SRAC Publication No. 454

November 1992

SouthernRegionalAquacultureCenter

Recirculating Aquaculture TankProduction Systems

Integrating Fish and Plant Culture

James E.

Recirculating aquaculture systemsare designed to raise large quanti-ties of fish in relatively small vol-umes of water by treating thewater to remove toxic waste prod-ucts and then reusing it. In theprocess of reusing the water manytimes, non-toxic nutrients and or-ganic matter accumulate. Thesemetabolic by- products need notbe wasted if they are channeledinto secondary crops that haveeconomic value or in some waybenefit the primary productionsystem. Systems that grow addi-tional crops by utilizing by-prod-ucts from the production of theprimary species are referred to asintegrated systems.

Plants are an ideal secondary cropin integrated systems because theygrow rapidly in response to thehigh levels of dissolved nutrientsthat are generated from the micro-bial breakdown of fish wastes. Inclosed recirculating systems,which employ very little dailywater exchange (1 to 5 percent),dissolved nutrients accumulateand approach the concentrationsthat are found in hydroponic nutri-ent solutions. Nitrogen, in particu-

lUniversity of The Virgin Islands;2North Carolina State University3Alabama Cooperative Extension Service

Rakocy 1, Thomas M. Losordo2 and Michael P. Masser3

lar, occurs at very high levels inrecirculating systems. Fish excretewaste nitrogen directly into thewater through their gills in theform of ammonia. Biofilter bacte-ria convert ammonia to nitrite andthen to nitrate (see SRAC Publica-tion No. 451 on critical considera-tions). Ammonia and nitrite aretoxic to fish, but nitrate is rela-tively harmless and is the pre-ferred form of nitrogen used byhigher plants, such as vegetables.

Integrated systems can be used forthe hydroponic culture of highvalue cash crops such as tomatoes,lettuce and sweet basil. Recirculat-ing systems may also be used forthe culture of aquatic plants.

Aquatic plantsAquatic plants grow rapidly inrecirculating systems that are lo-cated outdoors, in greenhouses orin buildings with adequate artifi-cial light. Plants are typicallygrown in shallow tanks that areseparate from the fish rearingtank. Three types of aquatic plantscan be cultured: floating, emer-gent and submerged.

Floating plants, which arenaturally buoyant, grow with theirroots suspended in the water andtheir leaves in the air. They repro-

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duce vegetatively (without seeds)by dividing in half and grow sorapidly that they can completelycover the surface of the tankwithin a short time. When this oc-curs, about one third of the plants

Leaf lettuce grown in deep flowing chan-nels is an ideal crop for integrated recir-culating systems.

should be removed to avoidcrowding and nutrient limitationsand to keep the remaining plantsin a state of rapid growth.

Common floating plants that canbe cultured in recirculating sys-tems include water hyacinth (Eich-horia crassipes), water lettuce (Pistiasp. ), duckweed (Spirodela andLemna spp.) and water fern (Azollasp. ). These plants can be sold as or-namental (water hyacinth, waterlettuce) or used as food (duck-weed, water fern) for herbivorousfish such as grass carp and tilapia.Floating plants are also useful ininhibiting the growth of phyto-plankton by blocking light penetra-tion into the water and removingexcess nitrogen from the system.Water hyacinth can remove morethan 1 gram of nitrogen per squaremeter of surface area per day dur-ing periods of optimum growth.

Emergent plants need an under-water substrate for their rootswhile their stems and leaves growabove the water surface. Emergentplants with economic value thatcan be cultured in a recirculatingsystem include watercress (Nastur-tium officinale), water chestnut(Eleocharis dulcis) and water spin-ach (Ipomoea aquatica). Substratesfor these plants may be mud, sandor an artificial medium (e. g., ver-miculite). Watercress may be cul-tured without a substrate if it issupported near the water surfaceon window screen, which allowsits roots to grow into the water col-umn. Watercress and water spin-ach grow very rapidly and can beharvested on a continuous basis.Water chestnut requires a 7-monthgrowing season to produce a thickstand of reed-like plants and ahigh density of underground tu-bers, which develop as theweather cools. Water chestnutyields of 0.6 pounds per squarefoot have been obtained in recircu-lating systems.

Submerged plants grow entirelyunderwater and are held in placeby their roots which grow in sub-strates of mud or sand. Submer-ged aquarium plants such as eelgrass (Vallisneria sp.) and anacha-

ris (Elodea sp.) can be grown suc-cessfully in recirculating systemswhere filters maintain clear water.These plants have economic valueas aquarium plants. They alsobenefit the system by inhibitingthe growth of phytoplankton andproducing dissolved oxygen onsunny days.

Hydroponic vegetablesThe production of vegetables with-out soil is referred to as hydropon-ics. Hydroponics requires mediaother than soil, or some structureto support the plant. Water mustcontain the 13 essential nutrientsfor plant growth.

Hydroponic culture is usuallyidentified by type of supportmedia that is used: water culture,sand culture or gravel culture.Water culture is further subdi-vided into nutrient film technique(NFT), deep flowing channels andaeroponics. NFT consists of manynarrow plastic troughs or guttersin which the plant roots are placedand exposed to a continuouslyflowing film of nutrient solution.Deep flowing channels are muchwider and deeper so that the plantroots are completely suspended ina slow moving stream of water.The plants are supported at thesurface with polystyrene sheets. Inaeroponics, the plant roots are sus-pended in air inside an opaquecontainer and sprayed with a finemist of nutrient solution.

Although these media have allbeen used successfully in inte-grated systems, fish culture waterhas some special characteristicsthat must be considered in thedesign of the hydroponic subsys-tem. Fish produce feces that mustbe removed from the systemthrough sedimentation or filtra-tion (see SRAC Publication No.453 on components). Althoughthese methods remove most of thesolid waste, a relatively smallamount remains suspended assmall (colloidal) particles. Hydro-ponic systems that use fine sup-port media for plants, such assand or gravel, may eventually be-come clogged with sludge. The

roots of plants grown by NFT maybecome fouled with a blanket ofsludge. Excessive sludge accumu-lation has a harmful affect onplant growth because it blocks theflow of water and creates zoneswithout oxygen. Plant roots needoxygen for healthy growth. Dis-solved oxygen is utilized by theroots for nutrient uptake and othervital cell functions.

There are techniques to preventsludge buildup and maintain ade-quate oxygen. A false bottom ofrigid screen can be used to sepa-rate the root growing area fromthe bottom of deep flowing chan-nels. The rigid screen can also sup-port plant roots in the NFTmethod or support growth mediasuch as gravel. Settleable solids ac-cumulate on the bottom of thetank under the root zone. If a fewfish are placed under the false bot-tom, their swimming action cankeep sludge from accumulating.To maintain adequate oxygen,gravel and sand media can beflooded intermittently, allowingtime for the culture water to drainout of the media and draw air(oxygen) into the root zone. Long,deep flowing channels must beaerated at intervals with diffusedair delivered through air stones.

Essential nutrientsEssential plant nutrients are di-vided into two categories; macro-nutrients which are required inrelatively large quantities, and mi-cronutrients which are needed invery small (trace) amounts. Macro-nutrients consist of nitrogen (N),phosphorus (P), potassium (K), cal-cium (Ca), sulfur (S), and magne-sium (Mg). Micronutrients includeiron (Fe), chlorine (Cl), manganese(Mn), boron (B), zinc (Zn), copper(Cu), and molybdenum (Mo). Foroptimum growth, plants need abalanced quantity of these essen-tial nutrients (Table 1).

Hydroponic solutions may contain10 to 20 percent of total nitrogenas ammonium (NH4), which stimu-lates vegetative growth. However,ammonium is never added to inte-grated systems since sufficient am-

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Table 1. Typical hydroponicnutrient solution.1

ConcentrationNutrient (mg/liter)

Ca 197Mg 44K 400N (as NO3) 145 (642)P (as PO4) 65 (199)S ( a s S O4) 197.5 (592)Cl —

Fe 2Mn 0.5Cu 0.03Zn 0.05B 0.5Mo 0.02

1Modified after Resh, 1989.

monium is produced by the fish.Although chloride ions are re-quired by plants, they are notadded to hydroponic or integratedsystems because sufficientamounts of this element occur infish feed and most water supplies.

The total concentration of nutrientsalts in this typical hydroponic so-lution is approximately 2,100mg/liter. Total nutrient concentra-tions that exceed this maximumsafe level become toxic to manyvegetable varieties and decreaseproduction. When this solution isused in standard hydroponic sys-tems, concentrations of all the es-sential elements decline as theyare absorbed by the plants. Eventu-ally the solution must be dis-carded and replaced with a new,balanced solution.

A different relationship exists in in-tegrated recirculating systems.Through the addition of feed andthe production of metabolicwastes by the fish, nutrients areusually generated faster than theycan be absorbed by the plants. Asa result, levels of nutrient saltssteadily increase, and when thetotal concentration exceeds 2,100mg/liter, the culture water mustbe exchanged or diluted. Waterthat is used for integrated systemsshould initially have low levels(<100 mg/liter) of total dissolvedsolids (salts). A good source is rain-

water, which has low salt levelsbut will require buffering to raisepH.

The rate of salt buildup can be de-creased by increasing the water ex-change rate or using the optimumratio between the number ofplants and the number of fish, ormore precisely, the daily feed allot-ment. An optimum ratio isachieved when plant production ismaximized and salt build-up islow. For example, the optimumratio for the number of leaf lettuceplants (summer Bibb) to fish (til-apia) is 1.9:1 in a closed recirculat-ing system using a deep flowingchannel with rainwater, a com-plete fish diet of 36 percent pro-tein, and a water exchange rate of1 percent/day. Lettuce productionis maximized at this ratio, which isequivalent to a daily feeding rateof 2.4 grams/plant. The waste nu-trients from this amount of feed be-come available to the plant afterthe feed is digested by the fish.This daily feeding rate is equiva-lent to 3.2 grams/cubicmeter/square meter when systemvolume and plant growing area(23.9 plants/ square meter) are con-sidered.

For example, at the optimum ratio,a 10,000-gallon integrated systemwith a daily feed input of 8.8pounds requires a plant produc-tion area of 800 square feet for2,100 heads of leaf lettuce. At the1.9 ratio, total salts still increase ata rate of 135 grams/kilogram offeed. Very little is presentlyknown about the optimum ratiofor other types of vegetables, fish,system designs, feeds, and watersources. In general, properlydesigned integrated systems havevery large plant growing areas re-lative to the fish production com-ponents.

When an integrated system is putinto operation, the fish should befed a few weeks before the firstplanting to allow time for all of theessential nutrients to accumulateto the minimal levels required forgood plant growth. Although nu-trient salts accumulate in inte-grated recirculating systems, not

all of the essential nutrients arepresent in sufficient quantities. Nu-trients that require supplementa-tion are potassium (as potassiumhydroxide), calcium (as calciumoxide or calcium hydroxide) andiron (as iron chelate containing 10percent iron by weight).

The addition of strong bases suchas potassium hydroxide and cal-cium oxide also neutralize acidsthat are produced when ammonia(NH3) is converted to nitrate(NO3) in the biofilter. These basesmust be added daily to maintainthe pH of the system near 7. As aresult, potassium and calcium con-tribute substantially to the build-up of nutrient salts, but they arealso required in large amounts byplants (Table 1). Iron is supple-mented less frequently (perhapsmonthly) by adding 2 mg/liter.Supplemental nutrients may be ap-plied directly to the plantsthrough foliar sprays. Nutrient de-ficiencies may vary depending onthe water source (chemistry), fishfeed and hydroponic substrateused.

Insect and disease controlInsect and disease control tech-niques for vegetables in integratedsystems are limited to the use of bi-ological control, resistant varieties,screening and specialized culturalpractices. One successful biologi-cal control method is the use of thebacterium Bacillus thuringiensis,which is effective in the control ofcaterpillars. In general, pesticidesshould not be used because mostare very toxic to fish, and nonehave been approved for use infood fish culture. Similarly, mosttherapeutants for treating fishparasites and diseases should notbe used in an integrated system be-cause they harm the biofilter andvegetables may readily absorb andconcentrate them.

A possible approach to avoid therestrictions of pesticide usage is toestablish the hydroponic unit as aflow- through system, thereby al-lowing the vegetables to be man-aged independently of the fish.

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Baskets of anacharis (Elodea) being harvested from an integrated recirculating system. In thebackground are stands of water chestnut (left) and water hyacinth (right).

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Annual production of leaf lettuce from the integrated recirculating system is projected to be35,000 heads.

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Water can be diverted to theplants from the recirculating sys-tem when optimum nutrient levelsare reached. Water should be di-verted at a rate (e.g., 5 percentdaily) that allows nutrients to bemaintained at optimum concentra-tions for plant growth.

Vegetable productionMore than 30 kinds of vegetableshave been raised in integrated sys-tems on an experimental basis. Awide range of systems has beenused in a number of different envi-ronments from greenhouses innorthern states that are heated inthe winter to outdoor systems inthe subtropics. Production levelshave varied greatly in response toenvironment, system design andmanagement. Table 2 lists yieldsof some of the vegetable varietiesthat have been grown in inte-grated systems.

Crops that have been studied mostintensively include tomatoes andlettuce. Tomatoes appear to bemost productive in outdoor gravelsystems with varieties that are de-terminant (i.e., they set their fruitduring a short period and are notpruned). Tomatoes require a longgrowing period and are thereforeat greater risk to damage frompests and diseases.

Lettuce is a popular plant for inte-grated production because it

grows well in response to high ni-trogen levels, yields a high propor-tion of edible product, and issubject to less pest pressure due toits short production cycle. Com-mon methods for growing lettucein integrated systems are NFT anddeep flowing channels with poly-styrene sheets for plant support.Plants are started in peat pellets intrays outside the system. After 3weeks, the seedlings are trans-ferred to the system for 3 moreweeks of growth, which will bringthem to harvest size under idealconditions. Two additional weeksof growth are required during thewinter in heated greenhouses. Let-tuce production is usually stag-gered so that one crop is harvestedeach week, and a new one is imme-diately planted.

Another crop with good potentialfor integrated systems is sweetbasil. Production data are notavailable, but sweet basil is beinggrown commercially in integratedsystems.

Vegetable yields in integrated sys-tems exceed that of field crops be-cause higher planting densities arepossible due to control of the nutri-ent solution, the constant availabil-ity of water and the absence ofcompetition from weeds. In somecases, as with lettuce, productionfrom integrated systems hasequaled that of standard hydro-

ponic systems. For the most part,however, production from inte-grated systems will probably fallslightly short of that from stand-ard hydroponics. Integrated sys-tems cannot be managed asefficiently because of pesticide userestrictions and complex nutrientdynamics involving build-ups ofsome elements and deficiencies inothers that must be supplemented.

The primary attraction for inte-grated systems is financial. Nutri-ent recovery from aquacultureeffluents reduces hydroponicchemical costs. High quality wateris repeatedly used to support thegrowth of both fish and vegeta-bles, further reducing costs. Addi-tional savings can be realized bysharing infrastructure (e.g., onepump can be used for both subsys-tems). Detailed economic studiesare needed to quantify the advan-tages of integration and determineprofitability, but a preliminary an-alysis with lettuce shows thatplants will generate most of the in-come and require most of thelabor.

ReferenceResh, H.M. 1989. Hydroponic

Food Production: A DefinitiveGuidebook of Soilless FoodGrowing Methods. WoodbridgePress Publishing Co., SantaBarbara, CA.

Table 2. Production of vegetable varieties in integrated recirculating aquiculture systems.

Crop Variety Density Growing Production in Ibs. per System(no./ft2) Period

(days)1 plant ft2

T o m a t o F l o r a d e l 0.36 133 20.0 3.3 GravelFloradade 0.17 112 20.0 3.3 GravelSunny 0.17 112 22.0 3.7 Gravel

Lettuce Summer Bibb 2.32 21 0.4 0.9 ChannelButtercrunch 2.32 21 0.4 1.0 Channel

Cucumber Triumph — — 8.9 — GravelBurpee Hybrid II 0.62 — 5.2 3.2 Sand

Squash Golden Bar — — 5.5 — GravelPac Choi Le Choi 1.77 28 1.1 1.8 Gravel

Pac Choi 1.77 28 0.9 1.6 GravelChinese 50-Day Hybrid 1.77 28 1.4 2.3 GravelCabbage Tropical Delight 1.77 28 1.3 2.2 Gravel1 From date of transplanting.

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The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 89-38500-4516 from the United StatesDepartment of Agriculture.

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