The Effect of Microbial Biofloc on Water Quality, Survival andGrowth of the Green Tiger Shrimp (Penaeus Semisulcatus) Fed
with Different crude Protein Levels
I: Sustainable Solution to the Dependency on Fish Oil, Fishmealand Environmental Problems
Mohamed E. MegahedDepartment of Aquaculture and Fish Resources, Faculty of
Environmental Agricultural Sciences, Suez Canal University, EL-Arish, North Sinai, Egypt.
ABSTRACT
On- farm trial were conducted to evaluate the effects of feeding on pelletswith different protein levels in the presence and absence of the bioflocs on waterquality, survival and growth of the green tiger shrimp (Penaeus semisulcatus) inintensive types of shrimp culture systems. Five different feeds were formulated. Fourbiofloc treatments (BFT) and one control were managed in 10 greenhouses earthenponds of 300 m2 each: BFT fed feeds of 31.15% crude protein (CP) (BFA31.15%),21.60% CP (BFB21.60%), 18.45% CP (BFC18.45%) and 16.25% CP (BFD16.25%) and acontrol without biofloc but fed a 42.95% CP feed. The bioflocs were developed in theBFT treatments using wheat flour as a carbon source. Thirty juvenile Penaeussemisulcatus with an average body weight of 1.55 ± 0.2 g were stocked per m2 andeach dietary treatment and the control was tested in two replicates over a 120 daysfeeding trial. Biofloc diets enhanced shrimp growth. There was a significantdifferences (P<0.05) between control and treatment groups in terms of final averagebody weight (ABW) at harvest (15.1 ± 1.6; 19.8 ± 0.1; 19.2 ± 0.3; 20.1 ± 0.3 and19.0 ± 1.1 for control; BFA31.15%; BFB21.60%; BFC18.45% and BFD16.25%, respectively)confirming the utilization of biofloc by shrimp as a food source. Concerning finalshrimp yield per dietary treatment, there was a significant differences (P<0.05)between control and treatment groups at harvest (114.1 ± 5.3; 151.5 ± 7.9; 152.6 ±7.4; 155.5 ± 6.9 and 153.6 ± 8.4 kg pond-1 for control; BFA31.15%; BFB21.60%;BFC18.45% and BFD16.25%, respectively). The FCR differs significantly between BFTtreatment and control (P<0.05). Growth (weight gain week-1) for control, BFA31.15%,BFB21.60%, BFC18.45% and BFD16.25% were respectively 0.85 ± 0.4; 0.98 ± 0.1; 1.00 ±0.2; 1.11± 0.2; and 1.00 ± 0.2. The shrimp survival % was not affected (P>0.05) bythe treatments and it ranged between 85.7% and 88.4%. The addition of carbohydrateto the water column (P<0.05) reduced the nitrate, nitrite-N levels and TAN in theexperimental greenhouses. There was a significant differences (P<0.05) in the nitrate,nitrite-N levels and TAN concentrations between treatments and the controlgreenhouses. Simple economic calculation revealed that the lowest CP% BFT
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treatment, BFD16.25% had a lowest total feed cost in addition to better water qualityand best shrimp economical production compared to conventional control diet. Theresults concluded also that the biofloc treatments succeeded to reduce the cost of kgshrimp production. Tukey's HSD test revealed also that the four microbial floc dietssignificantly (P<0.05) outperformed control in terms of final ABW, final yield (kgshrimp/pond), FCR, weight gain per week and water quality parameters with nodifferences in survival rate.
Keywords: Biofloc, sustainability, Alternative feeds, water quality, Green tigershrimp
INTRODUCTION
World Aquaculture is growingwith an annual rate of 8.9–9.1% sincethe 1970s. This high growth rate isneeded to solve the problem ofshortage in protein food supplies,which is particularly situated in thedeveloping countries (Gutierrez-Wingand Malone, 2006; Matos et al., 2006and Subasinghe, 2005). The globalshrimp market has expanded from lessthan $1 billion to $5.8 billion (US)from 2000 to 2005 (FAO, 2008). Tomeet this growing demand, the shrimpindustry is shifting from extensiverearing systems to more intensiverearing systems. However,environmental (i.e. discharge of farmeffluents) and economical limitations(Higher prices of feed ingredients,especially fishmeal) can hamper thisgrowth. The expansion of theaquaculture production is restricted dueto the pressure it causes on theenvironment by the discharge of wasteproducts in the water bodies and by itsdependence on fish oil and fishmeal(De Schryver et al, 2008).
In order for aquaculture to becompletely successful, the industry willneed to develop technology that willincrease economic and environmentalsustainability (Kuhn et al., 2010). Thistechnology implements cheaperalternative ingredient to fishmeal andthis will effectively reduce the costs offeed as feed costs can account for 50%of operational expense (Van Wyk etal., 1999) while reducing their impacton overexploited natural fisheries(Tacon et al., 2006 and Naylor et al.,2009). Thus, it is important todetermine if alternative ingredientsderived from biologically treating fishwaste, bioflocs (microbial flocs), couldbe a suitable replacement ingredient inmarine shrimp diets. If implementedsuccessfully, this option would offer asustainable option to fishmeal.
According to Kuhn et al. (2010),initial cost estimates for bioflocproduction is approximately $400 to$1000 per ton of dry ingredients whichis projected to be less than theingredients such as fishmeal and withinthe range for soybean meal. Over theperiod of January 2008 to May 2009,
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the global fishmeal market varied froma low mean of about $900 to a highmean of $1250 per metric ton. Duringthe same time frame, soybean mealvaried approximately from a low meanof $375 to a high mean of $550 (FAO,2009). Thus, we are suggesting that theuse of biofloc represents a viable andmore sustainable feed option due tocost, the manner in which it isgenerated, and the potential that it canease the pressure on wild fisheries byreducing at least some of the demandfor fishmeal.
Aquaculture produces largequantities of wastes that contain solids(e.g. feces and uneaten feed) andnutrients (e.g. nitrogen andphosphorus) which can be detrimentalto the environment, if managedimproperly. These solids and nutrientsoriginate from uneaten feed, feces, andanimal urea/ammonia (Maillard et al.,2005 and Sharrer et al., 2007), ifreleased directly to the environment,these solids and nutrients can bepollutants resulting in environmentalissues such as eutrophication (Wetzel,2001) or could be directly toxic toaquatic fauna (Timmons et al., 2002and Boardman et al., 2004). The mostcommon method for dealing with thispollution has been the use ofcontinuous replacement of the pondwater with new clean water from thewater source (Gutierrez-Wing andMalone, 2006). For instance, Penaeidshrimp require about 20 m3 fresh water
per kg shrimp produced (Wang, 2003).For an average farm with a productionof 1000 kg shrimp ha−1 yr−1 and totalpond surface of 5 ha, this correspondswith a water use of ca. 270 m3 day−1.
A relatively new alternative toprevious approaches is the bioflocstechnology (BFT) aquaculture(Avnimelech, 2006). In these systems,a co-culture of heterotrophic bacteriaand algae is grown in flocs undercontrolled conditions within the culturepond. The system is based on theknowledge of conventional domesticwastewater treatment systems and isapplied in aquaculture environments.Microbial biomass is grown on fishexcreta resulting in a removal of theseunwanted components from the water.The major driving force is the intensivegrowth of heterotrophic bacteria. Theyconsume organic carbon; 1.0 g ofcarbohydrate-C yields about 0.4 g ofbacterial cell dry weight-C; anddepending on the bacterial C/N-ratiothereby immobilize mineral nitrogen.As such, Avnimelech (1999) calculateda carbohydrate need of 20 g toimmobilize 1.0 g of N, based on amicrobial C/N-ratio of 4 and a 50% Cin dry carbohydrate.
The present study aimed toreduce the inorganic nitrogenaccumulating in an extensive shrimpculture system by (1) increasing theC/N ratio of the feed through reducingits protein content and by (2)
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increasing the C/N ratio furtherthrough carbohydrate addition to theponds. These manipulations shouldfacilitate the development ofheterotrophic bacteria and the relatedin situ protein synthesis, which in turnwill contribute to the protein intake ofthe shrimps. Also, this study aims toreduce the inclusion level of fishmealin the feeds of marine shrimp toenhance sustainable development ofmarine shrimp aquaculture in Egypt.
MATERIALS AND METHODS
Experimental designThe experiment was carried out
at the Shrimp and Fish InternationalCompany (SAFICO), South Sinai,Egypt, over a 120 days ( from 1st ofmay, 2010 to 1st of September, 2010)feeding trial in 10 greenhouses with anaverage area of 300 m2 and 1.0 maverage depth each. Postlarvae (PL15)shrimp Penaeus semisulcatus werepurchased from (Haraz’s Marine Fishand Shrimp Hatchery, El- Qantara,Ismailia). Shrimp were grown in anursery to an initial stocking weight of1.55 ± 0.2 g for the experimentalfeeding trial. The feeding experimentconsisted of four treatments (with tworeplicates) and one control (with tworeplications) was compared. In controlgreenhouses, the shrimp were fed withartificial feed with 42.95% crudeprotein (CP), whereas in the bioflocstreatments, the shrimp were fed with
bioflocs and artificial feeds withdifferent CP%. The description of thedifferent treatments and control used isas follow:1. Control: artificial feed with 42.95
CP%.2. Treatment A 31.15 CP%
(BFA31.15%).3. Treatment B 21.60 CP%
(BFB21.60%).4. Treatment C 18.45 CP%
(BFC18.45%).5. Treatment D 16.25% CP%
(BFD16.25%).
Feeds
A control feed (high proteincontent) in the absence of bioflocproduction was challenged against fourfeeds formulated with different levelsof crude protein (low protein diet) inthe presence of biofloc production. Thebasic guideline for the formulation ofthe feeds was to reduce the inclusion offishmeal. Different CP levels in allfeeds were achieved by manipulatingvarious inclusion levels of fishmealprotein. All feeds were produced at thefeed preparation section of the studyfarm.
The experimental feeds wereanalyzed for the proximatecomposition following Association ofAnalytical Chemist Methods(A.O.A.C., 2000). Moisture contentwas determined by drying in an oven at
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85oC to constant weight. Crude proteinwas determined indirectly from theanalysis of total nitrogen (crude protein= amount of Nitrogen x 6.25) usingKjeldahl method while crude lipid wasdetermined after soxhlet extraction ofdried samples with 1.25% H
2SO
4and
1.25% NaOH. Ash content after ashingin a porcelain crucible placed in a
muffle furnace at 550oC for 16 hours.
Proximate compositions ofexperimental feeds were presented inTable 1.
Feeding trial
At the start of the experiment, theshrimp had an average weight (±standard deviation) of 1.55 ± 0.2 g.The rearing units consisted of 10greenhouses and were stocked at an
initial stocking density of 30individuals /m2. Feed was given at 5%of the shrimp biomass. Feed weregiven in daily four meals. Wheat flourwas also added at a rate of 50% of feedapplied to each biofloc treatment tomaintain an optimum C:N ratio forbacteria (Goldman et al, 1987;Avnimelech, 1999 and Hargreaves,2006). Wheat flour was spread to thegreenhouses surfaces in the afternoonand completely mixed with the waterof each greenhouse by strong aerationsystem. Discharged water from thefarm of the present study was used asinoculum to develop the biofloc.Nutritional value of biofloc wasdetermined every 30 days in order toobtain information on its nutritionalcontribution to each biofloc treatment.
Table 1. Proximate composition of formulated feeds based on dry weight basis (g/100g)Constituent Test diets
The biofloc was collected from eachtreatment using a net and was dried inan oven at 85oC to constant weight.The amount of CP% was determinedby the (A.O.A.C., 2000) methods,lipid% were estimated by the Bligh and
Dyer (1959) Method as modified byKates (1986). Total n-3 PUFA (mg/gDW) and total n-6 PUFA (mg/g DW)were determined using the NOAAprotocol (1988) of National Marine
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Fisheries for the analysis of marine fishoil.
Shrimp performance indicators
Weekly sampling to estimatesurvival rate %, Feed conversion ratio(FCR), average final body weight(ABW), weight gain (g week-1) wereused to assess dietary effects on shrimpperformance. At the end of the 120-days experiment, the sameperformance indicators were estimatedin addition to final yield (kg shrimppond-1) per treatment basis.
Economic Analysis
The economic analysis wascomputed to estimate the cost of feedrequired to raise a kilogram of shrimpusing the various experimental feeds.The major assumption is that all otheroperating costs for commercial shrimpproduction will remain the same for allfeeds. Thus cost of feed was the onlyeconomic criterion in this case. Thecost was based on the current prices ofthe feed ingredients as at the time ofpurchase. The economic evaluations ofthe feeds were calculated from themethod of (New 1989 and Mazid et al.,1997) as:
Profit Index = Value of shrimp cropped(L.E)/Cost of Feed (L.E).
Net Profit = Total value of shrimpcropped (L.E) – Total Expenditure(L.E).
Benefit: Cost Ratio (BCR) = Totalvalue of shrimp cropped (L.E) /Total Expenditure (L.E).
Water quality
During the experimental period,water quality in the culture systemswas monitored daily for dissolvedoxygen (mg/L), salinity (ppt), pH andtemperature oC in all greenhouses atsunrise (05:30 – 06:00) using a YSI556MPS meter (Yellow SpringInstrument Co., Yellow Springs,OH,USA). Nitrite (NO2–N mg/L) andnitrate (NO3–N mg/L) were analyzedspectrophotometrically according tostandard methods (APHA, 1998).
Statistical analysis
Statistical analysis wasperformed using SAS v9.2 forWindows (Cary, North Carolina, US,2002–2004). Analysis of variance(ANOVA) was used for the dataanalysis. Tukey's HSD was employedto check for differences between meansaccording to the method described byZar (1996). The 5% significance levelwas used for all tests.
RESULTS
Biofloc consumption
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The shrimp in the bioflocstreatment were actually able toconsume the flocs. This was shown bygrazing behavior observed in the pondsand biofloc harvested materials withshrimp samples and also by the colorof their digestive tract.
Shrimp performance indicators
As presented in Table 2, nodifferences were observed (P>0.05)between survival rate % (85.7% to88.4%). The development of theBiofloc enhanced the shrimp growth.Tukey's HSD revealed that all fourbiofloc treatments significantlyoutperformed the control in terms offinal ABW (g), yield (kg shrimp pond-
1), FCR, weight gain (g week-1) atharvest confirming the utilization ofbiofloc by fish as food.
Economically speaking, as can beseen from economic parameters inTable 3, the biofloc treatmentsucceeded to reduce the cost comparedto the control diet. The highest feedcost was for the control feed and thelowest feed cost was for theBFD16.25%treatment. The best net profitof 5,785.76 was recorded from shrimp
fed BFC18.45% and the best Cost benefitratio of 3.99 were recorded fromshrimp fed BFD16.25%. This due to thehigher cost of high protein shrimp feedin control diet compared to otherdietary treatments in the presence ofthe biofloc formation. The totalrevenue from the harvested shrimp washigher in the dietary treatments withlow CP levels in the presence ofbiofloc than in control diet due to thecombined effect of better yield, lesscost and high price of shrimpsaccording to their marketable size(Tables 2 and 3).
Nutritional value of bioflocs
There were no significantdifferences (P>0.05) in the CP%, totallipids, total n-3 PUFA (mg/g D) andtotal n-6 PUFA (mg/g DW) betweenbiofloc treatments (Table 4).
Water quality
The average value of dissolvedoxygen, nitrate, nitrite, TAN, pH,temperature and salinity during thefeeding experiment are displayed inTable 5.
Table 2. Mean production parameters (mean ± SD) of shrimp P. semisulcatus after 120 days ofculture and fed four feeds with varying levels of CP% with biofloc grown with wheat flouras a carbon source and control feed without biofloc (42.95 g/100g).Parameters Treatment
Mean values in same row with different superscript differ significantly (P<0.05).*Analysis based on dry weight basis (g/100g)
There were no significantdifferences (P>0.05) in the pH, watertemperature and salinity between BFTand control greenhouses. Theconcentrations of nitrogen species(nitrate, nitrite and total ammonia –N(TAN)) in the control greenhouses wassignificantly (P<0.05) higher than thatof other treatments. There was asignificant difference (P<0.05) indissolved oxygen between control andall other BFT treatments. The additionof carbohydrate to the water column(P<0.05) reduced the nitrate, nitrite-Nlevels and TAN in the experimentalgreenhouses. The ANOVA resultsshowed that the protein level in the dietwas having a significant effect(P<0.05) on the water TAN, nitrite-Nand total nitrogen concentrations, ascan be seen from different dietarytreatments. There was a significantdifferences (P<0.05) between BFTponds and the control ponds (Table 5).
DISCUSSION
Biofloc: A sustainable solution forreduction of feed cost
Microbial flocs produced inthis study could offer the shrimpindustry a novel alternative feed andreduction in the dependency on fish oil
and fishmeal in feeding marine shrimp.In this study, microbial flocs wereproduced in intensive shrimpgreenhouses using wheat flour as acarbon source. Feed was applied at 5%of the total shrimp biomass in dailyfour rations. The nutritional quality ofbiofloc was appropriate for shrimp(Table 4). There was significantdifference (P<0.05) in shrimpgrowth/production between control andbiofloc treatments of varying lowprotein levels.
Survival and abnormality werecompared and no significantdifferences (P>0.05) between BFT andcontrol diet indicating no increasedshrimp stress due to the presence of
Table 5. Summary of water quality parameters observed over 120-days growingperiod in the shrimp culture system during the feeding experiment.
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Treatment Parameters
Dissolvedoxygen(mg/L)
Nitrate-N
(mg/L)
Nitrite-N(mg/L)
TAN(mg/L)
pH TemperatureoC
Salinity(ppt)
Control 5.80±2.43c 1.40±0.78d 0.1±0.23d 1.05±1.51c 8.07±0.86a 28.6±1.6a 44.1±1.3a
Mean values in same column with different superscript differ significantly(P<0.05).Values are (Mean ± SD).
biofloc. Overall shrimp growth andproduction was good in terms ofcommercial feasibility (Table 3).
During the feeding experiment,it can be seen that the shrimp in all thebiofloc treatments were able toconsume the flocs. This was visuallyobserved by the color of the digestivetract of the shrimp. The shrimp fedwith control feed showed greenishdigestive tracts similar to the color ofthe feed whereas those fed withbioflocs revealed whitish and brownishdigestive tracts, similar to the colors ofthe bioflocs.
There was no significantdifference (P>0.05) in survivalbetween the control and all othertreatments. The ABW of biofloctreatments was significantly (P<0.05)higher than control and there was nosignificant difference (P>0.05) amongbiofloc treatments. Based on visualobservation made during the
experiment, the shrimps in control dietreduced feed intake which showed bysampling, checking feeding trays andby observing the empty digestive tract.This can be due to several reasons suchas the palatability of feed, stress due todisease infection or water qualitydeterioration. Also, Tacon (1987)found that the absence of feedattractant and low palatability may alsohave been the cause of less feedconsumption in control diet.
During the culture period, thebiofloc treatments showed good flocformation. Crab et al. (2007) pointedout that at moderate mixing rate aspracticed in aquaculture system (1 – 10W/m3), microbial cells in permeableaggregates grow better than singledispersed cells due to higheraccessibility to the nutrients. Intenseaeration on the other hand minimizesthe advantage of growing in flocs andfree cells show a higher nutrientuptake.
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The differences in growthwhen the biofloc was included wereevident. Survival rates did not vary(P>0.05) among dietary treatments;however, shrimp growth wassignificantly improved (P<0.05) forshrimp fed microbial flocs (Table 2).Eventhough numerous studies havereported enhanced survival, health, andgrowth rates of shrimp raised in pondswith high activity of algae, microbialflocs, and other natural biota(Avnimelech, 1999; Moss et al., 2000;Moss et al., 2001; Tacon et al., 2002;Burford et al., 2004; Cuzon et al.,2004; Izquierdo et al., 2006 andWasielesky et al., 2006).
There was no abnormality ordisease symptoms observed in controland biofloc treatments. Growth ofshrimp as well as other aquaticorganisms is mostly affected by waterquality (Smith et al., 2000), culturesystems (Williams et al., 1996 andTacon et al., 2002), nutrition (Chen etal, 2006) and health condition(Rengpipat et al., 1998; Rodriguez andLe Moullac, 2000 and Argue et al.,2002).
The bacterial protein and newcells (single-cell protein) synthesizedby the heterotrophic bacterialpopulation are utilized directly as afeed source by the cultured fish andshellfish species, thus lowering the
demand for supplemental feed protein(Avnimelech, 1999). Hari et al (2004)reported that Penaeus monodon couldeffectively utilize the additional proteinderived from the increased bacterialbiomass as a result of carbohydrateaddition. Burford et al (2004)suggested that “flocculated particles”rich in bacteria and phytoplanktoncould contribute substantially to thenutrition of the Litopenaeus vannameiin intensive shrimp ponds. Bacterialflocculation was observed, probablysupporting filtering out by the shrimpand thus supplying protein that wasavailable and suitable for shrimpnutrition.
With regard to the proteinrequirement of white shrimp, Kureshyand Davis (2002) reported that 32%protein diet gave a better growth ofjuvenile and subadult Litopenaeusvannamei as compared to diets with alower (16%) and a higher (42%)protein content. It was found that theaddition of carbohydrates, essentiallychanging the high CP % protein feedmaterial to low CP% protein feed, ledto significant reduction of inorganicnitrogen accumulation; increasedutilization of protein feed; andsignificant reduction of feedexpenditure (Avnimelech et al., 1992and 1994).
In terms of shrimp growth andfeed utilization, it can be seen that
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shrimp growth was better in the lowCP treatment, most likely due to thelower concentrations of toxic inorganicnitrogen species. In addition to a lowerfeed conversion ratio (FCR).According to Avnimelech (1999), theprotein conversion ratio (PCR) wasmarkedly reduced in the 20% proteintreatment. The PCR in theconventional 30% protein feedtreatment was 4.35–4.38, meaning thatonly 23% of the feed protein wasrecovered by the fish. The PCR in thelow CP% was twice as high. Theincreased protein utilization is due toits recycling by the microorganisms. Itmay be said that the proteins are eatenby the fish twice, first in the feed andthen harvested again as microbialproteins. It is possible that proteinrecycling and utilization can be furtherincreased.
It is not known exactly howmicrobial flocs enhance growth.Izquierdo et al. (2006) suggested lipidcontributions of microbial flocs areimportant. It is also speculated thatmicrobial flocs are probiotics (Bairagiet al., 2002 and 2004 and Kesarcodi-Watson et al., 2008). Ultimately, morework needs to be done in order to fullyunderstand the contributions ofmicrobial flocs and natural organismsfound in ponds.
There were no significantdifferences (P>0.05) in the total CP%,
total lipids, total n-3 PUFAs and totaln-6 PUFAs among biofloc treatments.This may be caused by the similarcomposition of the microbiota in thebioflocs (e.g. marine microalgae).
As shrimp aquaculture isexpected to continue to increase in thecoming years, shrimp prices are likelyto continue to fall as productionexceeds demand, therefore challengingthe profitability of this industry. Onefactor considered to reduce shrimpproduction costs and increaseproducers profitability, is the use offeeds with low levels of fishmeal andhigh levels of less expensive, highquality plant protein sources.Commercial shrimp feeds arecommonly reported to include fishmealat levels between 25% and 50% of thetotal diet (Tacon and Barg, 1998 andDersjant-Li, 2002). However, recentstudies have shown that commercialshrimp feeds containing 30–35% crudeprotein can include levels as low as7.5–12.5% fishmeal withoutcompromising shrimp performance(Fox et al., 2004). Protein levels in L.vannamei diets have been reducedfrom the reported protein requirementsof 30% to 44% (Guillaume, 1997) to22% in high-intensity, zero-exchangeponds, based on the theory that theflocculated particles in these systemscontribute substantially to shrimpnutrition (Hopkins et al., 1995 andMcIntosh and Avnimelech, 2001).
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Due to the fact that proteinsare the expensive component of thefeed, its reduction was reflected in thetotal feed cost which decreased from1,862.76 L.E for control diet to249.27for the lowest protein feedBFD16.25% ( Table 3). On the otherhand, low CP% led to significantreduction of inorganic nitrogenaccumulation and improved shrimpgrowth.
The cost of production and thebenefits positively favored all BFTtreatments since the values computedare > 1.0 and the best values were forall treatments with biofloc whichshows an increase in the shrimp valueabove the amount invested. Thisachieves high profit for the farmerswhen BFD16.25% is used to reduce theinclusion level of fishmeal in the feedsof P. semisulcatus. This aquacultureutilization will promotes sustainableaquaculture in Egypt and helps in theattraction of new investments in thefield of marine shrimp aquaculture inEgypt.
Production results obtained inthis study are within the range ofcommercial shrimp production inintensive production systems. Thisstudy supports the theory that naturalbiota can provide a nitrogen source forshrimp, and that flocculated particles
are likely to be a significant proportionof this nitrogen source.
If this biofloc technologyproved to be successful, it could offerthe shrimp industry a new cultureoption. A very significant furtherjustification is the need to havealternative lower cost feeds replacingmarine fish and shrimp meals. Forthese reasons, this study investigated ifit would be possible to producemicrobial floc as a potential ingredientfor reducing fishmeal in shrimp feeds.
Biofloc: A sustainable solution tocontrol water quality and
environmentally friendly option
The accumulation of thenitrogen species (nitrate, nitrite, andTAN) in the culture systems is due tothe decomposition of uneaten feed andexcretion products. The nitrogenspecies concentration in control wassignificantly (P<0.05) higher thanother treatments. This suggested thatnitrification was likely to occur in thistreatment. In the biofloc treatments, thenitrification bacteria were possiblyoutcompeted by the heterotrophicbacteria. The presence of high TANconcentration in the control indicatedthat there was not sufficient organiccarbon available to convert allinorganic nitrogen into bacterialbiomass compared to all other biofloctreatments.
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Intensification of aquaculturesystems is inherently associated withthe enrichment of the water withrespect to ammonium and otherinorganic nitrogenous species. Themanagement of such systems dependson the developing methods to removethese compounds from the pond. Oneof the common solutions used toremove the excessive nitrogen is tofrequently exchange and replace thepond water. However, this approach islimited because environmentalregulations prohibit the release of thenutrient rich water into theenvironment; the danger of introducingpathogens into the external water; andthe high expense of pumping hugeamounts of water.
The practical usage of biofloctechnology is essential strategy forenvironmental protection due to strictlegislation regarding the discharge offarm effluents into the neighboringwater bodies (seas, rivers and lakes).Water quality values in the presentstudy were considered optimal forshrimp culture (Davis et al., 1993;Lawrence and He, 1999; Van Wyk etal., 1999; Cuzon et al., 2004 and Foxet al., 2006).
Nitrogen produced inaquaculture systems is controlled byfeeding bacteria with carbohydrates,and through the subsequent uptake of
nitrogen from the water, by thesynthesis of microbial protein. Therelationship between addingcarbohydrates, reducing ammoniumand producing microbial proteinsdepends on the microbial conversioncoefficient, the C/N ratio in themicrobial biomass and the carboncontents of the added material(Avnimelech, 1999).
Typically, only 20–25% of fedprotein is retained in the fish raised inintensive systems (Avnimelech, 2006),the remainder being lost to the systemas ammonia and organic N in feces andfeed residue. Microbial breakdown oforganic matter leads to the productionof new bacteria, amounting to 40–60%of the metabolized organic matter(Avnimelech, 1999). Under optimumC:N ratio, inorganic nitrogen isimmobilized into bacterial cell whileorganic substrates are metabolized. Theconversion of ammonium to microbialprotein needs less dissolved oxygencompared to oxygen requirement fornitrification (Avnimelech, 2006 andEbeling et al., 2006) suggesting thepreference of heterotrophic communityrather than nitrifying bacteria in theBFT system. In addition, the growthrate and microbial biomass yield perunit substrate of heterotrophs are afactor 10 higher than that of nitrifyingbacteria (Hargreaves, 2006).
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The new strategy that ispresently getting more attention is theremoval of ammonium from the waterthrough its assimilation into microbialproteins by the addition ofcarbonaceous materials to the system.This offer the potential utilization ofmicrobial protein as a source of feedprotein for fish or shrimp. The fact thatrelatively large microbial cell clustersare formed due to flocculation of thecells, alone or in combination with clayor feed particles (Harris and Mitchell,1973 and Avnimelech et al., 1982)additionally favors cell uptake by fish.
The conventional controlmeans for ponds are to intensivelyexchange the water, a strategy that isnot always practical, and to stopfeeding to slow down TAN build up.The proposed method enables keepinga high biomass and to have a correctivemeans in case of a failure ofconventional controls.
Added value of bioflocs technology forshrimp production
According to De Schryver et al.(2008), the added value that BFTbrings to aquaculture is represented bythe reduced costs for water treatmentthat is not needed anymore. Bioflocsdo not allow for a completereplacement of the traditional food butstill can bring about a substantialdecrease of the processing cost sincethe feed represents 40–50% of the total
production costs (Craig and Helfrich,2002).
The potential savings of feed thatcan be obtained by BFT in shrimpculture was theoretically calculated asdescribed in De Schryver et al. (2008).According to Eric De Muylder (2009),white shrimp can be produced withartificial feed with 35% CP at anaverage food conversion ratio (FCR) of2.0.
In a culture without application ofbioflocs technology, the FCR of 2.0with 35% CP, which means 2 kg of
feed, is required to produce 1 kgshrimp
2 x 0.35 = 0.7 kg protein is givenper kg shrimp produced
According to Eric De Muylder(2009), only 20% of the feed istaken up by the shrimp. Thus, 0.2 x0.7 = 0.14 kg protein is taken upper kg shrimp produced.
In a culture system with bioflocsapplication, the FCR of 2.0 with 35%
CP, which means 2 kg of feed, isrequired to produce 1 kg shrimp
Part of the feed will be recycledinto flocs, which can also be consumedby the shrimps as feed source.Therefore, less artificial feed is appliedto the system. Take F the amount of
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artificial feed added to the system ifBFT is applied: With flocs, F kg feed is given per
kg shrimp produced. (0.35 x F) kg protein is given per
kg shrimp produced 0.35 x (0.2 x F) = 0.07 x F kg
protein is taken up per kg shrimpproduced.
As much as 80% of the artificialfeed is thus unused and recycledinto the flocs
(0.8 x F) kg feed is recycled 0.8 x 0.35 x F is recycled = 0.28 x
F kg protein recycled Assume that the shrimp also take
only 20% of the flocs 0.2 x (0.28 x F) = 0.056 x F kg
protein is taken up out of the flocsper kg of shrimp produced
Calculation of the amount of externalfeed needed when BFT is applied.
Total requirement of protein by theshrimp is 0.14 kg protein per kg of
shrimp produced:
Total protein requirement = proteinobtained from the feed + proteinfrom the flocs = 0.14
Total protein requirement = 0.07 xF + 0.056 x F = 0.14
The amount of feed that still needsto be applied = 1.11 kg
Calculation of the amount of organiccarbon needed to grow the flocs
0.8 x 1.11 = 0.9 kg of the feed isunused per kg of shrimp produced
0.35 x 0.9 = 0.3 kg protein isunused per kg of shrimp produced(protein content of the feed is 35%)
0.16 x 0.3 = 0.05 kg nitrogen isunused per kg of shrimp produced(16% nitrogen content of protein)and is recycled into floc biomass.The flocs have a C/N ratio of 4(Avnimelech, 1999)
4 x 0.05 = 0.2 kg C in floc biomassis produced per kg of shrimpproduced. The yield of bacterialbiomass can be taken to be 0.5(Avnimelech, 1999)
0.2 / 0.5 = 0.4 kg C needs to beadded to the water for the flocs tobe able to assimilate the excessnitrogen per kg shrimp produced. If glycerol (40% of C) is used
as carbon source
0.4/0.4 = 1 kg of glycerol needs tobe added to the water per kgshrimp produced.1.1.1 Calculation of the cost
saving for shrimp cultureusing BFT is as follow
The feed cost for theproduction of 1 kg shrimpwithout BFT
2 kg feed per kg shrimp x 0.9 € perkg of feed = 1.8 € per kg shrimpproduced.
MICROBIAL BIOFLOC AND PROTEIN LEVELS IN GREEN TIGER SHRIMP
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The feed cost for theproduction of 1 kg shrimp withBFT
(1.11 kg feed per kg shrimpproduced x 0.9 € per kg of feed) +(1 kg glycerol per kg shrimpproduced x 0.15 € per kg glycerol)= 1.2 € per kg shrimp produced.Thus, the feed cost for theproduction of 1 kg shrimp withBFT is 33% less than without BFT.
Based on the theoretical andpractical calculation, the application ofBFT is believed to reduce the feedcost. This is in agreement to otherstudies (Avnimelech, 2007 and Hari etal., 2004 and 2006). Besides thereduction in feed cost, the applicationof BFT also brings other beneficialeffects such as better water quality,which, leads to the moreenvironmentally friendly aquaculturepractices and increase biosecuritywhich can control the transmission ofthe aquatic animal diseases (Tacon etal., 2002).
CONCLUSION ANDRECOMMENDATION
In conclusion, the presentstudy suggests that P. semisulcatus arecapable for ingesting and retainingnitrogen derived from natural biota.Based on the result, it can beconcluded that biofloc technologyprovide a solution to reduce feed costand minimize the environmental
impacts. At low CP%, the shrimp fedbiofloc showed better growth rate asthat of the control. This indicates thatbiofloc is a sustainable strategy toreduce feed cost, environmental controland supporting shrimp farming.
Based on practical experience andresults gained in this study, somerecommendations are suggested:
Other nutritional parameters ofthe bioflocs such as amino acidsprofile, lipid class, vitamins andminerals content should bemeasured.
The use of carbon sources withlow price such as molasses,tapioca flour, rice bran, etc.should be investigated.
The effect of biofloc on thesurvival and pathogenicity ofVibrio should be monitored.
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