum). The results can be summarized as follows:1. A trial was conducted to evaluate the effects of five inorganic phosphorus (dibasic calcium phosphate, calcium dihydrogen phosphate, calcium phosphate, sodium dihydrgen phosphate, potassium dihydrogen phosphate) on growth, body proximate composition, tissue phosphorus content,biochemical indexes to juvenile cobia. Diets of isonitrogenous and isoenergetic which contained dietary phosphorus 0.91%, without supplemental phosphorus as the control group (dietary total P 0.61%)were fed to juvenile cobia. The experiment indicated that weight gain ratio (WGR), special growth rate (SGR), fat factor (CF) and hepatosomatic index (HIS) of fish were significant differences among various dietary treatments (P<0.05). The highest WGR and CF were in the sodium dihydrgen phosphate group and calcium phosphate group, respectively. The control group was the lowest HIS; The body composition analysis showed that supplementation of different sources phosphorus had no significantly effect on moisture, protein contents in the carcass and muscle, but there was some effect on the carcass lipids and muscle lipids content, the lowest carcass lipids content was the sodium dihydrgen phosphate group; Ash and calcium, as well as phosphorus contents in the whole body, muscle, skin, skull and vertebrae,were significantly affected by dietary different sources phosphorus (P<0.05),the highest ash and calcium, as well as phosphorus contents in the carcass and muscle was the calcium dihydrogen phosphate group, the highest ash and calcium, as well as phosphorus contents in skin, skull and vertebrae was the sodium dihydrgen phosphate group. The analysis of plasma mineral elements indicated that the calcium, phosphorus, potassium and magnesium contents, were significantly affected by dietary phosphate sources (P<0.05). The biochemistry analysis showed that the triglycerides (TG), cholesterol (CHO), alkaline phosphatase (ALP) and creatine phosphoransferase (CK) in plasma, liver and muscle were significantly affected by sources of phosphorus, and the control group TG,CHO contents was significantly higher than the five supplemented groups; The highest ALP activity in plasma, liver and muscle was the control group, the calcium phosphate diet, the calcium dihydrogen phosphate diet;The highest CK activity in plasma, liver and muscle was the sodium dihydrgen phosphate diet, the calcium phosphate diet, the sodium dihydrgen phosphate diet. Within the inorganic phosphorus sources, ADC of P was significantly affected by supplement source. Mean P availability decreased in the following order∶sodium dihydrgen phosphate, 80.68%; potassium dihydrogen phosphate, 74.56%; phosphate dicalcium phosphate, 44.55%; monocalcium phosphate, 13.81%; tricalcium phosphate, -6.42%. Based on the index of growth performance and ADC of phosphorus, sodium dihydrgen phosphate was the optimal dietary phosphate.2. A trial was conducted to investigate the effects of dietary phosphorus levels on on growth, body proximate composition, tissue phosphorus content and its biochemical indexes of juvenile cobia. Six experimental isonitrogenous and isoenergetic purified diets ( casein-gelatin based) were formulated to contain 0.61%,0.76% ,0.91%,1.11% ,1.31%,1.51% phosphorus with calcium dihydrogen phosphate as source of phosphorus.Weight gain ratio (WGR), special growth rate (SGR) of fish were significantly increased with increasing phosphorus from 0.61 to 0.91% of diet (P<0.05), and then decreased. No significantly (P>0.05) changes exist in condition factor of juvenile cobia fed different levels of phosphorus, but the hepatosomatic index (HIS) was significantly improved with the increasing supplementation of dietary phosphorus. Survival of juvenile cobia for all dietary treatments ranged from 77.78 to 82.22%, and there were no significant differences among various dietary treatments (P>0.05). The body composition analysis showed that supplementation of dietary phosphorus had no significantly effect on the carcass moisture, carcass protein and the muscle moisture, muscle protein, muscle lipids, but there was some effect on the carcass lipids content, the carcass lipids content decreased with the increase of dietary phosphorus. Ash and calcium, as well as phosphorus contents in the whole body, skin, skull and vertebrae, increased with elevation of dietary phosphorus content (P<0.05). The blood biochemistry analysis showed that plasma calcium, phosphorus, potassium and magnesium contents increased with the increase of dietary available phosphorus levels (P<0.05), but plasma iron was negative.Dietary phosphorus supplementation had some influence on triglycerides (TG), cholesterol (CHO), alkaline phosphatase (ALP) and creatine phosphoransferase (CK) in plasma, liver and muscle. With the elevation of dietary phosphorus, plasma CHO and TG content decreased. Muscle CK, ALP and CK of plasma and liver increased with increasing phosphorus from 0.61 to 0.91% of diet and then decreased.Broken-line analysis based on WGR indicated that the minimum available phosphorus requirement for the optimal growth of juvenile cobia was 0.93%.Based on the ash content in vertebrae or the phosphorus content,the requirements were 0.91%and 0.95%,respectively.3. A trial was conducted to investigate the effects of dietary calcium(Ca)and phosphorus(P) ratio on growth performance ,body composition and biochemical indexes and optimal dietary Ca and P ratio of juvenile cobia.Diets were formulated to contain six Ca and P ratio as 1∶0.5,1∶0.8,1∶1,1∶2,1∶3,1∶4 using casein and gelatin as the main protein sources, Ca-lactate and NaH2PO4·2H2O as the Ca, P source, respectively. The growth trial suggested that Weight gain ratio (WGR), special growth rate (SGR) of fish were significantly affected by different dietary calcium and phosphorus ratios ,the highest WGR was the calcium and phosphorus ratio (1∶1) group, beyond this ratio the growth decreased. The fat factor (CF) and hepatosomatic index (HIS) of fish were not significant differences among various dietary treatments (P<0.05).The body composition analysis showed that dietary calcium and phosphorus ratios had no significantly effect on moisture, protein contents in the carcass and muscle, as well as muscle lipids content. The carcass lipids, carcass ash and calcium as well as phosphorus contents in the whole body, skull and vertebrae, were significantly affected by dietary phosphorus, not dietary calcium (P<0.05). The skin calcium content decreased with the increase of dietary phosphorus; As well, the ash and phosphorus contents in muscle increased with elevation of dietary phosphorus content, but no significant changes were found in the muscle ash and calcium content(P>0.05). The blood biochemistry analysis showed that that plasma calcium, phosphorus, potassium was negative among various dietary treatments, magnesium contents increased with the increase of dietary phosphorus levels (P<0.05), but plasma iron was decreased.With different dietary calcium and phosphorus ratios, significant differences were observed plasma triglycerides (TG), plasma cholesterol(CHO), alkaline phosphatase(ALP) and creatine phosphoransferase (CK) in plasma, liver and muscle (P<0.05). Based on the growth performance, dietary calcium and phosphorus ratio for juvenile cobia was 1∶1 might be optimum. |
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