Heterotrophic versus mixed BFT system: Impacts on water use,suspended solids production and growth performance of Litopenaeus vannamei

The Biofloc Technology System (BFT) is characterized by stimulating the development of a microbial community that acts mainly in the maintenance of water quality but also promotes other benefits such as increased productivity, biosafety and serves as a supplementary source of food for reared animals. Two main groups of bacteria are involved in nitrogen removal in this system: heterotrophic bacteria and autotrophic nitrifying bacteria, present in the aggregates. Different fertilization techniques can be used for the formation and maintenance of bioflocs, depending on which group of bacteria the predominance is preferred. This study aimed to analyze the effect of different organic fertilization techniques on the bioflocs establishment, amount of water used, the production of suspended solids and the growth performance of Litopenaeus vannamei reared in the BFT System. Shrimp juveniles were stocked in 150-liter tanks at a stocking density of 300 shrimps/m³. Three treatments (in triplicate) were tested using different fertilization techniques: 1) without supplementary organic fertilization; 2) organic fertilization according to nominal ammonia reading (heterotrophic/chemoautotrophic = “mixed” system) and 3) organic fertilization according to estimated ammonia production (heterotrophic). The temperature, salinity, dissolved oxygen, pH, ammonia, nitrite, nitrate, alkalinity and total suspended solids (TSS) of the water were monitored. The water quality parameters were influenced by the treatments with differences found in the concentrations of ammonia, nitrite, nitrate, pH, alkalinity and TSS. Ammonia levels were higher in control treatment since no organic fertilization was performed. Nitrite levels were lower in heterotrophic system since the nitrifying pathway was suppressed due to daily fertilization, also resulting in lower nitrate levels. There were significant differences in the growth performance parameters, with the highest final weight and yield, as well as the lowest FCR, found in the mixed treatment. There were no significant differences among survival. The mixed system treatment used less water during production cycle compared to other treatments while the volume of solids removed was almost four times greater in the heterotrophic treatment compared to the others. These results show that adopting a mixed heterotrophic/chemoautotrophic biofloc system improves shrimp growth performance, optimize water use and decrease solids production.     

凡纳滨对虾高位池养殖氮、磷收支研究及养殖效果分析

对凡纳滨对虾（Litopenaeus vannamei）高位池养殖氮（N）和磷（P）收支情况进行系统研究,比较分析不同放养季节、虾苗品系以及是否进行分段养殖引起养殖效果的差异。结果显示,饲料是最主要的N和P输入源,分别占池塘N和P总输入的91.76%~93.68%和94.55%~96.97%。收获对虾输出N和P分别占总输入的29.46%~40.46%和12.64%~17.41%,随养殖废水排出的N和P分别占24.63%~54.52%和23.03%~59.02%,沉积在池塘底部的N和P分别占14.10%~44.59%和27.59%~62.25%。放养季节和虾苗品系对养殖效果有显著影响。夏季组（ZS）对虾平均生长速度达到0.175g.d-1,分别比秋季组（ZF）和冬季普通组（ZW）高73.0%和139.3%。ZW成活率77.70%~87.75%,显著高于ZS和ZF。与相同养殖季节放养一代苗的ZW相比,放养本地苗的冬季组（BW）养殖成活率62.10%~72.30%,单位面积产量8821~9878kg.hm-2,均显著较低。采用分段养殖的冬季标粗组（ZWb）养成池塘单造使用周期缩短56.13%。     

凡纳滨对虾高位池养殖氮磷收支研究

对凡纳滨对虾（Litopenaeus vannamei）高位池养殖氮（N）和磷（P）收支情况进行系统研究,比较分析不同放养季节、虾苗品系以及是否进行分段养殖引起养殖效果的差异。结果显示,饲料是最主要的N和P输入源,分别占池塘N和P总输入的91.76%~93.68%和94.55%~96.97%。收获对虾输出N和P分别占总输入的29.46%~40.46%和12.64~17.41%,随养殖废水排出的N和P分别占24.63%~54.52%和23.03%~59.02%,沉积在池塘底部的N和P分别占14.10%~44.59%和27.59%~62.25%。放养季节和虾苗品系对养殖效果有显著影响。夏季组（ZS）对虾平均生长速度达到0.175 gd-1,分别比秋季组（ZF）和冬季普通组（ZW）高73.0%和139.3%。ZW成活率77.70%~87.75%,显著高于ZS和ZF。与相同养殖季节放养一代苗的ZW相比,放养本地苗的冬季组（BW）养殖成活率62.10%~72.30%,单位面积产量8 821~9 878 kghm-2,均显著较低。采用分段养殖的冬季标粗组（ZWb）养成池塘单造使用周期缩短56.13%。     

Effects of water exchange protocols on water quality,sedimentation rate and production performance of Penaeus monodon in earthen ponds

This study was carried out in farmers' fields to quantify the total water and consumptive water use in grow‐out culture of Penaeus monodon under recommended package of practice with two different water management protocols: T₁, with no water exchange and T₂, with regulated water exchange. Treatment‐wise estimated total water use, was 2.09 and 2.43 ha‐m 122 day^?1, while the computed consumptive water use index (m³ kg^?1 biomass) was 5.35 and 6.02 in T₁ and T₂ respectively. Lower rates of water exchange (T₂) showed significantly improved (P < 0.05) crop performance in terms of performance index (19.75 ± 0.75), production‐size index (74.1 ± 3.4), survival rate (80.13 ± 1.7%) and productivity (2.44 ± 0.08 t) over the zero water exchange. The shrimp pond water quality suitability index (WQSI) infers that regulated water exchange (T₂) improved the overall suitability of water quality for shrimp culture. WQSI up to 90 days of culture ranged between 7.5–9.0 in T₂, needs little management while in the last month of rearing, it was good with moderate management requirements. Treatment‐wise sediment load ranged between 50.4–56.3 m³ t^?1 shrimp biomass. High intensity of water exchange and low apparent feed conversion ratio influenced in lowering the sedimentation rate. Regulated water exchange protocol (T₂) performed well (higher net total water productivity and net consumptive water productivity) against no water exchange (T₁). A higher OV:CC ratio (ratio of the output value to the cost of cultivation) indicated that T₂ had a distinct edge over the T₁ protocol.     

凡纳滨对虾工厂化循环水养殖系统水质指标及微生物菌群结构的分析

为探究凡纳滨对虾(Litopenaeus vannamei)工厂化循环水养殖系统的养殖水体水质情况以及微生物菌群的组成结构,本研究利用高通量测序技术和生物信息学分析手段,测定凡纳滨对虾工厂化循环水养殖过程一级移动床生物净化、二级固定床生物净化、养殖水体的水质指标、水体和生物净化载体以及对虾肠道微生物菌群的组成。结果显示,水体的氨氮(NH₄⁺-N)和亚硝态氮(NO₂^–-N)质量浓度显著降低,分别为0.85和0.21 mg/L。养殖系统水体、生物净化载体和虾肠道样品中共有的优势菌为变形菌门(Proteobacteria)、拟杆菌门(Bacteroidetes),此外,一级、二级生物净化系统水体中的放线菌门(Actinobacteria)为优势菌,生物净化载体中浮霉菌门(Planctomycetes)和硝化螺旋菌门(Nitrospirae)为优势菌;对虾肠道中的厚壁菌门(Firmicutes)为优势菌。另外,对虾养殖循环水系统中生物净化载体上的细菌物种含量比水样中的细菌物种少,但微生物多样性高于养殖水体,...     

Dynamic stock model for analysing semi‐intensive production of whiteleg shrimp Litopenaeus (Penaeus) vannamei affected by the acute hepatopancreatic necrosis disease: Assessment of disease severity indicators and relationships with pond water quality parameters

The production of Litopenaeus vannamei was analysed when affected by the acute hepatopancreatic necrosis disease using a dynamic stock model and primary data of seven production cycles from a shrimp farm in Mexico from 2013 to 2016. Significant results (p < .05) of the correlation analysis indicated that during those years mortalities by the disease were more severe when water salinity was high and productivity was low. Significant results from ANOVA showed that throughout the period, disease severity and salinity diminished while pond productivity initially declined but subsequently improved. Significant results from regression analyses conducted for each production cycle also indicated the importance of salinity and productivity on disease severity and showed that early mortality by the disease occurred in ponds with warmer water. Within the observed range of water quality parameters, increases of 1 cm in water transparency and 1 g/L in salinity resulted in increments within 0.17%–0.25% and 1.7%–3.1% in shrimp mortality by the disease. When increases of 1°C in water temperature were recorded, outbreaks occurred 0.2–1.57 weeks earlier. In conclusion, the disease strongly determines the dynamics of shrimp production, and the role of salinity, productivity and temperature is worthy of further delving.     

Natural food evaluation and water quality in zero water exchange culture of Litopenaeus vannamei fertilized with wheat bran

The aim of this study was to analyze the influence of organic fertilizer on the availability of natural food (plankton and benthos) and water quality. Two fertilization protocols were adopted using inorganic and organic fertilizers with shrimp (stocked treatment) and their controls (unstocked treatment). Experimental units consisted of 12 circular fiberglass tanks (500 l) with estuarine sediment, individual aeration and no water exchange. In stocked treatments were used 40 juveniles/m², and they were fed with 35% crude protein marine shrimp ration, three times a day. Under the organic fertilization protocol, the plankton showed higher abundance of Nitzschia and rotifers, the phytobenthos consisted mainly of Nitzschia, Amphiprora and Oscillatoria, the epibenthos was represented mainly by nematodes and rotifers, and the macro-invertebrates were mainly oligochaetes. In relation to inorganic fertilization, the plankton was represented mainly by Coscinodiscus and rotifers, the phytobenthos consisted mainly of Amphiprora and Oscillatoria, the epibenthos was represented mainly by nematodes and rotifers, and the macro-invertebrates were mainly oligochaetes. Dissolved oxygen was higher for organic fertilizer (6.16 ± 0.98 mg/l) than for inorganic (5.92 ± 1.19 mg/l) while the other water quality parameters did not present significant differences. Survival was similar in the two fertilization regimes (96.6%). Final body weight was 11.89 ± 1.73 g for the inorganic fertilizers and 12.28 ± 1.71 g for organic fertilizer. It is concluded that wheat bran showed good performance in the water quality without exchange, in the availability of natural food, and in the growth and survival of the shrimps in the microcosms.     

A comparison between water exchange and settling tank as a method for suspended solids management in intensive biofloc technology systems: effects on shrimp (Litopenaeus vannamei) performance,water quality and water use

Biofloc systems rely on microbial processes in the water column to recycle animal waste products, reducing the need for water exchange. These increases biofloc concentration in the water and some form of removal is needed. An experiment was carried out to evaluate two management practices to control biofloc in Litopenaeus vannamei culture. Six tanks (48 m³) were divided into two treatments: water exchange and solid settler. Shrimp were stocked at 164 shrimp m^?2 and with 0.67 g of weight. After 61 days, shrimp under solid settler treatment demonstrated mean weight of 12.7 ± 0.5 g with survival of 73.8 ± 1.4%, and those under water exchange had a final weight of 10.1 ± 0.2 g and survival rate of 57.8 ± 11.1%. Total suspended solids did not differ between the treatments: 326.8 ± 24.9 mg L^?1 for water exchange and 310.9 ± 25.3 mg L^?1 for solid settlers. Settleable solids and productivity/respiration ratio was higher (P < 0.05) in water exchange treatment, indicating differences in physical and biological characteristics of bioflocs. Solids removal method influenced the water use, in which 1150 ± 249 L of water was necessary to produce one kilogram of shrimp using water exchange strategy, and 631 ± 25 L kg^?1 with the use of settlers. Our results indicate that continuous operation of settlers can reduce variability in solids characteristics and water quality variables such as ammonia. Both strategies are efficient in controlling biofloc concentrations of the water; however, settlers can reduce water use and improve shrimp production.     

Effect of different biofloc system on water quality,biofloc composition and growth performance in Litopenaeus vannamei (Boone, 1931)

The experiment was conducted with three biofloc treatments and one control in triplicate in 500 L capacity indoor tanks. Biofloc tanks, filled with 350 L of water, were fed with sugarcane molasses (BFT_S), tapioca flour (BFT_T), wheat flour (BFT_W) and clean water as control without biofloc and allowed to stand for 30 days. The postlarvae of Litopenaeus vannamei (Boone, 1931) with an Average body weight of 0.15 ± 0.02 g were stocked at the rate of 130 PL m^?2 and cultured for a period of 60 days fed with pelleted feed at the rate of 1.5% of biomass. The total suspended solids (TSS) level was maintained at around 500 mg L^?1 in BFT tanks. The addition of carbohydrate significantly reduced the total ammonia‐N (TAN), nitrite‐N and nitrate‐N in water and it significantly increased the total heterotrophic bacteria (THB) population in the biofloc treatments. There was a significant difference in the final average body weight (8.49 ± 0.09 g) in the wheat flour treatment (BFT_W) than those treatment and control group of the shrimp. Survival of the shrimps was not affected by the treatments and ranged between 82.02% and 90.3%. The proximate and chemical composition of biofloc and proximate composition of the shrimp was significantly different between the biofloc treatments and control. Tintinids, ciliates, copepods, cyanobacteria and nematodes were identified in all the biofloc treatments, nematodes being the most dominant group of organisms in the biofloc. It could be concluded that the use of wheat flour (BFT_W) effectively enhanced the biofloc production and contributed towards better water quality which resulted in higher production of shrimp.     

铜绿假单胞菌YY24对凡纳滨对虾生长及水质的影响

以凡纳滨对虾(Litopenaeus vannamei)为研究对象,探讨了水体中不同方式和不同浓度铜绿假单胞菌(Pseudomonas aeruginosa)YY24对凡纳滨对虾生长及水质的影响。试验设7个处理,每个处理3个重复,每个重复样本30尾,试验时间10 d,每2 d投放1次YY24,并保持其活性完好,有效养殖水体(50±3)L,控制温度(28±1)℃,盐度(20±1),每天每箱投料量按总重量的1%,溶解氧充足。试验期间,每24 h测定水体基础指标,在试验开始后的0、12、24、48、96、144、168、192、240 h测定水体理化指标。结果表明,与对照组相比,各试验组存活率与增重率均显著高于对照组(P0.05);各试验组均能明显净化水质,试验组的pH呈上升趋势,各试验组NH_4~+-N、NO_3~--N、NO_2~--N浓度均在96 h时就达到最低值;初始投放10~6CFU/m L浓度的YY24,在96 h内去除氨氮的效果较好,而每隔一天投10~4CFU/m L浓度YY24的持续作用效果更好。     

Effect of light limitation on the water quality,bacterial counts and performance of Litopenaeus vannamei postlarvae reared with biofloc at low salinity

The aim of this study was to evaluate the effect of light limitation on the water quality, bacterial counts and performance of Litopenaeus vannamei postlarvae reared with biofloc at low salinity (≈9 g L^?1). Two treatments were designed: T₁ = culture with natural sunlight and T₂ = culture in darkness. After 28 days, in both treatments, the final weight of shrimp was over 0.6 g with a specific growth rate over 7.4% d^?1, and a survival rate over 70%. In both treatments, Vibrio sp. concentration presented low values (culture with natural sunlight = 0.1 to 9.9 × 10² CFU mL^?1, culture in darkness = 0.4 to 11.7 × 10² CFU mL^?1) and Bacillus sp. had high values (culture with natural sunlight = 0.7 to 66.0 × 10⁴ CFU mL^?1, culture in darkness = 0.7 to 65.8 × 10⁴ CFU mL^?1). All water quality parameters remained within the ranges suitable for shrimp culture, except for alkalinity during the first stage of the study. Although in some sampling periods some significant differences were found in bacterial counts and water quality parameters, shrimp productive performance under culture with biofloc at low salinity was not affected significantly by light limitation.     

投喂频率对凡纳滨对虾生长和水质的影响

研究了不同投喂频率(1,2,3,4和5次·d-1)对凡纳滨对虾生长、存活率和水质的影响。试验在室内2m3的圆形水泥池(d=1.6m)中进行,采用全封闭养殖模式,凡纳滨对虾(初始重量为0.24g)养殖密度为50尾·m-3,投喂人工配合饲料(粗蛋白含量为42.98%)。通过7周的试验,结果表明,投喂频率为1～4次·d-1时,随着投喂频率的增加,对虾增重率显著增加;投喂频率由4次·d-1增加到5次·d-1时,对虾增重率略有增加,但差异不显著。凡纳滨对虾的成活率在1次·d-1时最低,2次·d-1时最高,2～5次·d-1时随着投喂频率的增加而降低(投喂频率为1和2次·d-1之间、3和4次·d-1之间差异显著)。投喂频率为3次·d-1时饲料系数最低,蛋白质效率最高。随着投喂频率1～3次·d-1增加,饲料系数逐渐显著降低,蛋白质效率显著递增;而投喂频率从3～5次·d-1增加时,饲料系数增大,蛋白质效率逐渐下降(3和4次·d-1差异显著;4和5次·d-1之间差异不显著)。投喂频率对水环境中的pH值、H2S含量、COD、盐度等水质指标没有显著影响;在养殖后期水环境中投喂频率3～5次·d-1的NH 4和NO-2浓度较投喂频率1～2次·d-1时高。     

Water quality,production parameters and nutritional condition of Litopenaeus vannamei (Boone, 1931) grown intensively in zero water exchange mesocosms with artificial substrates

The effect of artificial substrates on the water quality, production parameters and nutritional condition was assessed in experimental intensive cultures of Litopenaeus vannamei grown in mesocosm units with zero water exchange. The initial stocking densities in triplicate 1000 L tanks were 600 and 800 g of juvenile shrimp (2.7 g) per unit, with (W) and without (WO) artificial substrates. There were no significant differences between the water quality parameters of the four treatments. In the tanks with the lower stocking biomass, the best survival (96.7% vs. 87.0%), growth rate (1.69 vs. 1.35 g week⁻¹), final biomass (1969.6 vs. 1516.0 g m³) and feed conversion ratio (1.30 vs. 1.90) were obtained with artificial substrates. Similar results were observed for the higher stocking biomass (90.9% vs. 74.5%, 1.50 vs. 1.13 g week⁻¹, 2221.6 vs. 1560.8 g m³, and 1.50 vs. 2.60 respectively). The nutritional condition of shrimp was improved using artificial substrates. The protein content in muscle was higher (21.6% and 20.4%) in ponds with substrates as compared with the control (21.2% and 15.9%).     

The effects of different solids and biological filters in intensive pacific white shrimp (Litopenaeus vannamei) production systems

Biofloc systems rely on suspended solids in the water to house microbes that can remove or cycle nitrogenous wastes; however, nitrogen cycling can be inconsistent. In contrast, external biofilters are used in many recirculating systems to provide a more consistent environment for microbes to process nitrogen. Regardless of the biofiltration approach, solids levels must be controlled to prevent issues in shrimp such as gill fouling, low dissolved oxygen levels, and other negative impacts. The purpose of this study was to examine the effects of settling chambers versus foam fractionators for solids filtration and to compare external biofilters to the biofloc approach as biofiltration strategies. Sixteen 1-m³ round, polyethylene tanks were randomly assigned to four treatments, each of which had four replicate tanks. Eight biofloc systems were established: four using settling chambers for solids control (BF-S) and four using foam fractionators (BF-F). The other eight tanks used external biofilters; four had settling chambers (EB-S) and the other four had foam fractionators (EB-F). All 16 systems were stocked with 250 shrimp at an average size of 4.3 g which were grown for 85 days. There were no significant differences in shrimp production between treatments; however, variability was high in biofloc systems. Nitrite levels were significantly lower in systems with fractionators compared to systems with settling chambers. The concentrations of dissolved Na, Mg, Ca, Sr and Ba in the water were significantly reduced in treatments with settling chambers. The results of this study show that filtration choices significantly impact short- and long-term water quality and reusability but may not have much effect on shrimp production in the short-term.     

Food inputs, water quality and nutrient accumulation in integrated pond systems: A multivariate approach

A participatory on-farm study was conducted to explore the effects of food input patterns on water quality and sediment nutrient accumulation in ponds, and to identify different types of integrated pond systems. Ten integrated agriculture-aquaculture (IAA) farms, in which ponds associate with fruit orchards, livestock and rice fields were monitored in the Mekong delta of Vietnam. Pond mass balances for nitrogen (N), organic carbon (OC) and phosphorus (P) were determined, and pond water quality and sediment nutrient accumulation were monitored. Data were analyzed using multivariate canonical correlation analysis, cluster analysis and discriminant analysis. The main variability in pond water quality and sediment nutrients was related with food inputs and water exchange rates. Water exchange rate, agro-ecological factors, pond physical properties and human waste input were major variables used to classify ponds. Classification was into: (1) low water exchange rate ponds in the fruit-dominated area, (2) low water exchange rate ponds in the rice-dominated area receiving homemade feed, and (3) high water exchange rate ponds in the rice-dominated areas receiving wastes. Pond water exchange rate was human-controlled and a function of food input patterns, which were determined by livelihood strategies of IAA-households. In the rice-dominated area with deep ponds, higher livestock and human wastes were found together with high water exchange rates. In these ponds, large organic matter loads reduced dissolved oxygen and increased total phosphorus concentrations in the water and increased nutrient (N, OC and P) accumulation in the sediments. In the rice-dominated area with wide ponds, higher homemade feed amounts were added to the ponds with low water exchange rate. This resulted in high phytoplankton biomass and high primary productivity. The contrary occurred in the fruit-dominated area, where fish were grown in shallow and narrow ponds, receiving more plant residue which resulted in lower phytoplankton biomass and lower sediment nutrient accumulation.     

温度、盐度对凡纳滨对虾精荚再生和精子质量的影响

研究了3种不同温度和盐度对凡纳滨对虾精荚再生和精子质量的影响。通过称量精荚以及采用染色或显微镜观察养殖在不同温、盐度条件下的雄虾的精子数量和正常率来评价精子质量。结果表明:随着实验养殖时间的延长和精荚再生次数的增加,26℃组的精子质量明显好于22℃组和30℃组,30℃组的活精子数由第1次再生的14.0×106个下降到4.3×106个,畸形精子百分率由6.1%增加到56.6%。温度越高,蜕皮周期越短,精荚再生时间也越短,但精子质量下降也很迅速。盐度30组的精荚重量显著高于盐度5组和15组,但盐度15的精子总数和活精子数却显著高于盐度5组和30组,随着时间的延长,3组盐度的畸形精子百分率都有所增加,其中盐度5组增加得最快。     

Comparison of growth and reproduction performance of broodstock Pacific white shrimp Litopenaeus vannamei reared in oceanic and brackish water

This study provides a preliminary evaluation of feasibility of brackish water breeding. Shrimps were initially cultured at brackish water. Shrimps were reared at different stages (5, 6, 7 and 8 months, respectively), and 300 female shrimps were selected from each stage for growth at an enhanced salinity of 30 ppt. When the shrimps were sexually mature, the reproductive characteristics (mating rate, egg number and nauplii number) of the broodstock shrimps, quality of nauplii (malformation rate of nauplii) and quality of postlarvae (survival rate and desalination stress) were tested to evaluate of the effects of brackish water (10 ppt) on these shrimps. Oceanic water (30 ppt) for rearing shrimps was used as control group. Histopathological analysis of gonads and hepatopancreases from oceanic and brackish water shrimp was performed. The results showed that shrimps cultured for a maximum of 8 months in brackish water remained sexually mature. There were no significant differences regarding the quality of shrimps in the oceanic water and brackish water. However, the brackish water cultured shrimps were easily adapted to salinity changes. Our results demonstrate the good feasibility of the present scheme, in which the broodstock shrimps were cultured at a salinity of 10 ppt.     

Effects of pond preparation and feeding rate on production of Penaeus monodon Fabricius, water quality, bacteria and benthos in model farming ponds

Fibreglass pools with sediment were used as model farming ponds to investigate the interactive effects of pond preparation and feeding rate on prawn production, water quality, bacterial dynamics, abundance of benthos and prawn feeding behaviour. Pools were either fertilised 1 month (prepared) or 2 days (unprepared) prior to stocking and either ‘high’ or ‘low’ feeding rates were used. The ‘high’ rate was 5.0% (range 4–8%) wet prawn biomass/day and was similar to that recommended for commercial farms. The ‘low’ rate was 2.5% (range 2–4%) wet prawn biomass/day. Juvenile Penaeus monodon (2.0–7.5 g) were stocked at 15 prawns/m² and were cultured for 71 days. With the exception of one prepared, high feeding-rate pool where mass mortality (> 80%) of prawns occurred following an interruption to aeration, prawn survival was high (> 86%) and was unaffected by preparation, feeding rate or their interaction. Pond preparation improved growth and biomass gain by about 20%. Growth was 4% higher with the higher feeding rate but biomass gain was not affected and, as food conversion ratio was much worse, use of the lower feeding rate offers considerable scope to reduce production costs, especially during cooler periods. There was no interaction in relation to growth between pond preparation and feeding rate. Meiofauna were more abundant, and prawns grew faster, in prepared pools than unprepared pools at the start of the experiment. However, changes in bacterial dynamics or meiofauna abundance over time did not explain reductions in prawn growth over time. In general, water quality was reduced in pools receiving the high feeding rate compared with low feeding rate pools. Other interactive effects of pond preparation and feeding rate on water quality, bacteria, benthos and prawn feeding behaviour are discussed.     

Effect of different biofloc levels on microbial activity,water quality and performance of Litopenaeus vannamei in a tank system operated with no water exchange

In zero-exchange superintensive culture systems, flocculated particles (bioflocs) accumulate in the water column. Consequently, some control over the concentration of these particles must be performed. The objective of this study is to evaluate the effects of three concentrations of bioflocs on microbial activity, selected water quality indicators and performance of Litopenaeus vannamei in a tank system operated with no water exchange. A 44-day study was conducted with juvenile (6.8 g) shrimp stocked in twelve 850 L tanks at a stocking density of 459 shrimp m⁻³. Biofloc levels were expressed as three presets of total suspended solids (TSS) concentrations, as follows: 200 mg L⁻¹ (T200), 400–600 mg L⁻¹ (T400–600), and 800–1000 mg L⁻¹ (T800–1000). TSS levels were controlled by attaching a 40 L settling tank to each culture tank. Reduction of TSS to concentrations close to 200 mg L⁻¹ decreased the time of bacterial cell residence and significantly reduced the nitrification rates in the water (P < 0.05). The tanks in the T200 treatment had a greater variability of ammonia and nitrite (P < 0.05), which led to the need to increase the C:N ratio of the organic substrate to control ammonia through its assimilation into heterotrophic bacterial biomass. But the higher production of heterotrophic bacteria in T200 (P < 0.05) increased the dissolved oxygen demand. Nitrification rates were higher (P < 0.05) in tanks with TSS concentrations above 400 mg L⁻¹, and ammonia and nitrite were significantly lower than in the T200 tanks. We suggest that ammonia and nitrite in the T400–600 and T800–1000 tanks were controlled primarily by nitrifying bacteria, which provided higher stability of these parameters and of dissolved oxygen. Regarding shrimp performance, the reduction of TSS to levels close to 200 mg L⁻¹ was associated with better nutritional quality of bioflocs. Nevertheless, differences in biofloc levels and nutritional quality were not sufficient to affect the weight gain by shrimp. The rate of shrimp survival and the final shrimp biomass were lower (P < 0.05) when the TSS concentrations were higher than 800 mg L⁻¹. Analysis of the shrimps’ gills showed a higher degree of occlusion in the T800–1000 treatment (P < 0.05), which suggests that the shrimp have an intolerance to environments with a solids concentration above 800 mg L⁻¹. Our results show that intermediate levels of bioflocs (TSS between 400 and 600 mg L⁻¹) appear to be more suitable to superintensive culture of L. vannamei since they create factors propitious for maintaining the system’s productivity and stability