Aquaponics, the combined culture offish and plants in recirculating sys-tems, has become increasingly popu-lar. Now a news group (aquaponics-request@townsqr.com — type sub-scribe) on the Internet discussesmany aspects of aquaponics on adaily basis. Since 1997, a quarterlyperiodical (Aquaponics Journal) haspublished informative articles, con-ference announcements and productadvertisements. At least two largesuppliers of aquaculture and/orhydroponic equipment have intro-duced aquaponic systems to theircatalogs. Hundreds of school districtsare including aquaponics as a learn-ing tool in their science curricula. Atleast two short courses on aquapon-ics have been introduced, and thenumber of commercial aquaponicoperations, though small, is increas-ing.Aquaponic systems are recirculatingaquaculture systems that incorporatethe production of plants without soil.Recirculating systems are designedto raise large quantities of fish in rel-atively small volumes of water bytreating the water to remove toxicwaste products and then reusing it.In the process of reusing the water

many times, non-toxic nutrients andorganic matter accumulate. Thesemetabolic by-products need not bewasted if they are channeled intosecondary crops that have economicvalue or in some way benefit the pri-mary fish production system.Systems that grow additional cropsby utilizing by-products from the pro-duction of the primary species arereferred to as integrated systems. Ifthe secondary crops are aquatic orterrestrial plants grown in conjunc-tion with fish, this integrated systemis referred to as an aquaponic system(Fig. 1). Plants grow rapidly with dissolved

nutrients that are excreted directlyby fish or generated from the micro-bial breakdown of fish wastes. Inclosed recirculating systems withvery little daily water exchange (lessthan 2 percent), dissolved nutrientsaccumulate in concentrations similarto those in hydroponic nutrient solu-tions. Dissolved nitrogen, in particu-lar, can occur at very high levels inrecirculating systems. Fish excretewaste nitrogen, in the form of ammo-nia, directly into the water throughtheir gills. Bacteria convert ammoniato nitrite and then to nitrate (seeSRAC Publication No. 451,“Recirculating Aquaculture TankProduction Systems: An Overview ofCritical Considerations”). Ammoniaand nitrite are toxic to fish, butnitrate is relatively harmless and isthe preferred form of nitrogen forgrowing higher plants such as fruit-ing vegetables.

Aquaponic systems offer several ben-efits. Dissolved waste nutrients arerecovered by the plants, reducing dis-charge to the environment andextending water use (i.e., by remov-ing dissolved nutrients through plantuptake, the water exchange rate canbe reduced). Minimizing waterexchange reduces the costs of operat-ing aquaponic systems in arid cli-mates and heated greenhouses wherewater or heated water is a significantexpense. Having a secondary plantcrop that receives most of its required

VIPR

November 2006Revision

SRAC Publication No. 454

1 Agricultural Experiment Station, University of the Virgin Islands

2 Department of Wildlife and Fisheries Sciences, Texas A&M University

3

Biological and Agricultural Engineering Department, North Carolina State University

Recirculating Aquaculture Tank ProductionSystems: Aquaponics—Integrating Fish and

Plant CultureJames E. Rakocy1, Michael P. Masser2 and Thomas M. Losordo3

Figure 1. Nutrients from red tilapiaproduce a valuable crop of leaf let-tuce in the UVI aquaponic system.

nutrients at no cost improves a sys-tem’s profit potential. The dailyapplication of fish feed provides asteady supply of nutrients to plantsand thereby eliminates the need todischarge and replace depleted nutri-ent solutions or adjust nutrient solu-tions as in hydroponics. The plantsremove nutrients from the culturewater and eliminate the need forseparate and expensive biofilters.Aquaponic systems require substan-tially less water quality monitoringthan separate hydroponic or recircu-lating aquaculture systems. Savingsare also realized by sharing opera-tional and infrastructural costs suchas pumps, reservoirs, heaters andalarm systems. In addition, theintensive, integrated production offish and plants requires less landthan ponds and gardens. Aquaponicsystems do require a large capitalinvestment, moderate energy inputsand skilled management. Niche mar-kets may be required for profitabili-ty.

System designThe design of aquaponic systemsclosely mirrors that of recirculatingsystems in general, with the additionof a hydroponic component and thepossible elimination of a separatebiofilter and devices (foam fractiona-tors) for removing fine and dissolvedsolids. Fine solids and dissolvedorganic matter generally do notreach levels that require foam frac-tionation if aquaponic systems havethe recommended design ratio. Theessential elements of an aquaponicsystem are the fish-rearing tank, asettleable and suspended solidsremoval component, a biofilter, ahydroponic component, and a sump(Fig. 2). Effluent from the fish-rearing tank istreated first to reduce organic matterin the form of settleable and sus-pended solids. Next, the culturewater is treated to remove ammoniaand nitrate in a biofilter. Then, waterflows through the hydroponic unitwhere some dissolved nutrients aretaken up by plants and additionalammonia and nitrite are removed bybacteria growing on the sides of thetank and the underside of the poly-styrene sheets (i.e., fixed-film nitrifi-cation). Finally, water collects in areservoir (sump) and is returned tothe rearing tank. The location of thesump may vary. If elevated hydro-ponic troughs are used, the sump

can be located after the biofilter andwater would be pumped up to thetroughs and returned by gravity tothe fish-rearing tank. The system can be configured sothat a portion of the flow is divertedto a particular treatment unit. Forexample, a small side-stream flowmay go to a hydroponic componentafter solids are removed, while mostof the water passes through a biofil-ter and returns to the rearing tank.The biofilter and hydroponic compo-nents can be combined by usingplant support media such as gravelor sand that also functions as biofil-ter media. Raft hydroponics, whichconsists of floating sheets of poly-styrene and net pots for plant sup-port, can also provide sufficientbiofiltration if the plant productionarea is large enough. Combiningbiofiltration with hydroponics is adesirable goal because eliminatingthe expense of a separate biofilter isone of the main advantages ofaquaponics. An alternative designcombines solids removal, biofiltra-tion and hydroponics in one unit.The hydroponic support media (peagravel or coarse sand) captures solidsand provides surface area for fixed-film nitrification, although with thisdesign it is important not to overloadthe unit with suspended solids.As an example, Figures 3 and 4 showthe commercial-scale aquaponic sys-tem that has been developed at theUniversity of the Virgin Islands(UVI). It employs raft hydroponics.

Fish productionTilapia is the fish species most com-monly cultured in aquaponic sys-tems. Although some aquaponic sys-tems have used channel catfish,largemouth bass, crappies, rainbowtrout, pacu, common carp, koi carp,

goldfish, Asian sea bass (barramun-di) and Murray cod, most commer-cial systems are used to raise tilapia.Most freshwater species, which cantolerate crowding, will do well inaquaponic systems (including orna-mental fish). One species reported toperform poorly is hybrid stripedbass. They cannot tolerate high lev-els of potassium, which is often sup-plemented to promote plant growth. To recover the high capital cost andoperating expenses of aquaponic sys-tems and earn a profit, both the fish-rearing and the hydroponic veg-etable components must be operatedcontinuously near maximum pro-duction capacity. The maximum bio-mass of fish a system can supportwithout restricting fish growth iscalled the critical standing crop.Operating a system near its criticalstanding crop uses space efficiently,maximizes production and reducesvariation in the daily feed input tothe system, an important factor insizing the hydroponic component.There are three stocking methodsthat can maintain fish biomass nearthe critical standing crop: sequentialrearing, stock splitting and multiplerearing units.

Sequential rearing

Sequential rearing involves the cul-ture of several age groups (multiplecohorts) of fish in the same rearingtank. When one age group reachesmarketable size, it is selectively har-vested with nets and a grading sys-tem, and an equal number of finger-lings are immediately restocked inthe same tank. There are three prob-lems with this system: 1) the period-ic harvests stress the remaining fishand could trigger disease outbreaks;2) stunted fish avoid capture andaccumulate in the system, wastingspace and feed; and 3) it is difficult

Rearingtank

Solidsremoval Biofilter

Hydroponicsubsystem Sump

Combined

Combined

Figure 2. Optimum arrangement of aquaponic system components (not toscale).

ing crop of the initial rearing tank isreached. The fish are either herdedthrough a hatch between adjoiningtanks or into “swimways” connect-ing distant tanks. Multiple rearingunits usually come in modules oftwo to four tanks and are connectedto a common filtration system. Afterthe largest tank is harvested, all ofthe remaining groups of fish aremoved to the next largest tank andthe smallest tank is restocked withfingerlings. A variation of the multi-ple rearing unit concept is the divi-sion of a long raceway into compart-ments with movable screens. As thefish grow, their compartment isincreased in size and moved closerto one end of the raceway wherethey will eventually be harvested.These should be cross-flow race-ways, with influent water enteringthe raceway through a series of portsdown one side of the raceway andeffluent water leaving the racewaythrough a series of drains down theother side. This system ensures thatwater is uniformly high qualitythroughout the length of the race-way. Another variation is the use of sever-al tanks of the same size. Each rear-ing tank contains a different agegroup of fish, but they are notmoved during the production cycle.This system does not use space effi-ciently in the early stages of growth,but the fish are never disturbed andthe labor involved in moving thefish is eliminated. A system of four multiple rearingtanks has been used successfullywith tilapia in the UVI commercial-scale aquaponic system (Figs. 3 and5). Production is staggered so one of

to maintain accurate stock recordsover time, which leads to a highdegree of management uncertaintyand unpredictable harvests.

Stock splitting

Stock splitting involves stocking veryhigh densities of fingerlings and peri-odically splitting the population inhalf as the critical standing crop ofthe rearing tank is reached. Thismethod avoids the carryover prob-lem of stunted fish and improvesstock inventory. However, the movescan be very stressful on the fishunless some sort of “swimway” isinstalled to connect all the rearingtanks. The fish can be herded intothe swimway through a hatch in thewall of a rearing tank and maneu-vered into another rearing tank bymovable screens. With swimways,dividing the populations in halfinvolves some guesswork becausethe fish cannot be weighed or count-

ed. An alternative method is tocrowd the fish with screens andpump them to another tank with afish pump.

Multiple rearing units

With multiple rearing units, theentire population is moved to largerrearing tanks when the critical stand-

Sump

ClarifierFilter tanks

Base additionDegassing

Fish rearing tanks Hydroponic tanksEffluent line

Return line

The UVI Aquaponic System

Tank dimensionsRearing tanks: Diameter: 10 ft, Height:4 ft, Water volume: 2,060 gal eachClarifiers: Diameter: 6 ft, Height ofcylinder: 4 ft, Depth of cone: 3.6 ft,Slope: 45º, Water volume: 1,000 galFilter and degassing tanks: Length: 6ft, Width: 2.5 ft, Depth: 2 ft, Water vol-ume: 185 galHydroponic tanks: Length: 100 ft,Width: 4 ft, Depth: 16 in, Water volume:3,000 gal, Growing area: 2,304 ft2

Sump: Diameter: 4 ft, Height: 3 ft,Water volume: 160 galBase addition tank: Diameter: 2 ft,Height: 3 ft, Water volume: 50 galTotal system water volume: 29,375galFlow rate: 100 GPM Water pump: 1⁄2 hpBlowers: 11⁄2 hp (fish) and 1 hp (plants)Total land area: 1⁄8 acre

Figure 3. Layout of UVI aquaponic system with tank dimensions and pipe sizes(not to scale).

Fig. 4. An early model of the UVIaquaponic system in St. Croix show-ing the staggered production of leaflettuce in six raft hydroponic tanks.

Figure 5. The UVI aquaponic system atthe New Jersey EcoComplex at RutgersUniversity. Effluent from four tilapia-rearing tanks circulates through eightraft hydroponic tanks, producing toma-toes and other crops.

Pipe sizesPump to rearing tanks: 3 inRearing tanks to clarifier: 4 inClarifiers to filter tanks: 4 inBetween filter tanks: 6 inFilter tank to degassing tank: 4 inDegassing to hydroponic tanks: 6 in

Between hydroponic tanks: 6 inHydroponic tanks to sump: 6 inSump to pump: 3 inPipe to base addition tank 0.75 inBase addition tank to sump: 1.25 in

the rearing tanks is harvested every6 weeks. At harvest, the rearing tankis drained and all of the fish areremoved. The rearing tank is thenrefilled with the same water andimmediately restocked with finger-lings for a 24-week production cycle.Each circular rearing tank has awater volume of 2,060 gallons and isheavily aerated with 22 air diffusers.The flow rate to all four tanks is 100gallons/minute, but the flow rate toindividual tanks is apportioned sothat tanks receive a higher flow rateas the fish grow. The average rearingtank retention time is 82 minutes.Annual production has been 9,152pounds (4.16 mt) for Nile tilapia and10,516 pounds (4.78 mt) for redtilapia (Table 1). However, produc-tion can be increased to 11,000pounds (5 mt) with close observationof the ad libitum feeding response. In general, the critical standing cropin aquaponic systems should notexceed 0.50 pound/gallon. This densi-ty will promote fast growth and effi-cient feed conversion and reducecrowding stress that may lead to dis-ease outbreaks. Pure oxygen is gener-ally not needed to maintain this den-sity. The logistics of working with bothfish and plants can be challenging.In the UVI system, one rearing tankis stocked every 6 weeks. Therefore,it takes 18 weeks to fully stock thesystem. If multiple units are used,fish may be stocked and harvestedas frequently as once a week.Similarly, staggered crop productionrequires frequent seeding, trans-planting, harvesting and marketing.Therefore, the goal of the designprocess is to reduce labor whereverpossible and make operations as sim-ple as possible. For example, pur-chasing four fish-rearing tanks addsextra expense. One larger tank couldbe purchased instead and partially

harvested and partially restockedevery 6 weeks. However, this opera-tion requires additional labor, whichis a recurring cost and makes man-agement more complex. In the longrun, having several smaller tanks inwhich the fish are not disturbeduntil harvest (hence, less mortalityand better growth) will be more costeffective.

SolidsMost of the fecal waste fish generateshould be removed from the wastestream before it enters the hydro-ponic tanks. Other sources of partic-ulate waste are uneaten feed andorganisms (e.g., bacteria, fungi andalgae) that grow in the system. If thisorganic matter accumulates in thesystem, it will depress dissolved oxy-gen (DO) levels as it decays and pro-duce carbon dioxide and ammonia.If deep deposits of sludge form, theywill decompose anaerobically (with-out oxygen) and produce methaneand hydrogen sulfide, which arevery toxic to fish. Suspended solids have special signifi-cance in aquaponic systems.Suspended solids entering the hydro-ponic component may accumulateon plant roots and create anaerobiczones that prevent nutrient uptakeby active transport, a process thatrequires oxygen. However, someaccumulation of solids may be bene-ficial. As solids are decomposed bymicroorganisms, inorganic nutrientsessential to plant growth are releasedto the water, a process known asmineralization. Mineralization sup-plies several essential nutrients.Without sufficient solids for mineral-ization, more nutrient supplementa-tion is required, which increases theoperating expense and managementcomplexity of the system. However,it may be possible to minimize or

eliminate the need for nutrient sup-plementation if fish stocking andfeeding rates are increased relativeto plants. Another benefit of solids isthat the microorganisms that decom-pose them are antagonistic to plantroot pathogens and help maintainhealthy root growth. SRAC Publication No. 453(“Recirculating Aquaculture TankProduction Systems: A Review ofComponent Options”) describessome of the common devices used toremove solids from recirculating sys-tems. These include settling basins,tube or plate separators, the combi-nation particle trap and sludge sepa-rator, centrifugal separators, micro-screen filters and bead filters.Sedimentation devices (e.g., settlingbasins, tube or plate separators) pri-marily remove settleable solids(>100 microns), while filtrationdevices (e.g., microscreen filters,bead filters) remove settleable andsuspended solids. Solids removaldevices vary in regard to efficiency,solids retention time, effluent charac-teristics (both solid waste and treatedwater) and water consumption rate. Sand and gravel hydroponic sub-strates can remove solid waste fromsystem water. Solids remain in thesystem to provide nutrients to plantsthrough mineralization. With thehigh potential of sand and gravelmedia to clog, bed tillage or periodicmedia replacement may be required.The use of sand is becoming lesscommon, but one popular aquaponicsystem uses small beds (8 feet by 4feet) containing pea gravel rangingfrom 1⁄8 to 1⁄4 inch in diameter. Thehydroponic beds are flooded severaltimes daily with system water andthen allowed to drain completely,and the water returned to the rear-ing tank. During the draining phase,air is brought into the gravel. Thehigh oxygen content of air (com-

Table 1. Average production values for male mono-sex Nile and red tilapia in the UVI aquaponic system.Nile tilapia are stocked at 0.29 fish/gallon (77 fish/m3) and red tilapia are stocked at 0.58 fish/gallon (154fish/m3).

Harvest weight Initial Final Growth Harvest weight per unit weight weight rate Survival

Tilapia per tank (lbs) volume (lb/gal) (g/fish) (g/fish) (g/day) (%) FCRNile 1,056 (480 kg) 0.51 (61.5 kg/m3) 79.2 813.8 4.4 98.3 1.7Red 1,212 (551 kg) 0.59 (70.7 kg/m3) 58.8 512.5 2.7 89.9 1.8

pared to water) speeds the decompo-sition of organic matter in the gravel.The beds are inoculated with redworms (Eisenia foetida), whichimprove bed aeration and assimilateorganic matter.

Solids removal

The most appropriate device forsolids removal in a particular systemdepends primarily on the organicloading rate (daily feed input andfeces production) and secondarily onthe plant growing area. For example,if large numbers of fish (high organicloading) are raised relative to theplant growing area, a highly efficientsolids removal device, such as amicroscreen drum filter, is desirable.Microscreen drum filters capture fineorganic particles, which are retainedby the screen for only a few minutesbefore backwashing removes themfrom the system. In this system, thedissolved nutrients excreted directlyby the fish or produced by mineral-ization of very fine particles and dis-solved organic matter may be suffi-cient for the size of the plant growingarea. If small amounts of fish (loworganic loading) are raised relative tothe plant growing area, then solidsremoval may be unnecessary, as moremineralization is needed to producesufficient nutrients for the plants.However, un-stabilized solids (solidsthat have not undergone microbialdecomposition) should not be allowedto accumulate on the tank bottomand form anaerobic zones. A recipro-cating pea gravel filter (subject toflood and drain cycles), in whichincoming water is spread evenly overthe entire bed surface, may be themost appropriate device in this situa-tion because solids are evenly distrib-uted in the gravel and exposed tohigh oxygen levels (21 percent in airas compared to 0.0005 to 0.0007 per-cent in fish culture water) on thedrain cycle. This enhances microbialactivity and increases the mineraliza-tion rate. UVI’s commercial-scale aquaponicsystem relies on two cylindro-conicalclarifiers to remove settleable solids.The fiberglass clarifiers have a vol-ume of 1,000 gallons each. The cylin-drical portion of the clarifier is situat-ed above ground and has a central

baffle that is perpendicular to theincoming water flow (Fig. 6). Thelower conical portion has a 45-degreeslope and is buried below ground. Adrain pipe is connected to the apex ofthe cone. The drain pipe rises verti-cally out of the ground to the middleof the cylinder and is fitted with aball valve. Rearing tank effluententers the clarifier just below thewater surface. The incoming water isdeflected upward by a 45-degree pipeelbow to dissipate the current. Aswater flows under the baffle, turbu-lence diminishes and solids settle onthe sides of the cone. The solids accu-mulate there and form a thick matthat eventually rises to the surface ofthe clarifier. To prevent this, approxi-mately 30 male tilapia fingerlings arerequired to graze on the clarifierwalls and consolidate solids at thebase of the cone. Solids are removedfrom the clarifier three times daily.Hydrostatic pressure forces solidsthrough the drain line when the ballvalve is opened. A second, smallerbaffle keeps floating solids frombeing discharged to the filter tanks. The fingerlings serve another pur-pose. They swim into and throughthe drain lines and keep them clean.Without tilapia, the 4-inch drain lineswould have to be manually cleanednearly every day because of bacterialgrowth in the drain lines, which con-stricts water flow. A cylindricalscreen attached to the rearing tankdrain keeps fingerlings from enteringthe rearing tank. The cylindro-conical clarifier removes

approximately 50 percent of the totalparticulate solids produced by thesystem and primarily removes largesettleable solids. Although fingerlingsare needed for effective clarifier per-formance, their grazing and swim-ming activities are also counterpro-ductive in that they resuspend somesolids, which exit through the clarifi-er outlet. As fingerlings become larg-er (>200 g), clarifier performancediminishes. Therefore, clarifier fishmust be replaced with small finger-lings (50 g) periodically (once every 4months).With clarification as the sole methodof solids removal, large quantities ofsolids would be discharged to thehydroponic component. Therefore,another treatment stage is needed toremove re-suspended and fine solids.In the UVI system, two rectangulartanks, each with a volume of 185 gal-lons, are filled with orchard/bird net-ting and installed after each of thetwo clarifiers (Fig. 7). Effluent fromeach clarifier flows through a set oftwo filter tanks in series. Orchardnetting is effective in removing finesolids. The filter tanks remove theremaining 50 percent of total particu-late solids. The orchard netting is cleaned onceor twice each week. Before cleaning,a small sump pump is used to care-fully return the filter tank water tothe rearing tanks without dislodgingthe solids. This process conserveswater and nutrients. The netting iscleaned with a high-pressure waterspray and the sludge is discharged tolined holding ponds. Effluent from the UVI rearing tanksis highly enriched with dissolvedorganic matter, which stimulates thegrowth of filamentous bacteria in thedrain line, clarifier and screen tank.The bacteria appear as translucent,gelatinous, light tan filaments. Tilapiaconsume the bacteria and control itsgrowth in the drain line and clarifier,but bacteria do accumulate in the fil-ter tanks. Without the filter tanks,the bacteria would overgrow plantroots. The bacteria do not appear tobe pathogenic, but they do interferewith the uptake of dissolved oxygen,water and nutrients, thereby affect-ing plant growth. The feeding rate tothe system and the flow rate from

A

B

CD

E

Figure 6. Cross-sectional view (not toscale) of UVI clarifier showing drainlines from two fish rearing tanks (A),central baffle (B) and discharge baffle(C), outlet to filter tanks (D), sludgedrain line (E) and direction of waterflow (arrows).

the rearing tank determine the extentto which filamentous bacteria grow,but they can be contained by provid-ing a sufficient area of orchard net-ting, either by adjusting screen tanksize or using multiple screen tanks.In systems with lower organic load-ing rates (i.e., feeding rates) or lowerwater temperature (hence, less bio-logical activity), filamentous bacteriadiminish and are not a problem. The organic matter that accumulateson the orchard netting betweencleanings forms a thick sludge.Anaerobic conditions develop in thesludge, which leads to the formationof gases such as hydrogen sulfide,methane and nitrogen. Therefore, adegassing tank is used in the UVIsystem to receive the effluent fromthe filter tanks (Fig. 7). A number ofair diffusers vent the gasses into theatmosphere before the culture waterreaches the hydroponic plants. Thedegassing tank has an internal stand-pipe well that splits the water flowinto three sets of hydroponic tanks.Solids discharged from aquaponicsystems must be disposed of appro-priately. There are several methodsfor effluent treatment and disposal.Effluent can be stored in aeratedponds and applied as relatively dilutesludge to land after the organic mat-ter in it has stabilized. This method isadvantageous in dry areas wheresludge can be used to irrigate and fer-tilize field crops. The solid fraction of

sludge can be separated from waterand used with other waste productsfrom the system (vegetable matter) toform compost. Urban facilities mighthave to discharge solid waste intosewer lines for treatment and dispos-al at the municipal wastewater treat-ment plant.

BiofiltrationA major concern in aquaponic sys-tems is the removal of ammonia, ametabolic waste product excretedthrough the gills of fish. Ammoniawill accumulate and reach toxic lev-els unless it is removed by theprocess of nitrification (referred tomore generally as biofiltration), inwhich ammonia is oxidized first tonitrite, which is toxic, and then tonitrate, which is relatively non-toxic.Two groups of naturally occurringbacteria (Nitrosomonas andNitrobacter) mediate this two-stepprocess. Nitrifying bacteria grow as afilm (referred to as biofilm) on thesurface of inert material or theyadhere to organic particles. Biofilterscontain media with large surfaceareas for the growth of nitrifyingbacteria. Aquaponic systems haveused biofilters with sand, gravel,shells or various plastic media assubstrate. Biofilters perform optimal-ly at a temperature range of 77 to86 °F, a pH range of 7.0 to 9.0, satu-rated DO, low BOD (<20 mg/liter)and total alkalinity of 100 mg/liter or

more. Nitrification is an acid-produc-ing process. Therefore, an alkalinebase must be added frequently,depending on feeding rate, to main-tain relatively stable pH values.Some method of removing deadbiofilm is necessary to preventmedia clogging, short circuiting ofwater flow, decreasing DO valuesand declining biofilter performance.A discussion of nitrification princi-ples and a description of variousbiofilter designs and operating proce-dures are given in SRAC PublicationNos. 451, 452 and 453.Four major biofilter options (rotatingbiological contactors, expandablemedia filters, fluidized bed filtersand packed tower filters) are dis-cussed in SRAC Publication No. 453.If a separate biofilter is required or ifa combined biofilter (biofiltrationand hydroponic substrate) is used,the standard equations used to sizebiofilters may not apply to aquapon-ic systems, as additional surface areais provided by plant roots and a con-siderable amount of ammonia istaken up by plants. However, thecontribution of various hydroponicsubsystem designs and plant speciesto water treatment in aquaponicsystems has not been studied.Therefore, aquaponic system biofil-ters should be sized fairly close tothe recommendations for recirculat-ing systems. Nitrification efficiency is affected bypH. The optimum pH range for nitri-fication is 7.0 to 9.0, although moststudies indicate that nitrification effi-ciency is greater at the higher end ofthis range (high 8s). Most hydroponicplants grow best at a pH of 5.8 to6.2. The acceptable range for hydro-ponic systems is 5.5 to 6.5. The pHof a solution affects the solubility ofnutrients, especially trace metals.Essential nutrients such as iron,manganese, copper, zinc and boronare less available to plants at a pHhigher than 7.0, while the solubilityof phosphorus, calcium, magnesiumand molybdenum sharply decreasesat a pH lower than 6.0. Compromisebetween nitrification and nutrientavailability is reached in aquaponicsystems by maintaining pH close to7.0.Nitrification is most efficient whenwater is saturated with DO. The UVI

Figure 7. Components of the UVI aquaponic system at the New Jersey EcoComplexat Rutgers University.

commercial-scale system maintainsDO levels near 80 percent saturation(6 to 7 mg/L) by aerating the hydro-ponic tanks with numerous small airdiffusers (one every 4 feet) distrib-uted along the long axis of the tanks.Reciprocating (ebb and flow) gravelsystems expose nitrifying bacteria tohigh atmospheric oxygen levels dur-ing the dewatering phase. The thinfilm of water that flows through NFT(nutrient film technique) channelsabsorbs oxygen by diffusion, butdense plant roots and associatedorganic matter can block water flowand create anaerobic zones, whichprecludes the growth of nitrifyingbacteria and further necessitates theinstallation of a separate biofilter.Ideally, aquaponic systems should bedesigned so that the hydroponic sub-system also serves as the biofilter,which eliminates the capital cost andoperational expense of a separatebiofilter. Granular hydroponic mediasuch as gravel, sand and perlite pro-vide sufficient substrate for nitrifyingbacteria and generally serve as thesole biofilter in some aquaponic sys-tems, although the media has a ten-dency to clog. If serious cloggingoccurs from organic matter overload-ing, gravel and sand filters can actu-ally produce ammonia as organicmatter decays, rather than remove it.If this occurs, the gravel or sandmust be washed and the systemdesign must be modified byinstalling a solids removal devicebefore the media, or else the organicloading rate must be decreased bystocking fewer fish and reducingfeeding rates. Raft hydroponics, which consists ofchannels (with 1 foot of water depth)covered by floating sheets of poly-styrene for plant support, also pro-vides sufficient nitrification if solidsare removed from the flow before itreaches the hydroponic component.The waste treatment capacity of rafthydroponics is equivalent to a feed-ing ratio of 180 g of fish feed/m2 ofplant growing area/day. (Note: 1 m2

= 10.76 ft2 and 454 g = 1 lb.) This isequivalent to about 1.2 pounds offeed for each 8-foot x 4-foot sheet ofpolystyrene foam. After an initialacclimation period of 1 month, it isnot necessary to monitor ammoniaand nitrite values in the UVI raft sys-

tem. A significant amount of nitrifi-cation occurs on the undersides ofthe polystyrene sheets, especially inthe areas exposed to strong currentsabove air diffusers where the biofilmis noticeably thicker. Aquaponic systems using nutrientfilm technique (NFT) as the hydro-ponic component may require a sep-arate biofilter. NFT consists of nar-row plastic channels for plant sup-port with a film of nutrient solutionflowing through them (Fig. 8). Thewater volume and surface area ofNFT are considerably smaller than inraft culture because there is just athin film of water and no substantialside wall area or raft underside sur-face area for colonization by nitrify-ing bacteria.

Hydroponic subsystemsA number of hydroponic subsystemshave been used in aquaponics.Gravel hydroponic subsystems arecommon in small operations. Toensure adequate aeration of plantroots, gravel beds have been operatedin a reciprocating (ebb and flow)mode, where the beds are alternatelyflooded and drained, or in a non-flooded state, where culture water isapplied continuously to the base ofthe individual plants through small-diameter plastic tubing. Dependingon its composition, gravel can pro-vide some nutrients for plant growth(e.g., calcium is slowly released asthe gravel reacts with acid producedduring nitrification).Gravel has several negative aspects.The weight of gravel requires strongsupport structures. It is subject toclogging with suspended solids,

microbial growth and the roots thatremain after harvest. The resultingreduction in water circulation,together with the decomposition oforganic matter, leads to the forma-tion of anaerobic zones that impairor kill plant roots. The small, plastictubes used to irrigate gravel are alsosubject to clogging with biologicalgrowth. Moving and cleaning gravelsubstrate is difficult because of itsweight. Planting in gravel is also dif-ficult, and plant stems can be dam-aged by abrasion in outdoor systemsexposed to wind. Gravel retains verylittle water if drained, so a disruptionin flow will lead to the rapid onset ofwater stress (wilting). The sturdyinfrastructure required to supportgravel and the potential for clogginglimits the size of gravel beds. One popular gravel-based aquaponicsystem uses pea gravel in small bedsthat are irrigated through a distribu-tion system of PVC pipes over thegravel surface. Numerous small holesin the pipes distribute culture wateron the flood cycle. The beds areallowed to drain completely betweenflood cycles. Solids are not removedfrom the culture water and organicmatter accumulates, but the beds aretilled between planting cycles so thatsome organic matter can be dis-lodged and discharged. Sand has been used as hydroponicmedia in aquaponic systems and isan excellent substrate for plantgrowth. In an experimental system,sand beds (25 feet long by 5 feetwide by 1.6 feet deep) were con-structed on slightly sloped groundcovered by polyethylene sheets adja-cent to in-ground rearing tanks, withthe tank floors sloping to one side. Apump in the deep end of the rearingtank was activated for 30 minutesfive times daily to furrow irrigate theadjacent sand bed. The culture waterpercolated through the sand andreturned to the rearing tank. Acoarse grade of sand is needed toreduce the potential for clogging overtime and some solids should beremoved before irrigation.Perlite is another media that hasbeen used in aquaponic systems.Perlite is placed in shallow alu-minum trays (3 inches deep) with abaked enamel finish. The trays varyfrom 8 inches to 4 feet wide and can

Figure 8. Using nutrient film technique,basil is produced in an aquaponic sys-tem at Bioshelters, Inc. in Amherst,Massachusetts.

be fabricated to any length, with 20feet the maximum recommendedlength. At intervals of 20 feet, adjoin-ing trays should be separated by 3inches or more in elevation so thatwater drops to the lower tray andbecomes re-aerated. A slope of 1inch in 12 feet is needed for waterflow. A small trickle of water entersat the top of the tray, flows throughthe perlite and keeps it moist, anddischarges into a trough at the lowerend. Solids must be removed fromthe water before it enters the perlitetray. Full solids loading will clog theperlite, form short-circuiting chan-nels, create anaerobic zones and leadto non-uniform plant growth.Shallow perlite trays provide mini-mal area for root growth and are bet-ter for smaller plants such as lettuceand herbs. Nutrient film technique (NFT) hasbeen successfully incorporated into anumber of aquaponic systems. NFTconsists of many narrow, plastictroughs (4 to 6 inches wide) in whichplant roots are exposed to a thin filmof water that flows down thetroughs, delivering water, nutrientsand oxygen to the roots of the plants.The troughs are lightweight, inex-pensive and versatile. Troughs canbe mounted over rearing tanks toefficiently use vertical greenhousespace. However, this practice is dis-couraged if it interferes with fish andplant operations such as harvesting.High plant density can be main-tained by adjusting the distancebetween troughs to provide opti-mum plant spacing during the grow-ing cycle. In aquaponic systems thatuse NFT, solids must be removed sothey do not accumulate and killroots. With NFT, a disruption inwater flow can lead quickly to wilt-ing and death. Water is delivered atone end of the troughs by a PVCmanifold with discharge holes aboveeach trough; it is collected at theopposite, down-slope end in an openchannel or large PVC pipe. The useof microtubes, which are used incommercial hydroponics, is not rec-ommended because they will clog.The holes should be as large as prac-tical to reduce cleaning frequency. A floating or raft hydroponic sub-system is ideal for the cultivation ofleafy green and other types of veg-

etables. The UVI system uses threesets of two raft hydroponic tanksthat are 100 feet long by 4 feetwide by 16 inches deep and contain12 inches of water. The channelsare lined with low-density polyeth-ylene liners (20 mil thick) and cov-ered by expanded polystyrenesheets (rafts) that are 8 feet long by4 feet wide by 1.5 inches thick. Netpots are placed in holes in the raftand just touch the water surface.Two-inch net pots are generallyused for leafy green plants, while 3-inch net pots are used for largerplants such as tomatoes or okra.Holes of the same size are cut intothe polystyrene sheet. A lip at thetop of the net pot secures it andkeeps it from falling through thehole into the water. Seedlings arenursed in a greenhouse and thenplaced into net pots. Their rootsgrow into the culture water whiletheir canopy grows above the raftsurface. The system provides maxi-mum exposure of roots to the cul-ture water and avoids clogging. Thesheets shield the water from directsunlight and maintain lower thanambient water temperature, whichis a beneficial feature in tropicalsystems. A disruption in pumpingdoes not affect the plant’s watersupply as in gravel, sand and NFTsubsystems. The sheets are easilymoved along the channel to a har-vesting point where they can belifted out of the water and placedon supports at an elevation that iscomfortable for workers (Fig. 9). A disadvantage of rafts in anaquaponic system is that roots areexposed to harmful organisms asso-ciated with aquaculture systems. Iftilapia fry gain access to the hydro-

ponic tanks, they consume plantroots and severely stunt plantgrowth, although it is relatively easyto keep fish from entering by placinga fine mesh screen at the entry pointof water into the degassing tank.Similarly, blooms of zooplankton,especially ostracods, will consumeroot hairs and fine roots, retardingplant growth. Other pests are tad-poles and snails, which consumeroots and nitrifying bacteria. Theseproblems can be surmounted byincreasing water agitation to preventroot colonization by zooplankton andby stocking some carnivorous fishsuch as red ear sunfish (shellcrack-ers) in hydroponic tanks to prey ontadpoles and snails.

SumpWater flows by gravity from gravel,sand and raft hydroponic subsystemsto a sump, which is the lowest pointin the system. The sump contains apump or pump inlet that returns thetreated culture water to the rearingtanks. If NFT troughs or perlite traysare located above the rearing tanks,the sump would be positioned infront of them so that water could bepumped up to the hydroponic com-ponent for gravity return to the rear-ing tanks. There should be only onepump to circulate water in anaquaponic system. The sump should be the only tankin the system where the waterlevel decreases as a result of over-all water loss from evaporation,transpiration, sludge removal andsplashing. An electrical or mechan-ical valve is used to automaticallyadd replacement water from a stor-age reservoir or well. Municipalwater should not be used unless itis de-chlorinated. Surface watershould not be used because it maycontain disease organisms. A watermeter should be used to recordadditions. Unusually high waterconsumption indicates a leak.The sump is a good location for theaddition of base to the system.Soluble base such as potassiumhydroxide causes high and toxic pHlevels in the sump. However, aswater is pumped into the rearingtank, it is diluted and pH decreasesto acceptable levels.

Figure 9. Leaf lettuce being harvestedfrom a raft hydroponic tank in the UVIaquaponic system in St. Croix.

The UVI system has a separate baseaddition tank located next to thesump. As water is pumped from thesump to the fish-rearing tanks, asmall pipe, tapped into the mainwater distribution line, delivers asmall flow of water to the base addi-tion tank, which is well aerated withone large air diffuser. When base isadded to this tank and dissolves, theresulting high pH water slowly flowsby gravity into the sump, where it israpidly diluted and pumped to fish-rearing tanks. This system prevents arapid pH increase in the fish-rearingtank.

Construction materialsMany materials can be used to con-struct aquaponic systems. Budgetlimitations often lead to the selectionof inexpensive and questionablematerials such as vinyl-lined, steel-walled swimming pools. Plasticizersused in vinyl manufacture are toxicto fish, so these liners must bewashed thoroughly or aged withwater for several weeks before fishcan be added safely to a tank of cleanwater. After a few growing periods,vinyl liners shrink upon drying,become brittle and crack, while thesteel walls gradually rust. Nylon-rein-forced, neoprene rubber liners arenot recommended either. Tilapia eatholes in rubber liners at the folds asthey graze on microorganisms.Moreover, neoprene rubber liners arenot impervious to chemicals. If herbi-cides and soil sterilants are appliedunder or near rubber liners, thesechemicals can diffuse into culturewater, accumulate in fish tissue andkill hydroponic vegetables.

Fiberglass is the best constructionmaterial for rearing tanks, sumps andfilter tanks. Fiberglass tanks are stur-dy, durable, non-toxic, movable andeasy to plumb. Polyethylene tanks arealso very popular for fish rearing andgravel hydroponics because of theirlow cost. NFT troughs made fromextruded polyethylene are specificallydesigned to prevent the puddling andwater stagnation that lead to rootdeath and are preferable to makeshiftstructures such as PVC pipes. Plastictroughs are commercially available

for floating hydroponic subsystems,but they are expensive. A good alter-native is the 20-mil polyethylene lin-ers that are placed inside concrete-block or poured-concrete side walls.They are easy to install, relativelyinexpensive and durable, with anexpected life of 12 to 15 years. A soilfloor covered with fine sand will pre-vent sharp objects from puncturingthe liners. Lined hydroponic tankscan be constructed to very largesizes—hundreds of feet long and upto 30 feet wide.

Component ratiosAquaponic systems are generallydesigned to meet the size require-ments for solids removal (for thosesystems requiring solids removal)and biofiltration (if a separate biofil-ter is used) for the quantity of fishbeing raised (see SRAC PublicationNo. 453, “Recirculating AquacultureTank Production Systems: A Reviewof Component Options”). After thesize requirements are calculated, it isprudent to add excess capacity as asafety margin. However, if a separatebiofilter is used, the hydroponiccomponent is the safety factorbecause a significant amount ofammonia uptake and nitrificationwill occur regardless of hydroponictechnique. Another key design criterion is theratio between the fish-rearing andhydroponic components. The key isthe ratio of daily feed input to plantgrowing area. If the ratio of dailyfeeding rate to plants is too high,nutrient salts will accumulate rapid-ly and may reach phytotoxic levels.Higher water exchange rates will berequired to prevent excessive nutri-ent buildup. If the ratio of dailyfeeding rate to plants is too low,plants will develop nutrient deficien-cies and need more nutrient supple-mentation. Fortunately, hydroponicplants grow well over a wide rangeof nutrient concentrations. The optimum ratio of daily fishfeed input to plant growing areawill maximize plant productionwhile maintaining relatively stablelevels of dissolved nutrients. A vol-ume ratio of 1 ft3 of fish-rearingtank to 2 ft3 of pea gravel hydro-ponic media (1⁄8 to 1⁄4 inch in diam-

eter) is recommended for recipro-cating (flood and drain) gravelaquaponic systems. This ratiorequires that tilapia be raised to afinal density of 0.5 pound/gallonand fed appropriately. With therecommended ratio, no solids areremoved from the system. Thehydroponic beds should be culti-vated (stirred up) between cropsand inoculated with red worms tohelp break down and assimilatethe organic matter. With this sys-tem, nutrient supplementationmay not be necessary. As a general guide for raft aquapon-ics, a ratio in the range of 60 to 100g of fish feed/m2 of plant growingarea per day should be used. Ratioswithin this range have been usedsuccessfully in the UVI system forthe production of tilapia, lettuce,basil and several other plants. In theUVI system all solids are removed,with a residence time of <1 day forsettleable solids (>100 micrometers)removed by a clarifier, and 3 to 7days for suspended solids removedby an orchard netting filter. The sys-tem uses rainwater and requiressupplementation for potassium, cal-cium and iron. Another factor to consider indetermining the optimum feedingrate ratio is the total water volumeof the system, which affects nutri-ent concentrations. In raft hydro-ponics, approximately 75 percentof the system water volume is inthe hydroponic component,whereas gravel beds and NFTtroughs contain minor amounts ofsystem water. Theoretically, insystems producing the same quan-tity of fish and plants, a dailyfeeding rate of 100 g/m2 wouldproduce total nutrient concentra-tions nearly four times higher ingravel and NFT systems (e.g.,1,600 mg/L) than in raft systems(e.g., 400 mg/L), but total nutrientmass in the systems would be thesame. Nutrient concentrations out-side acceptable ranges affect plantgrowth. Therefore, the optimumdesign ratio varies with the typeof hydroponic component. Graveland NFT systems should have afeeding rate ratio that is approxi-mately 25 percent of the recom-mended ratio for raft hydroponics.

Other factors in determining theoptimum feeding rate ratio are thewater exchange rate, nutrient levelsin the source water, degree andspeed of solids removal, and type ofplant being grown. Lower rates ofwater exchange, higher source-waternutrient levels, incomplete or slowsolids removal (resulting in therelease of more dissolved nutrientsthrough mineralization), and slow-growing plants would allow a lowerfeeding rate ratio. Conversely, higherwater exchange rates, low source-water nutrient levels, rapid and com-plete solids removal, and fast-grow-ing plants would allow a higherfeeding rate ratio. The optimum feeding rate ratio isinfluenced by the plant culturemethod. With batch culture, allplants in the system are planted andharvested at the same time. Duringtheir maximum growth phase, thereis a large uptake of nutrients, whichrequires a higher feeding rate ratioduring that period. In practice, how-ever, a higher feeding rate ratio isused throughout the productioncycle. With a staggered productionsystem, plants are in different stagesof growth, which levels out nutrientuptake rates and allows good pro-duction with slightly lower feedingrate ratios. In properly designed aquaponic sys-tems, the surface area of the hydro-ponic component is large comparedto the surface area of the fish-rearingtank (stocked at commercially rele-vant densities). The commercial-scale unit at UVI has a ratio of 7.3:1.The total plant growing area is 2,304ft2 and the total fish-rearing surfacearea is 314 ft2.

Plant growth requirementsFor maximum growth, plants inaquaponic systems require 16 essen-tial nutrients. These are listed belowin the order of their concentrationsin plant tissue, with carbon and oxy-gen being the highest. The essentialelements are arbitrarily divided intomacronutrients, those required inrelatively large quantities, andmicronutrients, those required inconsiderably smaller amounts. Threeof the macronutrients—carbon (C),oxygen (O) and hydrogen (H)—are

supplied by water (H2O) and carbondioxide gas (CO2). The remainingnutrients are absorbed from the cul-ture water. Other macronutrientsinclude nitrogen (N), potassium (K),calcium (Ca), magnesium (Mg),phosphorus (P) and sulfur (S). Theseven micronutrients include chlo-rine (Cl), iron (Fe), manganese (Mn),boron (B), zinc (Zn), copper (Cu) andmolybdenum (Mo). These nutrientsmust be balanced for optimum plantgrowth. High levels of one nutrientcan influence the bioavailability ofothers. For example, excessiveamounts of potassium may interferewith the uptake of magnesium orcalcium, while excessive amounts ofeither of the latter nutrients mayinterfere with the uptake of theother two nutrients.Enriching the air in an unventilatedgreenhouse with CO2 has dramati-cally increased crop yields in north-ern latitudes. Doubling atmosphericCO2 increases agricultural yields byan average of 30 percent. However,the high cost of energy to generateCO2 has discouraged its use. Anaquaponic system in a tightlyenclosed greenhouse is ideal becauseCO2 is constantly vented from theculture water. There is a growing body of evidencethat healthy plant developmentrelies on a wide range of organiccompounds in the root environment.These compounds, generated bycomplex biological processes involv-ing microbial decomposition oforganic matter, include vitamins,auxins, gibberellins, antibiotics,enzymes, coenzymes, amino acids,organic acids, hormones and othermetabolites. Directly absorbed andassimilated by plants, these com-pounds stimulate growth, enhanceyields, increase vitamin and mineralcontent, improve fruit flavor andhinder the development ofpathogens. Various fractions of dis-solved organic matter (e.g., humicacid) form organo-metallic complex-es with Fe, Mn and Zn, therebyincreasing the availability of thesemicronutrients to plants. Althoughinorganic nutrients give plants anavenue to survival, plants not onlyuse organic metabolites from theenvironment, but also need these

metabolites to reach their fullgrowth potential. Maintaining high DO levels in theculture water is extremely importantfor optimal plant growth, especiallyin aquaponic systems with theirhigh organic loads. Hydroponicplants are subject to intense root res-piration and draw large amounts ofoxygen from the surrounding water.If DO is deficient, root respirationdecreases. This reduces waterabsorption, decreases nutrientuptake, and causes the loss of celltissue from roots. The result isreduced plant growth. Low DO lev-els correspond with high concentra-tions of carbon dioxide, a conditionthat promotes the development ofplant root pathogens. Root respira-tion, root growth and transpirationare greatest at saturated DO levels. Climatic factors also are importantfor hydroponic plant production.Production is generally best inregions with maximum intensityand daily duration of light. Growthslows substantially in temperategreenhouses during winter becausesolar radiation is low. Supplementalillumination can improve winterproduction, but is not generally costeffective unless an inexpensive ener-gy source is available.Water temperature is far moreimportant than air temperature forhydroponic plant production. Thebest water temperature for mosthydroponic crops is about 75 °F.However, water temperature can goas low as the mid-60s for most com-mon garden crops and slightly lowerfor winter crops such as cabbage,brussel sprouts and broccoli.Maintaining the best water tempera-ture requires heating during thewinter in temperate greenhousesand year-round cooling in tropicalgreenhouses. In addition to evapora-tive cooling of tropical greenhouses,chillers are often used to cool thenutrient solution. In tropical outdoorsystems, complete shading of thefish-rearing and filtration compo-nents lowers system water tempera-ture. In raft hydroponics, the poly-styrene sheets shield water fromdirect sunlight and maintain temper-atures that are several degrees lowerthan those in open bodies of water.

Crop varieties may need to beadjusted seasonally for both temper-ate and tropical aquaponic produc-tion. Plants cultured in outdooraquaponic systems must be protect-ed from strong winds, especiallyafter transplanting when seedlingsare fragile and most vulnerable todamage.

Nutrient dynamicsDissolved nutrients are measuredcollectively as total dissolved solids(TDS), expressed as ppm, or as thecapacity of the nutrient solution toconduct an electrical current (EC),expressed as millimhos/cm(mmho/cm). In a hydroponic solu-tion, the recommended range forTDS is 1,000 to 1,500 ppm (1.5 to 3.5mmho/cm). In an aquaponic system,considerably lower levels of TDS(200 to 400 ppm) or EC (0.3 to 0.6mmho/cm) will produce good resultsbecause nutrients are generated con-tinuously. A concern with aquaponicsystems is nutrient accumulation.High feeding rates, low waterexchange and insufficient plantgrowing areas can lead to the rapidbuildup of dissolved nutrients topotentially phytotoxic levels.Phytotoxicity occurs at TDS concen-trations above 2,000 ppm or ECabove 3.5 mmho/cm. Becauseaquaponic systems have variableenvironmental conditions such asdaily feed input, solids retention,mineralization, water exchange,nutrient input from source water orsupplementation, and variable nutri-ent uptake by different plant species,it is difficult to predict the exact levelof TDS or EC and how it is chang-ing. Therefore, the culturist shouldpurchase an inexpensive conductivi-ty meter and periodically measureTDS or EC. If dissolved nutrients aresteadily increasing and approach2,000 ppm as TDS or 3.5 mmho/cmas EC, increasing the water exchangerate or reducing the fish stockingrate and feed input will quicklyreduce nutrient accumulation.However, because these methodseither increase costs (i.e., more waterconsumed) or lower output (i.e., lessfish produced), they are not goodlong-term solutions. Better but morecostly solutions involve removingmore solids (i.e., upgrade the solids

removal component) or enlargingthe plant-growing areas.The major ions that increase conduc-tivity are nitrate (NO3

-), phosphate(PO4

-2), sulfate (SO4-2), K+, Ca+2 and

Mg+2. Levels of NO3-, PO4

-2 and SO4-2

are usually sufficient for good plantgrowth, while levels of K+ and Ca+2

are generally insufficient. Potassiumis added to the system in the form ofpotassium hydroxide (KOH) and Cais added as calcium hydroxide[Ca(OH)2]. In the UVI commercial-scale system, KOH and Ca(OH)2 areadded in equal amounts (usually 500to 1,000 g). The bases are addedalternately several times weekly tomaintain pH near 7.0. Adding basiccompounds of K and Ca serves thedual purpose of supplementingessential nutrients and neutralizingacid. In some systems Mg also maybe limiting. Magnesium can be sup-plemented by using dolomite[CaMg(CO3)2] as the base to adjustpH. The addition of too much Cacan cause phosphorous to precipitatefrom culture water in the form ofdicalcium phosphate [CaHPO4].Sodium bicarbonate (NaHCO3)should never be added to anaquaponic system for pH controlbecause a high Na+ level in the pres-ence of chloride is toxic to plants.The Na+ concentration in hydropon-ic nutrient solutions should notexceed 50 mg/L. Higher Na+ levelswill interfere with the uptake of K+

and Ca+2. In lettuce, reduced Ca+2

uptake causes tip-burn, resulting inan unmarketable plant. Tip-burnoften occurs during the warmermonths. Salt (NaCl) is added to fishfeed during manufacture. A produc-er who orders large quantities offeed could request that salt not beadded if this does not affect fishhealth. If Na+ exceeds 50 mg/L andthe plants appear to be affected, apartial water exchange (dilution)may be necessary. Rainwater is usedin UVI’s systems because thegroundwater of semiarid islands gen-erally contains too much salt foraquaponics. The accumulation of too muchnitrate in aquaponic systems issometimes a concern as fruitingplants set less fruit and produce

excess vegetative growth whennitrate levels are high. The filtertanks in the UVI commercial-scalesystem have a mechanism for con-trolling nitrate levels through denitri-fication, the reduction of nitrate ionsto nitrogen gas by anaerobic bacte-ria. Large quantities of organic mat-ter accumulate on the orchard net-ting between cleanings. Denitrifica-tion occurs in anaerobic pockets thatdevelop in the sludge. Water movesthrough the accumulated sludge,which provides good contactbetween nitrate ions and denitrifyingbacteria. The frequency of cleaningthe netting regulates the degree ofdenitrification. When the netting iscleaned often (e.g., twice per week),sludge accumulation and denitrifica-tion are minimized, which leads toan increase in nitrate concentrations.When the netting is cleaned lessoften (e.g., once per week), sludgeaccumulation and denitrification aremaximized, which leads to adecrease in nitrate levels. Nitrate-nitrogen levels can be regulatedwithin a range of 1 to 100 mg/L ormore. High nitrate concentrationspromote the growth of leafy greenvegetables, while low nitrate concen-trations promote fruit developmentin vegetables such as tomatoes. The micronutrients Fe+2, Mn+2,Cu+2, B+3 and Mo+6 do not accumu-late significantly in aquaponic sys-tems with respect to cumulative feedinput. The Fe+2 derived from fishfeed is insufficient for hydroponicvegetable production and must besupplemented with chelated Fe+2 sothat the concentration of Fe+2 is 2.0mg/L. Chelated Fe+2 has an organiccompound attached to the metal ionto prevent it from precipitating outof solution and making it unavailableto plants. The best chelate is Fe-DTPA because it remains soluble atpH 7.0. Fe-EDTA is commonly usedin the hydroponics industry, but it isless stable at pH 7.0 and needs to bereplenished frequently. Fe+2 also canbe applied in a foliar spray directlyto plant leaves. A comparison ofMn+2, B+3 and Mo+6 levels withstandard nutrient formulations forlettuce shows that their concentra-tions in aquaponic systems are sev-eral times lower than their initial lev-

els in hydroponic formulations.Deficiency symptoms for Mn+2, B+3

and Mo+6 are not detected inaquaponic systems, so their concen-trations appear to be adequate fornormal plant growth. Concentrationsof Cu+2 are similar in aquaponic sys-tems and hydroponic formulations,while Zn+2 accumulates in aquapon-ic systems to levels that are four tosixteen times higher than initial lev-els in hydroponic formulations.Nevertheless, Zn+2 concentrationsusually remain within the limit thatis safe for fish.

Vegetable selectionMany types of vegetables have beengrown in aquaponic systems.However, the goal is to culture a veg-etable that will generate the highestlevel of income per unit area perunit time. With this criterion, culi-nary herbs are the best choice. Theygrow very rapidly and commandhigh market prices. The incomefrom herbs such as basil, cilantro,chives, parsley, portulaca and mint ismuch higher than that from fruitingcrops such as tomatoes, cucumbers,eggplant and okra. For example, inexperiments in UVI’s commercial-scale system, basil production was11,000 pounds annually at a value of$110,000, compared to okra produc-tion of 6,400 pounds annually at avalue of $6,400. Fruiting crops alsorequire longer culture periods (90days or more) and have more pestproblems and diseases. Lettuce isanother good crop for aquaponic sys-tems because it can be produced in ashort period (3 to 4 weeks in the sys-tem) and, as a consequence, has rela-tively few pest problems. Unlikefruiting crops, a large portion of theharvested biomass is edible. Othersuitable crops are Swiss chard, pakchoi, Chinese cabbage, collard andwatercress. The cultivation of flow-ers has potential in aquaponic sys-tems. Good results have beenobtained with marigold and zinnia inUVI’s aquaponic system. Traditionalmedicinal plants and plants used forthe extraction of modern pharmaceu-ticals have not been cultivated inaquaponic systems, but there may bepotential for growing some of theseplants. All plant production has to becoupled to the producer’s ability to

market the final product.

Crop production systemsThere are three strategies for pro-ducing vegetable crops in the hydro-ponic component. These are stag-gered cropping, batch cropping andintercropping. A staggered crop pro-duction system is one in whichgroups of plants in different stagesof growth are cultivated simultane-ously. This allows produce to be har-vested regularly and keeps theuptake of nutrients from the culturewater relatively constant. This sys-tem is most effective where cropscan be grown continuously, as in thetropics, subtropics, or temperategreenhouses with environmentalcontrol. At UVI, the production ofleaf lettuce is staggered so that acrop can be harvested weekly onthe same day, which facilitates mar-keting arrangements. Bibb lettucereaches market size 3 weeks aftertransplanting. Therefore, threegrowth stages of Bibb lettuce arecultivated simultaneously, and one-third of the crop is harvested week-ly. Red leaf lettuce and green leaflettuce require 4 weeks to reachmarketable size. The cultivation offour growth stages of these lettucevarieties allows one-fourth of thecrop to be harvested weekly. In 3years of continuous operation, UVIhas harvested 148 crops of lettuce,which demonstrates the system’ssustainability. Leafy green vegeta-bles, herbs and other crops withshort production periods are wellsuited for continuous, staggered pro-duction systems.A batch cropping system is moreappropriate for crops that are grownseasonally or have long growingperiods (>3 months), such as toma-toes and cucumbers. Various inter-cropping systems can be used inconjunction with batch cropping.For example, if lettuce is inter-cropped with tomatoes and cucum-bers, one crop of lettuce can be har-vested before the tomato plantcanopy begins to limit light.

Pest and disease controlPesticides should not be used to con-trol insects on aquaponic plantcrops. Even pesticides that are regis-

tered would pose a threat to fish andwould not be permitted in a fish cul-ture system. Similarly, therapeutantsfor treating fish parasites and dis-eases should not be used becausevegetables may absorb and concen-trate them. The common practice ofadding salt to treat fish diseases orreduce nitrite toxicity is detrimentalto plant crops. Nonchemical meth-ods of integrated pest managementmust be used. These include biologi-cal control (resistant cultivars, preda-tors, pathogens, antagonistic organ-isms), physical barriers, traps, andmanipulation of the physical envi-ronment. There are more opportuni-ties to use biological control meth-ods in enclosed greenhouse environ-ments than in exterior installations.Parasitic wasps and ladybugs can beused to control white flies andaphids. In UVI’s systems, caterpil-lars are effectively controlled bytwice weekly spraying with Bacillusthuringiensis, a bacterial pathogenthat is specific to caterpillars. Fungalroot pathogens (Pythium), which areencountered in summer at UVI andreduce production, dissipate in win-ter in response to lower water tem-perature. The prohibition on the use of pesti-cides makes crop production inaquaponic systems more difficult.However, this restriction ensuresthat crops from aquaponic systemswill be raised in an environmentallysound manner and be free of pesti-cide residues. A major advantage ofaquaponic systems is that crops areless susceptible to attack from soil-borne diseases. Plants grown inaquaponic systems may be moreresistant to diseases that affect plantsgrown in standard hydroponics. Thisresistance may be due to the pres-ence of some organic matter in theculture water that creates a stablegrowing environment with a widediversity of microorganisms, some ofwhich may be antagonistic to plantroot pathogens (Fig. 10).

Approaches to system designThere are several ways to design anaquaponic system. The simplestapproach is to duplicate a standardsystem or scale a standard systemdown or up, keeping the compo-nents proportional. Changing aspects

of the standard design is not recom-mended because changes often leadto unintended consequences. How-ever, the design process often startswith a production goal for either fishor plants. In those cases there aresome guidelines that can be fol-lowed. Use an aquaponic system that isalready designed. The easiestapproach is to use a system designthat has been tested and is in com-mon use with a good track record. Itis early in the development ofaquaponics, but standard designs willemerge. The UVI system has beenwell documented and is being stud-ied or used commercially in severallocations, but there are other systemswith potential. Standard designs willinclude specifications for layout, tanksizes, pipe sizes, pipe placement,pumping rates, aeration rates, infra-structure needs, etc. There will beoperation manuals and projected pro-duction levels and budgets for vari-ous crops. Using a standard designwill reduce risk. Design for available space. If a limitedamount of space is available, as in anexisting greenhouse, then that spacewill define the size of the aquaponicsystem. A standard design can bescaled down to fit the space. If ascaled-down tank or pipe size fallsbetween commercially availablesizes, it is best to select the largersize. However, the water flow rateshould equal the scaled-down ratefor best results. The desired flow ratecan be obtained by buying a highercapacity pump and installing abypass line and valve, which circu-lates a portion of the flow back to

the sump and allows the desiredflow rate to go from the pump to thenext stage of the system. If morespace is available than the standarddesign requires, then the systemcould be scaled up within limitationsor more than one scaled-down sys-tem could be installed. Design for fish production. If the pri-mary objective is to produce a cer-tain amount of fish annually, the firststep in the design process will be todetermine the number of systemsrequired, the number of rearingtanks required per system, and theoptimum rearing tank size. The num-ber of harvests will have to be calcu-lated based on the length of the cul-ture period. Assume that the finaldensity is 0.5 pound/gallon for anaerated system. Take the annual pro-duction per system and multiply itby the estimated feed conversionratio (the pounds of feed required toproduce 1 pound of fish). Convertthe pounds of annual feed consump-tion to grams (454 g/lb) and divide by365 days to obtain the average dailyfeeding rate. Divide the average dailyfeeding rate by the desired feedingrate ratio, which ranges from 60 to100 g/m2/day for raft culture, todetermine the required plant produc-tion area. For other systems such asNFT, the feeding rate ratio should bedecreased in proportion to the watervolume reduction of the system asdiscussed in the component ratio sec-tion. Use a ratio near the low end ofthe range for small plants such asBibb lettuce and a ratio near the highend of the range for larger plantssuch as Chinese cabbage or romainelettuce. The solids removal compo-nent, water pump and blowersshould be sized accordinglySample problem: This example illustrates only themain calculations, which are simpli-fied (e.g., mortality is not considered)for the sake of clarity. Assume thatyou have a market for 500 pounds oflive tilapia per week in your city andthat you want to raise lettuce withthe tilapia because there is a goodmarket for green leaf lettuce in yourarea. The key questions are: Howmany UVI aquaponic systems doyou need to harvest 500 pounds oftilapia weekly? How large should therearing tanks be? What is the appro-

priate number and size of hydroponictanks? What would the weekly let-tuce harvest be? 1. Each UVI system contains four

fish-rearing tanks (Fig. 3). Fishproduction is staggered so thatone fish tank is harvested every 6weeks. The total growing periodper tank is 24 weeks. If 500pounds of fish are requiredweekly, six production systems(24 fish-rearing tanks) are need-ed.

2. Aquaponic systems are designedto achieve a final density of 0.5pound/gallon. Therefore, thewater volume of the rearingtanks is 1,000 gallons.

3. In 52 weeks, there will be 8.7harvests (52 ÷ 6 = 8.7) per sys-tem. Annual production for thesystem, therefore, is 4,350pounds (500 pounds per harvest× 8.7 harvests).

4. The usual feed conversion ratiois 1.7. Therefore, annual feedinput to the system is 7,395pounds (4,350 lb × 1.7 = 7,395lb).

5. The average daily feed input is20.3 pounds (7,395 lb/year ÷ 365days = 20.3 lb).

6. The average daily feed input con-verted to grams is 9,216 g (20.3lb × 454 g/lb = 9216 g).

7. The optimum feeding rate ratiofor raft aquaponics ranges from60 to 100 g/m2/day. Select 80g/m2/day as the design ratio.Therefore, the required lettucegrowing area is 115.2 m2 (9,216g/day ÷ 80 g/m2/day =115.2 m2).

8. The growing area in square feetis 1,240 (115.2 m2 × 10.76 ft2/m2

= 1,240 ft2). 9. Select a hydroponic tank width

of 4 feet. The total length of thehydroponic tanks is 310 feet(1,240 ft2 ÷ 4 ft = 310 ft).

10. Select four hydroponic tanks.They are 77.5 feet long (310 ft ÷4 = 77.5 ft). They are roundedup to 80 feet in length, which isa practical length for a standardgreenhouse and allows the use often 8-foot sheets of polystyreneper hydroponic tank.

11. Green leaf lettuce produces

Figure 10. Healthy roots of Italian pars-ley cultured on rafts in a UVI aquapon-ic system at the Crop DiversificationCenter South in Alberta, Canada.

good results with plant spacingof 48 plants per sheet (16/m2).The plants require a 4-weekgrowth period. With staggeredproduction, one hydroponic tankis harvested weekly. Each hydro-ponic tank with ten polystyrenesheets produces 480 plants. Withsix aquaponic production sys-tems 2,880 plants are harvestedweekly.

In summary, the weekly productionof 500 pounds of tilapia results inthe production of 2,880 green leaflettuce plants (120 cases). Sixaquaponic systems, each with four1,000-gallon rearing tanks (water vol-ume), are required. Each system willhave four raft hydroponic tanks thatare 80 feet long by 4 feet wide. Design for plant production. If the pri-mary objective is to produce a cer-tain quantity of plant crops annually,the first step in the design processwill be to determine the arearequired for plant production. Thearea needed will be based on plantspacing, length of the productioncycle, number of crops per year orgrowing season, and the estimatedyield per unit area and per cropcycle. Select the desired feeding rateratio and multiple by the total areato obtain the average daily feedingrate required. Multiply the averagedaily feeding rate by 365 days todetermine annual feed consumption.Estimate the feed conversion ratio(FCR) for the fish species that will becultured. Convert FCR to feed con-version efficiency. For example, ifFCR is 1.7:1, then the feed conver-sion efficiency is 1 divided by 1.7 or0.59. Multiply the annual feed con-sumption by the feed conversionefficiency to determine net annualfish yield. Estimate the average fishweight at harvest and subtract theanticipated average fingerling weightat stocking. Divide this number intothe net annual yield to determine thetotal number of fish produced annu-ally. Multiply the total number offish produced annually by the esti-mated harvest weight to determinetotal annual fish production. Dividetotal annual fish production by thenumber of production cycles peryear. Take this number and divide by0.5 pound/gallon to determine thetotal volume that must be devoted to

fish production. The required watervolume can be partitioned amongmultiple systems and multiple tanksper system with the goal of creatinga practical system size and tankarray. Divide the desired individualfish weight at harvest by 0.5pound/gallon to determine the vol-ume of water (in gallons) requiredper fish. Divide the number of gal-lons required per fish by the watervolume of the rearing tank to deter-mine the fish stocking rate. Increasethis number by 5 to 10 percent toallow for expected mortality duringthe production cycle. The solidsremoval component, water pumpand blowers should be sized accord-ingly.Sample problem:Assume that there is a market for1,000 Bibb lettuce plants weekly inyour city. These plants will be soldindividually in clear, plastic,clamshell containers. A portion ofthe root mass will be left intact toextend self life. Bibb lettuce trans-plants are cultured in a UVI raft sys-tem for 3 weeks at a density of 29.3plants/m2. Assume that tilapia willbe grown in this system. The keyquestions are: How large should theplant growing area be? What will bethe annual production of tilapia?How large should the fish-rearingtanks be?1. Bibb lettuce production will be

staggered so that 1,000 plantscan be harvested weekly.Therefore, with a 3-week grow-ing period, the system mustaccommodate the culture of3,000 plants.

2. At a density of 29.3 plants/m2,the total plant growing area willbe 102.3 m2 (3,000 plants ÷29.3/m2 = 102.3 m2). This areais equal to 1,100 square feet(102.3 m2 × 10.76 ft2/m2 = 1,100ft2).

3. Select a hydroponic tank widthof 8 feet. The total hydroponictank length will be 137.5 feet(1,100 ft2/8 ft = 137.5 ft).

4. Multiples of two raft hydroponictanks are required for the UVIsystem. In this case only twohydroponic tanks are required.Therefore, the minimum lengthof each hydroponic tank will be

68.75 feet (137.5 ft ÷ 2 = 68.75ft). Since polystyrene sheetscome in 8-foot lengths, the totalnumber of sheets per hydropon-ic tank will be 8.59 sheets (68.75ft ÷ 8 ft/sheet = 8.59 sheets).To avoid wasting material,round up to nine sheets.Therefore, the hydroponic tankswill be 72 feet long (9 sheets × 8ft per sheet = 72 ft).

5. The total plant growing areawill then be 1,152 ft2 (72 ft × 8ft per tank × 2 tanks = 1,152ft2). This is equal to 107 m2

(1,152 ft2 ÷ 10.76 ft2/m2). 6. At a planting density of 29.3

plants/m2, a total of 3,135 plantswill be cultured in the system.The extra plants will provide asafety margin against mortalityand plants that do not meetmarketing standards.

7. Assume that a feeding rate of 60g/m2/day provides sufficientnutrients for good plant growth.Therefore, daily feed input tothe system will be 6,420 g (60g/m2/day × 107 m2 = 6,420 g).This is equal to 14.1 pounds offeed (6,420 g ÷ 454 g/lb = 14.1lb).

8. Annual feed input to the systemwill be 5,146 pounds (14.1lb/day × 365 days = 5,146 lb)

9. Assume the feeding conversionratio is 1.7. Therefore, the feedconversion efficiency is 0.59 (1lb of gain ÷ 1.7 lb of feed =0.59).

10. The total annual fish produc-tion gain will be 3,036 pounds(5,146 lb × 0.59 feed conversionefficiency = 3,036 lb).

11. Assume that the desired harvestweight of the fish will be 500 g(1.1 lb) and that 50-g (0.11-lb)fingerlings will be stocked.Therefore, individual fish willgain 450 g (500 g harvest weight- 50 g stocking weight = 450 g).The weight gain per fish will beapproximately 1 pound (454 g).

12. The total number of fish har-vested will be 3,036 (3,036 lb oftotal gain ÷ 1 lb of gain per fish= 3,036 fish).

13. Total annual production will be

3,340 pounds (3,036 fish × 1.1lb/fish = 3,340 lb) when the ini-tial stocking weight is considered.

14. If there are four fish-rearingtanks and one tank is harvestedevery 6 weeks, there will be 8.7harvests per year (52 weeks ÷ 6weeks = 8.7).

15. Each harvest will be 384 pounds(3,340 lb per year ÷ 8.7 harvestsper year = 384 lb/harvest).

16. Final harvest density should notexceed 0.5 pound/gallon.Therefore, the water volume ofeach rearing tank should be 768gallons (384 lb ÷ 0.5 lb/gal =768 gal). The tank should be larg-er to provide a 6-inch freeboard(space between the top edge ofthe tank and the water levels).

17. Each fish requires 2.2 gallons ofwater (1.1 lb ÷ 0.5 lb of fish/gal= 2.2 gal per fish).

18. The stocking rate is 349 fish pertank (768 gal ÷ 2.2 gal/fish =349 fish).

19. To account for calculated mortali-ty, the stocking rate (349 fish pertank) should be increased by 35fish (349 fish × 0.10 = 34.9) toattain an actual stocking of 384fish per tank.

In summary, two hydroponic tanks(each 72 feet long by 8 feet wide) willbe required to produce 1,000 Bibblettuce plants per week. Four fish-rearing tanks with a water volume of768 gallons per tank will be required.The stocking rate will be 384 fish pertank. Approximately 384 pounds oftilapia will be harvested every 6weeks, and annual tilapia productionwill be 3,340 pounds.

EconomicsThe economics of aquaponic systemsdepends on specific site conditionsand markets. It would be inaccurateto make sweeping generalizationsbecause material costs, constructioncosts, operating costs and marketprices vary by location. For example,an outdoor tropical system would beless expensive to construct and oper-ate than a controlled-environmentgreenhouse system in a temperate cli-mate. Nevertheless, the economicpotential of aquaponic systems looks

promising based on studies with theUVI system in the Virgin Islands andin Alberta, Canada.The UVI system is capable of produc-ing approximately 11,000 pounds oftilapia and 1,400 cases of lettuce or11,000 pounds of basil annuallybased on studies in the VirginIslands. Enterprise budgets for tilapiaproduction combined with either let-tuce or basil have been developed.The U.S. Virgin Islands represent asmall niche market with very highprices for fresh tilapia, lettuce andbasil, as more than 95 percent of veg-etable supplies and nearly 80 percentof fish supplies are imported. Thebudgets were prepared to show rev-enues, costs and profits from six pro-duction units. A commercial enter-prise consisting of six productionunits is recommended because onefish-rearing tank (out of 24) could beharvested weekly, thereby providinga continuous supply of fish for mar-ket development. The enterprise budget for tilapia andlettuce shows that the annual returnto risk and management (profit) forsix production units is US$185,248.The sale prices for fish ($2.50/lb) andlettuce ($20.00/case) have been estab-lished through many years of marketresearch at UVI. Most of the lettuceconsumed in the Virgin Islands isimported from California. It is trans-ported by truck across the UnitedStates to East Coast ports and thenshipped by ocean freighters toCaribbean islands. Local productioncapitalizes on the high price ofimports caused by transportationcosts. Locally produced lettuce is alsofresher than imported lettuce.Although this enterprise budget isunique to the U.S. Virgin Islands, itindicates that aquaponic systems canbe profitable in certain niche mar-kets. The enterprise budget for tilapia andbasil shows that the annual return torisk and management for six produc-tion units is US$693,726. Aquaponicsystems are very efficient in produc-ing culinary herbs such as basil (Fig.11) and a conservative sale price forfresh basil with stems in the U.S.Virgin Islands is $10.00/pound.However, this enterprise budget isnot realistic in terms of market

demand. The population (108,000people) of the U.S. Virgin Islandscannot absorb 66,000 pounds of freshbasil annually, although there areopportunities for provisioning shipsand exporting to neighboring islands.A more realistic approach for a six-unit operation is to devote a portionof the growing area to basil to meetlocal demand while growing othercrops in the remainder of the system.The break-even price for theaquaponic production of tilapia in theVirgin Islands is $1.47/pound, com-pared to a sale price of $2.50/pound.The break-even prices are $6.15/casefor lettuce (sale price = $20.00/case)and $0.75/pound for basil (sale price= $10.00/pound). The break-evenprices for tilapia and lettuce do notcompare favorably to commodityprices. However, the cost of construc-tion materials, electricity, water, laborand land are very high in the U.S.Virgin Islands. Break-even prices fortilapia and lettuce could be consider-ably lower in other locations. Thebreak-even price for basil comparesfavorably to commodity pricesbecause fresh basil has a short shelflife and cannot be shipped great dis-tances. A UVI aquaponic system in an envi-ronmentally controlled greenhouse atthe Crops Diversification CenterSouth in Alberta, Canada, was evalu-ated for the production of tilapia anda number of plant crops. The cropswere cultured for one productioncycle and their yields were extrapo-lated to annual production levels.Based on prices at the Calgary whole-sale market, annual gross revenuewas determined for each crop perunit area and per system with a plantgrowing area of 2,690 ft2 (Table 2).

Figure 11. Basil production in the UVIaquaponic system.

Table 2. Preliminary production and economic data from the UVI aquaponic system at the CropDiversification Center South, Alberta, Canada.1 (Data courtesy of Dr. Nick Savidov)

Annual production Wholesale price Total valueCrop lb/ft2 tons/2690 ft2 Unit $ $/ft2 $/2690 ft2

Tomatoes 6.0 8.1 15 lb 17.28 6.90 18,542Cucumbers 12.4 16.7 2.2 lb 1.58 8.90 23,946Eggplant 2.3 3.1 11 lb 25.78 5.33 14,362Genovese basil 6.2 8.2 3 oz 5.59 186.64 502,044Lemon basil 2.7 3.6 3 oz 6.31 90.79 244,222Osmin basil 1.4 1.9 3 oz 7.03 53.23 143,208Cilantro 3.8 5.1 3 oz 7.74 158.35 425,959Parsley 4.7 6.3 3 oz 8.46 213.81 575,162Portulaca 3.5 4.7 3 oz 9.17 174.20 468,618

1Ecomonic data based on Calgary wholesale market prices for the week ending July 4, 2003.

The information given herein is for educational purposes only. Reference tocommercial products or trade names is made with the understanding thatno discrimination is intended and no endorsement by the Southern RegionalAquaculture Center or the Cooperative Extension Service is implied.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Centerthrough Grant No. 2003-38500-12997 from the United States Department of Agriculture, Cooperative StateResearch, Education, and Extension Service.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software SteeringCommittee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have notbeen revised are considered to reflect the current state of knowledge.

Annual production levels based onextrapolated data from short produc-tion cycles are subject to variation.Similarly, supply and demand willcause wholesale prices to fluctuateduring the year. Nevertheless, thedata indicate that culinary herbs ingeneral can produce a gross incomemore than 20 times greater than thatof fruiting crops such as tomatoesand cucumbers. It appears that justone production unit could provide alivelihood for a small producer.However, these data do not showcapital, operating and marketingcosts, which will be considerable.Furthermore, the quantity of herbsproduced could flood the market anddepress prices. Competition from

current market suppliers will alsolead to price reductions.

OverviewAlthough the design of aquaponicsystems and the choice of hydropon-ic components and fish and plantcombinations may seem challenging,aquaponic systems are quite simpleto operate when fish are stocked at arate that provides a good feedingrate ratio for plant production.Aquaponic systems are easier tooperate than hydroponic systems orrecirculating fish production systemsbecause they require less monitoringand usually have a wider safetymargin for ensuring good water

quality. Operating small aquaponicsystems can be an excellent hobby.Systems can be as small as anaquarium with a tray of plants cov-ering the top. Large commercialoperations comprised of many pro-duction units and occupying severalacres are certainly possible if mar-kets can absorb the output. The edu-cational potential of aquaponic sys-tems is already being realized inhundreds of schools where studentslearn a wide range of subjects byconstructing and operating aquapon-ic systems. Regardless of scale orpurpose, the culture of fish andplants through aquaponics is a grati-fying endeavor that yields usefulproducts—food.