Cytogenetic analysis of hybrids and hybrid triploids between the river puffer,Takifugu obscurus,and the tiger puffer,Takifugu rubripes

Cytogenetic analyses of the river puffer, Takifugu obscurus, the tiger puffer, Takifugu rubripes and hybrids produced between female T. obscurus and male T. rubripes and their hybrid triploids (produced by cold shock treatment at 4°C) were performed. T. obscurus had 2n = 44 chromosomes and 1.84 ± 0.019 pg DNA/cell, T. rubripes had 2n = 44 and 2.64 ± 0.015, the hybrids had 2n = 44 and 2.15 ± 0.010 and the hybrid triploids had 3n = 66 and 3.22 ± 0.010. The erythrocyte values of the hybrids were more similar to those for T. obscurus, whereas the hepatocyte, midgut and proximal tubule kidney cell values of the hybrids fell down between those for the parental species (p < .05). The erythrocyte, proximal tubule, hepatocyte and midgut epithelial cell sizes for the hybrid triploids were 1.5‐fold larger than those for the hybrids (p < .05). The thickness of retina and each layer in the hybrid triploids were 1.1‐fold larger than those of the hybrids (p < .05) and did not differ significantly among T. obscurus, T. rubripes and the hybrids (p > .05); however, the hybrid triploids had fewer cell nucleus outer layers than the hybrids (p < .05). Gonad development in the hybrids and hybrid triploids was less matured than in T. obscurus and T. rubripes. The metaphase nucleolus organizer regions (NORs) and the gill cells of T. obscurus, T. rubripes and the hybrids contained two satellite telocentrics, whereas the hybrid triploids contained three satellite telocentrics.     

Effect of supplemental phytase on growth, phosphorus digestibility and bone mineralization of common carp (Cyprinus carpio L)

The effect of dietary phytase on phosphorus (P) digestibility, growth, bone mineralization and plasma P of common carp (Cyprinus carpio) was investigated. Five diets, D0, DP, PHYT1, PHYT2 and PHYT4, were used. D0 contained no supplement. DP was supplemented with 2.6 g P (Na₂HPO₄× 2H₂O) kg⁻¹. PHYT1, PHYT2 and PHYT4 comprised 1000, 2000 and 4000 U phytase [Ronozyme P (CT)] kg⁻¹. D0, PHYT1, PHYT2, PHYT4 had average of 7.95 g P kg⁻¹; DP contained 10.3 g P kg⁻¹. After 126 days of feeding, fish grew from 115 to 347 g (D0), 583.7 g (DP), 352.6 g (PHYT1), 393.3 g (PHYT2) and 406.4 g (PHYT4). Addition of phytase significantly increased daily feed intake, but only led to a marginal improvement in the weight gain, SGR and FCR of fish fed PHYT2 and PHYT4 compared with fish fed D0. Fish fed DP showed the best (P<0.05) growth performances. Fish fed DP and PHYT4 significantly digested dietary P more than the fish fed D0. Fish fed D0, PHYT1, PHYT2 and PHYT4 retained higher (P<0.05) Zn in their bones than fish fed DP. However, there was no further effect on bone mineralization. Plasma P scarcely improved by between 14% and 26% in fish fed PHYT2 and PHYT4 relative to fish fed D0. P concentration of the faeces was reduced by about 9–22% in fish fed PHYT1 and PHYT4 compared with fish fed D0.     

Effects of estradiol‐17β on survival,growth performance,gonadal structure and sex ratio of the tiger puffer,Takifugu rubripes (Temminck & Schlegel, 1850), fingerlings

Systematic research was performed to assess the survival, growth performance, gonadal structure and sex ratio during and after sex reversal induced by estradiol‐17β in tiger puffers. The fish of 20 days after hatching (dah) were immersed in different doses (0.2, 2, 20, 100 ppb) for 60 days. There were significant differences in survival rates among groups (p < .05). With the increase in the estradiol‐17β dose, the survival rates of tiger puffers decreased, revealing a linear regression relationship with log₁₀ (estradiol + 1; p < .05). The growth tendency of fish in the low‐dose group was similar to those in the control group over the same period. The growth in the 20 ppb dose group was significantly slower than in the low‐dose and control groups in fish of same age (p < .05), but was significantly faster than in the 100 ppb dose group (p < .05) after 60 days. Intersex fish were categorized into intersex I, II and III types, according to the feminization degree from low to high. Phenotypic intersex puffer fish were found in the 2, 20 and 100 ppb dose groups. Intersex ratios in 2, 20 and 100 ppb were 10%, 43.3% and 46.7%, respectively, the ratios except the ones in 2 ppb group being significantly higher than those in the control group (p < .05). Moreover, 100.0% male induction for intersex II and III types occurred in 20 and 100 ppb groups. Together, these results indicated that estradiol‐17β at a dose of 20 ppb was most effective for pseudo‐female production.     

Histopathological studies on Vibrio harveyi– infected tiger puffer,Takifugu rubripes (Temminck et Schlegel), cultured in Japan

Vibrio harveyi infection occurred with a moderate mortality in tiger puffer, Takifugu rubripes (Temminck et Schlegel), in autumn 2007, at a mariculture farm in western Japan. The diseased fish showed nodular lesions in the branchial chamber and the inner surface of the operculum. Histopathologically, the lesions comprised granulation tissue containing many suppurative foci allowing propagation of the bacteria and granuloma encapsulating abscesses with a decrease in bacteria. The bacteria were disseminated in visceral organs including the spleen, kidney, liver, and myocardium, resulting in the formation of granulomatous lesions. Two groups of tiger puffer juveniles were artificially infected by an intramuscular injection with an isolate (1.0_10⁸ CFU/fish). During the experimental period, 20% mortality occurred within 4–6 days post‐infection (d.p.i). The fish sampled on 4 d.p.i showed abscesses in the lateral musculature at the injection site. The fish sampled 5 d.p.i. displayed the production of granulation tissue containing many suppurative foci, which replaced the necrotic dermis and lateral musculature. Surviving fish (15 d.p.i.) had granulomatous lesions in the lateral musculature at the injection site. Pyogranulomatosis is pathognomonic in V. harveyi infection of tiger puffer.     

Pathological changes induced by three myxosporeans in the intestine of cultured tiger puffer, Takifugu rubripes (Temminck and Schlegel)

Pathological changes in cultured tiger puffer, Takifugu rubripes , with emaciation disease in Kyushu, Japan were studied histologically. In most cases, diseased fish were heavily infected with at least one of three myxosporeans ( Myxidium fugu , Myxidium sp. and Leptotheca fugu ) and two unidentified hyperparasitic microsporeans, attached to, or in, the intestinal epithelium. Myxidium fugu attached to the surface of the epithelium, caused no noticeable effects on the host tissue, irrespective of its infection with the hyperparasite. Myxidium sp., which proliferated in the epithelium, induced severe pathological changes including accumulation of cell debris between the epithelium and lamina propria and resultant detachment of the epithelium. Leptotheca fugu, another histozoic myxosporean, induced degeneration of the epithelium, associated with massive infiltration of macrophages into the epithelium to encapsulate parasites. When L. fugu was infected with its hyperparasitic microsporean, shortened villi were also observed. This is probably because passage of macrophage-parasite aggregates through the basement membrane of the epithelium severely damaged the epithelial structure. It is evident histologically that, unlike epicellular M. fugu , histozoic Myxidium sp. and L. fugu with or without hyperparasitic microsporeans, were highly pathogenic to host fish. This strongly suggests that they are causative agents of the emaciation disease.     

Dietary inorganic phosphorus or microbial phytase supplementation improves growth,nutrient utilization and phosphorus mineralization of juvenile red sea bream,Pagrus major,fed soybean‐based diets

A 2 × 3 factorial design with triplicates examined the interaction between dietary inorganic phosphorus (IP) and phytase on growth, mineral utilization and phosphorus (P) mineralization in juvenile red sea bream. The treatments were three levels of dietary IP supplementation at 0, 2.5 and 5 g kg^?1, either without or with phytase supplementation [2000 FTU kg^?1; phytase unit is defined as the amount of enzyme activity which liberates 1 micromol of inorganic phosphorus per minute at pH 5.5 and 37 °C at a substrate concentration (sodium phytate) of 5.1 mmol L^?1]. Juvenile red sea bream (IBW = 1.3 g ± 0.1) were stocked twelve fish per tank and fed for 50 days. Growth and feed efficiency were significantly (P < 0.05) enhanced by both dietary P and phytase supplementation. Feed intake and survival rate were not significantly affected by the dietary treatments. Both dietary IP and phytase supplementation significantly increased plasma IP and Mg levels. Concentration of vertebral mineral and scale P was significantly increased by both dietary treatments. A skeletal malformation syndrome of scoliosis occurred in fish fed both non‐IP and non‐phytase supplemented diet. Interaction between main dietary effects was detected for vertebral Zn, scale P and whole‐body ash and Mg content. With regard to growth and other examined productivity traits, phosphorus requirement of juvenile red sea bream can be met if supplemented with 2000 FTU phytase kg^?1 or in the absence of phytase, by dietary inclusion of 2.5–5 g kg^?1 of IP.     

Temperature effects on the development of Enteromyxum spp. (Myxozoa) in experimentally infected tiger puffer, Takifugu rubripes (Temminck & Schlegel)

The effects of water temperature on the development of the enteric myxosporeans, Enteromyxum fugu and Enteromyxum leei, were investigated in experimentally infected tiger puffer, Takifugu rubripes. After naïve tiger puffer were fed gut tissue infected with both E. fugu and E. leei, they were divided into separate tanks and kept at different constant temperature regimes between 10 and 25 °C. Regardless of the water temperature tested, E. fugu was consistently detected with a high prevalence of infection (60–100%), although no sporulation occurred at 10 and 15 °C. Development of E. leei and the onset of disease were suppressed by low water temperatures (<15 °C). However, a temperature increase to 20 °C promoted the development of E. leei, followed by an increase of disease rate in the fish. The present study demonstrates that water temperatures below 15 °C have an inhibitory effect on the development of E. fugu and E. leei, resulting in suppression of enteromyxosis at low temperatures.     

Effect of dietary DP/DE ratio on apparent digestibility, growth and nitrogen and phosphorus retention in rainbow trout, Oncorhynchus Mykiss (Walbaum)

The effect of DP/DE ratio in diets for rainbow trout, Oncorhynchus mykiss (Walbaum), was investigated. To evaluate growth and body composition, groups of trout were fed three experimental diets with a constant level of gross energy (25.4 ± 0.12 MJ kg^?1 dry matter (DM)) and different digestible protein/digestible energy (DP/DE) ratios (diet A, 16. 35; diet B, 17.21; dietC, 18.23 g Mr^?1). Fat, protein and energy digestibility coefficients were not affected by the DP/DE ratio of the diets. Growth and feed utilization improved markedly as dietary DP/DE ratio increased (P < .01). The efficiency of fat, protein and energy utilization tended to increase with increasing DP/DE ratio of the diets. Nitrogen discharge in effluent water per kg of weight gain was not affected by dietary treatments (mean values for: diet A, 29.9; diet B, 29.8; diet C, 29.1 g N kg^?1 weight gain) while phosphorus discharge in effluent water fell using diets with a higher DP/DE ratio (mean values for: diet A, 7.3; diet B, 6.7; diet C, 5.9 g P kg^?1 weight gain).     

Dietary taurine impacts the growth,amino acid profile and resistance against heat stress of tiger puffer (Takifugu rubripes)

A 56‐day feeding trail was conducted to evaluate the effects of taurine (Tau) supplementation on growth performance, amino acid profile and heat stress resistance capacity in juvenile Takifugu rubripes (initial body weight: 32.28 ± 0.20 g). Tiger puffer were distributed into five groups and each received one of the following diets: each diet was supplemented with 0 (T1), 5 (T2), 10 (T3), 20 (T4) or 50 (T5) g Tau/kg, respectively. Growth performances were significantly ameliorated with fed diet T3 (p < .05). The Tau content of muscle increased significantly as dietary Tau increased (p < .05). Compared to the control group, fish fed diets T4 and T5 showed significantly higher concentration of triglyceride and urea, respectively (p < .05), while taurine transporter (TauT) gene and IGF‐1 gene expression increased significantly in fish fed diets T3 and T4, respectively (p < .05). After acute heat stress, HSP70 gene expression levels were significantly greater in the livers and spleens of T. rubripes (p < .05) fed diet supplemented with Tau. Levels of complement C4 were highest in fish fed 10 g/kg Tau (p < .05). These data suggested that dietary supplemented with taurine indicated positive effects on growth performances, amino acid profile and resistance against heat stress for T. rubripes.     

Effect of dietary microbial phytase supplementation on growth and nutrient digestibility of Pangasius pangasius (Hamilton) fingerlings

A feeding trial was conducted for 60 days to study the effects of exogenous microbial phytase supplementation on the growth and nutrient digestibility of Pangasius pangasius fingerlings. Eight isocaloric and isoprotein experimental diets (35.67% crude protein and 3870 kcal kg^?1) were prepared with graded levels of phytase, e.g., 0 (T₁), 150 (T₂), 250 (T₃), 350 (T₄), 500 (T₅), 1000 (T₆) and 2000 (T₇) FTU (phytase units) kg^?1. Three hundred and fifteen fingerlings of P. pangasius (1.97–2.05 g) were randomly distributed in seven treatments with three replicates each. Maximum weight gain (350.72%), specific growth rate (2.51%), protein efficiency ratio (2.1), apparent net protein utilization (27.85%), energy retention value (88.47%) and feed conversion efficiency were observed in T₅ group supplemented with 500 FTU phytase kg^?1 diet. Apparent dry matter and protein digestibility in phytase‐supplemented groups were significantly (P<0.01) higher at a minimum supplement of 500 FTU kg^?1 or higher. Liver alkaline phosphatase activity increased significantly in treatment groups. Supplementation at 500 FTU kg^?1 of phytase is optimum in the diet of P. pangasius fingerlings.     

Dietary astaxanthin differentially affected the lipid accumulation in the liver and muscle of the marine teleost,tiger puffer Takifugu rubripes

The lipid‐regulating effects of astaxanthin (ASTX) have been widely reported in terrestrial animals. However, little relevant information has been available in fish although ASTX has been used as exogenous pigment and antioxidant in fish feed. A 74‐day feeding study was conducted to investigate the effects of dietary ASTX on lipid accumulation in the marine teleost tiger puffer. Four experimental diets differing only in ASTX supplementation, that is, 0 (control), 50 (ASTX50), 100 (ASTX100) and 500 (ASTX500) mg kg^‐1, were randomly assigned to 12 tanks of juvenile tiger puffer. Compared to control, the liver lipid content in group ASTX50 was significantly higher, while those in groups ASTX100 and ASTX500 were lower. The muscle lipid contents in group ASTX500 were significantly higher compared to control. Group ASTX50 had the best growth performances, while diet ASTX500 seemed to have adverse effects. In the liver, compared to control, groups ASTX50 and ASTX100 showed significantly lower mRNA expressions of genes related to triglycerol synthesis and fatty acid synthesis, transport and uptake, but higher expressions of genes related to β‐oxidation and monoglycerol hydrolysis. In the muscle, compared to control, ASTX100 showed higher expressions of genes related to β‐oxidation. ASTX50 resulted in higher contents of saturated and monounsaturated fatty acids but lower contents of n‐3 polyunsaturated fatty acids in fish. In conclusion, astaxanthin in diets for tiger puffer differentially regulated the lipid accumulation in the liver and muscle, both in dose‐dependent manners. Excess dietary astaxanthin (500 mg/kg) had adverse effects on tiger puffer.     

饲料中淀粉和糊精对红鳍东方鲀生长和低氧耐受性的影响

在水产养殖鱼粉鱼油供应相对紧缺的背景下,充分挖掘碳水化合物(糖类)的营养功能具有重要意义。低氧是水产养殖中一种常见的环境应激。大量研究表明,鱼类在低氧条件下主要利用糖来提供能量,但饲料糖源对红鳍东方鲀(Takifugu rubripes)的生长和急性低氧耐受性尚未见报道。本研究分别在饲料中添加玉米淀粉或糊精(玉米淀粉水解物)作为糖源,养殖8周后,测定生长、急性低氧存活率、代谢产物和低氧诱导因子(HIF)信号通路等指标。结果显示,与淀粉组相比,糊精虽然不会影响红鳍东方鲀的生长性能和鱼体成分,但显著提高了其在急性低氧条件下的存活率。在常氧条件下,淀粉组和糊精组肝脏的糖原和乳酸含量均无显著差异(P>0.05),但在低氧条件下,糊精组肝脏的乳酸含量和乳酸脱氢酶基因表达量均显著高于淀粉组(P<0.05)。同时,糊精组肝脏hif-1α (hypoxia inducible factor 1 subunit alpha)和hif-3α (hypoxia inducible factor 1 subunit alpha, like)、肌肉hif-1α和vegfa (vascular endothelial growth factor A)的表达量均显著高于淀粉组(P<0.05)。在低氧条件下,糊精组血清甘油三酯(TG)含量显著升高,但淀粉组血清和肝脏的TG含量在低氧后却显著下降(P<0.05)。与常氧组相比,糊精组在低氧后的肌肉总蛋白含量、肝脏蛋白激酶B (v-akt murine thymoma viral oncogene homolog 1, akt1)和哺乳动物雷帕霉素靶蛋白(mechanistic target of rapamycin kinase, mtor)基因表达量均显著降低,但淀粉组肝脏的mtor基因表达在低氧后却显著升高(P<0.05)。综上所述,用易消化的糊精替代玉米淀粉激活了HIF信号通路介导的无氧糖酵解来为鱼体提供更多的能量,同时还可以抑制脂肪的分解和蛋白质的合成,减少了O2的消耗,从而提高了红鳍东方鲀对急性低氧的耐受性。本研究结果为鱼类耐低氧饲料的设计和健康养殖提供了重要参考。     

饲料蛋白能量比对红鳍东方鲀幼鱼的生长性能、饲料利用和体组成的影响

为探求红鳍东方鲀(Takifugu rubripes)幼鱼饲料的最适蛋白质能量比,以鱼粉和豆粕作为主要蛋白源,鱼油和豆油作为主要脂肪源,配制粗蛋白含量分别为36％、42％和48％,粗脂肪含量分别为8％、12％和16％,蛋能比为17.06～24.20 mg/kJ的9组饲料.投喂初始体重为14.95 g的红鳍东方鲀幼鱼56...     

Dietary microbial phytase and citric acid synergistically enhances nutrient digestibility and growth performance of Labeo rohita (Hamilton) juveniles at sub-optimal protein level

A 2 × 2 × 2 factorial experiment was designed to study the main effect of dietary microbial phytase, citric acid, crude protein (CP) level and their interactions on growth performance, nutrient digestibility and body composition of Labeo rohita juveniles. Two basal diets were formulated using plant‐based ingredients containing either sub‐optimum (25%) or normal (35%) CP levels. Both the diets were supplemented with microbial phytase (U kg⁻¹) and citric acid (%) at the level of 0, 0; 500, 0; 0, 3; and 500, 3 respectively. One hundred and twenty L. rohita juveniles (average weight 12.61–13.72 g) were randomly distributed in eight treatments, each with three replicates. Dietary microbial phytase alone had no significant effect on whole‐body ash content but addition of citric acid (3%) in the diet activated the phytase as evidenced by their significant interaction. A significant interaction between citric acid and microbial phytase was also found on weight gain%, specific growth rate, protein efficiency ratio. Increasing the dietary CP level from 25% to 35% significantly (P<0.01) decreased phosphorus and dry matter digestibility. Thus, these results showed that the growth‐promoting effect was higher in groups fed a sub‐optimum protein (25%) diet containing both 3% citric acid and 500 U kg⁻¹ of microbial phytase than those fed 35% CP diets. This suggests that microbial phytase and citric acid in sub‐optimum CP diet had a synergistic effect on nutrient digestibility and growth performance of L. rohita juveniles.     

Effect of partial replacement of dietary monocalcium phosphate with neutral phytase on growth performance and phosphorus digestibility in gibel carp,Carassius auratus gibelio (Bloch)

A feeding trial was conducted to study the effect of partial replacement of dietary monocalcium phosphate (MCP) with neutral phytase on growth performance and phosphorus digestibility in gibel carp, Carassius auratus gibelio (Bloch). Control diet was prepared with 2% MCP but without phytase (P0). Other three experimental diets were prepared by replacement of MCP by 25%, 50% and 75% respectively in comparison with control with supplementation of neutral phytase at 500 U kg^?1 diet in each and designated as P25, P50 and P75 respectively. Gibel carp (initial body weight of 30.22 ± 1.98 g) were reared in twelve 300‐L cylindrical fibreglass tanks provided with filtered flow‐through tap water at 26–28°C. After 8‐week experiment, gibel carp fed with P50 had no obvious differences from the control group on weight gain (WG), specific growth rate (SGR), feed conversion ratio (FCR), protein efficiency rate (PER) and survival rate. Phytase supplementation did not affect body compositions or muscle compositions. Crude protein and phosphorus (P) contents in the faeces of fish fed with the phytase‐supplemented diets were significantly lower than those of the control group. The apparent digestibility coefficients (ADCs) of crude protein and P in gibel carp were increased when fish fed with the diets in which MCP was replaced by neutral phytase. This study suggested that partial replacement of dietary MCP at 50% with neutral phytase was considered as a recommended dietary supplemental level and increased dietary P and protein availability.     

江蓠、浒苔、藻渣和菌渣替代鱼粉对红鳍东方鲀幼鱼生长性能、相关生化指标的影响

本实验旨在研究江蓠(Gracilaria verrucosa)、浒苔(Enteromorpha prolifera)、藻渣和菌渣替代鱼粉对红鳍东方鲀(Takifugu rubripes)幼鱼生长性能、血清和肝脏生化指标和体组成的影响。实验共制成6种等氮、等脂的饲料,以含鱼粉60%为对照组1和45%鱼粉的为对照组2,分别将10%的江蓠、浒苔、藻渣、菌渣与植物蛋白质配合后替代对照组1饲料中30%的鱼粉,投喂初始体重为(17.33±0.55) g的红鳍东方鲀幼鱼56 d。结果显示,1)浒苔组特定生长率(SGR)显著高于其他5组(P<0.05),藻渣组SGR显著高于对照组2和江蓠组(P<0.05),与对照组1和菌渣组无显著差异(P>0.05);对照组1和浒苔组饲料效率(FER)显著高于对照组2和菌渣(P<0.05),与江蓠和藻渣组无显著差异(P>0.05);对照组1的蛋白质沉积率(PPV)和蛋白质效率(PER)显著高于对照组2和菌渣组(P<0.05),与江蓠、浒苔和藻渣组无显著差异(P>0.05)。2)各组血清和肝脏中谷丙转氨酶(GPT)和谷草转氨酶(GOT)活性均无显著差异(P>0.05)。各组血清丙二醛(MDA)含量和超氧化物歧化酶(SOD)活性均无显著差异(P>0.05)。3)菌渣组鱼体粗蛋白质含量显著高于藻渣组(P<0.05),与对照组1、对照组2、江蓠和浒苔无显著差异(P>0.05);浒苔和藻渣组鱼体粗脂肪含量显著高于对照组2 (P<0.05),与对照组1、江蓠和菌渣组无显著差异(P>0.05)。综合来看,将10%江蓠、浒苔、藻渣和菌渣与植物蛋白质配合后替代饲料中30%的鱼粉对红鳍东方鲀幼鱼的生长性能无不良影响,并且浒苔可以显著提高红鳍东方鲀的生长性能。     

Effect of microbial phytase on apparent nutrient digestibility of barley, canola meal, wheat and wheat middlings, measured in vivo using rainbow trout (Oncorhynchus mykiss)

Microbial phytase (Natuphos 5000G) was supplemented into barley, canola meal, wheat and wheat middlings at 500 FTU kg^–1 diet to test the effectiveness of the phytase on digestibility of crude protein (CP), gross energy (GE) and minerals for rainbow trout. A total of 180 rainbow trout (initial mean body weight 223.8 ± 17.9 g) were stocked into ten 40-L fibreglass digestibility tanks with 18 fish per tank. Duplicate tanks were assigned to a reference diet and a single tank was assigned randomly to each of the eight diets made from these four ingredients supplemented with and without phytase. The collection of faeces lasted for 2 weeks. Faeces collected in each week represented a replicate and they were analysed separately. The mean apparent digestibility coefficients (average of four ingredients) in phytase supplemented and nonsupplemented ingredients were: CP, 97.3% and 96.5%; GE, 62.8% and 53.1%; calcium (Ca), 38.3% and –5.3%; magnesium (Mg), 72.4% and 50.3%; manganese (Mn), 31.0% and –4.2%; total-phosphorus (total-P), 71.1% and 47.6%; phytate-phosphorus (phytate-P), 87.5% and 22.3%; copper (Cu), 38.3% and 26.4%; potassium (K), 97.1% and 97.0%; sulphur (S), 90.2% and 88.6%; zinc (Zn), 12.3% and –16.6%. A two-tailed t -test indicated that phytase supplementation significantly ( P  < 0.05) improved the digestibility of Ca, Mg, Mn, total-P, phytate-P, and GE. The efficacy of phytase on nutrient digestibility also depended on the type of ingredient as measured in rainbow trout.     

Dietary available phosphorus requirement for tambaqui,Colossoma macropomum,juveniles based on growth,haematology and bone mineralization

An economical and efficient approach to reduce the impact of P discharge by aquaculture industry is to adjust the P level in fish feeds to the precise nutrient requirement at different growth stages in a digestible nutrient basis. However, P requirement seems to be species specific and affected by several physiological, dietary and environmental factors. Based on the importance of tambaqui (Colossoma macropomum) to Latin American aquaculture, we designed a 63‐day trial to evaluate the effect of available P (AP) levels on growth, nutrient digestibility, haematology and blood biochemical parameters, carcass proximate composition and bone mineralization. Quadruplicate groups of tambaqui juveniles (144 ± 2.0 g) were fed five isonitrogenous (278 g kg^?1 digestible protein) and isocaloric (13.5 Mj DE kg^?1) diets containing graded AP levels (3.0, 5.6, 7.5, 9.1 and 11.0 g kg^?1) following a completely randomized design. Dicalcium phosphate (DCP) was used as the main P source. No mortalities and signs of P deficiency were observed among the dietary treatments. A remarkably high P digestibility was observed in all plant‐based diets with a tendency of decreasing P digestibility with the increase in total P levels. Tambaqui seems to be able to grow well without inorganic P supplementation during the trials; however, this species required 7.0 g AP kg^?1 diet for proper bone mineralization. P supplementation had a limited effect on haematology and blood biochemistry of tambaqui. Ecological implications of natural feeding habit and evolutionary position of this species are further discussed, and new hypothesis are drawn based on our results.