Effects Of Dietary Phosphorus And Calcium Levels On Growth,Tissue Deposition And Digestive Physiology Of Juvenile Discus Fish(symphysodon Haraldi)

An 8-week feeding trial was conducted to investigate the effects of dietary calcium and phosphorus levels on growth,body composition,tissues mineralization and digestive physiology of juvenile discus fish Symphysodon haraldi.Fish were fed twelve different diets with four calcium levels(0,5,10 and 15 g kg^-1,dry matter)from calcium chloride?CaCl₂?and three phosphorus levels(0,5 and 10 g kg^-1,dry matter)from potassium phosphate?KH₂PO₄?.After 2 weeks acclimating in rearing water,a total of 360 juveniles were then selected with similar size?initial weight of 4.98±0.26 g and initial body length of 4.55±0.25 cm?and randomly assigned into 36 tanks?35×45×35 cm,basis on effective water?,three replicates for each group with a density of 10 fish per tank.Tanks were attached in a recirculating system.During the trial period,rearing water was automatically aerated maintaining over 5 mg L^-1 dissolved oxygen and controlled in a steady temperature at 29.0±0.5?,and the pH was 7.8±0.3.The calcium and phosphorus content were 36.5±0.48 and 1.42±0.04 mg L^-1 respectively.The result at the end of the trial showed that:1.Dietary calcium and phosphorus level had no affection on hepatosomatic index,visceralsomatic index and condition factor?P>0.05?,but the survival rate significantly reduced with dietary phosphorus supply?P<0.05?.Dietary calcium or phosphorus supplementation obviously affected final body weight?FBW?,weight gain rate?WGR?,specific growth rate of weight?SGR_W?,feed conversation ratio ?FCR?and protein efficiency rate?PER?of discus.Without phosphorus supply,the FBW,WGR,SGR_W and PER slightly increased with dietary calcium supplement,and all of them obtained highest mean value in presence of 10 g kg^-1 calcium supplied,but there's no significant difference compared with control group?P>0.05?.In terms of 5 or 10 g kg^-1 phosphorus supplied in diets,the FBW,WGR,SGR and PER sharply declined rather than improving with dietary calcium addition?P<0.05?,and these parameters of fish fed diets with 15 g kg^-1calcium were significantly lower than those of fish fed diets without calcium supply.While the FCR appeared an opposite tendency.In presence of 5 or 10 g kg^-1 phosphorus supplied in diets,the body length BL),body height?BH?and specific growth rate of length?SGR_L?showed a eduction with increasing calcium level.The BL/BH ratio of fish fed diet with 10 g g^-1 phosphorus and without calcium supply was significantly lower than other roups,and the highest SGR_L was also located in this group.2.Dietary calcium and phosphorus supplementation significantly affected biochemical omposition in whole fish and muscle of juvenile discus.In presence of 5 g kg^-1 hosphorus supply,the moisture content in whole fish increased with dietary calcium upplement.Both calcium and phosphorus supply reduced crude fat content?P<0.05? nd improved ash content in whole fish.As 5 g kg^-1 phosphorus supplied,dietary alcium addition significantly reduced whole fish crude protein content?P<0.05?, nd obtained highest?15.25%?and lowest?12.60%?value in terms of 0 and 15 g kg^- 1 calcium supply,respectively.The moisture content in muscle increased with dietary hosphorus supply while crude protein and ash content showed an opposite tendency.hen 5 g kg^-1 phosphorus added,the calcium supplementation showed an obvious eduction on crude fat content in muscle.3.Whole fish fat deposition rate significantly reduced with dietary phosphorus supply P<0.05),and with dietary calcium supplement,crude fat deposition rate firstly educed and then increased.Without phosphorus supply,crude protein deposition rate n whole fish showed an increasing followed by a reduction;in presence of 5 or 10 g g^-1 phosphorus supply,whole fish protein deposition rate significantly reduced with etary calcium supplementation.Protein deposition rate of fish fed diets without alcium supply significantly higher than that of fish fed diet with 15 g kg^-1 calcium pply,but not significantly differ from the group fed diet with 5 g kg^-1 calcium upply.In terms of 0 g kg^-1 phosphorus supplied in diets,whole fish ash deposition ate firstly increased and then reduced with calcium supplementation.As with hosphorus supplied in diets,ash deposition rate reduced with calcium supply.And he group fed diet with 10 g kg^-1 phosphorus and 15 g kg^-1 calcium showed lowest ean data of ash deposition rate in whole fish.4.Whole fish calcium deposition rate obviously reduced with dietary calcium supply.s with 0 or 5 g kg^-1 calcium supplied in diets,calcium deposition rate improved ith dietary phosphorus level.Excepted the groups fed with 15 g kg^-1 calcium,the alcium deposition rate value in other groups were all over 100%.Fish fed diet with 0 g kg^-1 phosphorus and 15 g kg^-1 calcium presented the lowest phosphorus eposition rate;and the highest phosphorus deposition rate located in group fed diet ith 0 g kg^-1 phosphorus and 10 g kg^-1 calcium.The calcium/phosphorus ratio in hole fish,moisture,vertebra and scale ranged in 1.37-1.99,0.44-1.52,2.11-2.48 nd 1.86-2.12,respectively.Dietary supplementation of calcium and phosphorus howed significant affection on calcium/phosphorus ratio in whole fish and muscle P<0.05),while no affection was found in scale?P>0.05?.5.Both protease and lipase specific activities of intestinal tract significantly reduced ith dietary calcium supplementation?P<0.05?.Individual phosphorus or calcium upply did not affect the amylase specific activity of intestinal tract,while they bviously showed an interaction.In presence of 10 g kg^-1 phosphorus supplied in iets,amylase activity increased followed by reducing.Glutamic-oxalacetic ransaminase?GOT?activity firstly reduced with calcium supplementation followed y increasing,and showed a decline again.Dietary supplementation also showed an nteraction on glutamic oxalacetic transaminase?GPT?activity,and individual alcium supplement significantly reduced GPT activity without phosphorus supply.Above all,under present condition,dietary supplementation of phosphorus(5 or 10g kg^-1)improved growth performance and the deposition of calcium and phosphorus in fish,while excessive phosphorus level(10 g kg^-1)in diet showed obvious reduction on survival rate.Discus fish could absorb dissolved calcium directly from surrounding environment,rather than intake from feed to meet their requirement for normal growth in present condition.Dietary calcium supply presented inhibiting effect on growth while phosphorus supplied over 5 g kg^-1(dry matter,actual content was 3.33 g kg^-1 as wet basis).Therefore,we suggested that 5 g kg^-1 phosphorus without calcium supplementation in diet was optimal for the growth and survival of S.haraldi?4.98±0.26 g?,the actual content of calcium and phosphorus in diet were 0.28 and 3.33 g kg^-1?wet basis?,respectively.