Paper1_Managing_Ammonia_in_Fish_Ponds_2004

8/7/2019 Paper1_Managing_Ammonia_in_Fish_Ponds_2004

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Amm onia is toxic to fish if allowedto accumulate in fish productionsystems. When amm onia accumu -lates to toxic levels, fish can notextract energy from feed efficient-ly. If the amm onia concentrationgets high enough, the fish willbecome lethargic and eventuallyfall into a coma and die.

In properly managed fish ponds,amm onia seldom accum ulates tolethal concentrations. However,ammonia can have so-called sub-

lethal effectssuch as red ucedgrowth, p oor feed conversion, andreduced disease resistanceatconcentrations that are lower thanlethal concentrations.

Effects of pH andtemperature onammonia toxicity

Amm onia in water is either un -ionized am monia (NH 3) or theammonium ion (NH 4

+). The tech-niques used to measure ammonia

provide a value that is the sum ofboth forms. The value is reportedas total ammonia or simplyamm onia. (In this pu blication,

amm onia refers to the sum ofboth forms; the specific forms w illbe referred to as appropriate.)The relative proportion of the twoforms present in water is mainlyaffected by pH. Un-ionizedammonia is the toxic form andpredominates when pH is high.

Amm onium ion is relatively non-toxic and predom inates wh en pHis low. In general, less than 10% ofammonia is in the toxic form whenpH is less than 8.0. However, thisproportion increases dramaticallyas pH increases (Fig. 1).

VIPRDecember 2004

SRAC Publication No. 4603

Managing Ammonia in Fish Ponds

John A. Hargreaves1 and Craig S. Tucker2

1 Louisiana State University Agricultural

Center2 Mississippi State University

Figure 1. The proportion of toxic, un-ionized ammonia increases as a function ofpH an d temp erature. To determine the propor tion of un-ionized amm onia in awater sam ple, draw a line from the pH of the water straight up to the line that isclosest to the water temp erature. From that point, dra w a line to the right u ntil itintersects the graphs vertical axis. That point is an estimate of the percentage ofun-ionized amm onia in the water sample. Now, simp ly multiply that number(divided by 100) by the total amm onia concentration to estimate the u n-ionizedamm onia concentration.


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In pond s, pH fluctuates with thephotosynthesis (which increasespH ) and respiration (whichreduces pH) of pond organisms.Therefore, the toxic form ofammonia predominates duringthe late afternoon and earlyevening (Fig. 2) and ammoniumpredominates from before sunrise

through early morning.The equilibrium between N H3and NH4

+ is also affected by tem-perature. At any given pH, moretoxic amm onia is present inwarm er water than in coolerwater (Fig. 1).

Ammonia dynamics infish ponds

The measurement of ammoniaconcentration (and that of manyother water quality variables) pro-vides only a snapshot of condi-

tions at the time a water sample iscollected. A single measurementprovides no insight into theprocesses that affect ammoniaconcentrations; it is simply the netresult of processes that produceamm onia and processes thatremove or transform am monia.The relationships among these

processes are complex, but theimportant p oint is that the rateschange differentially throughoutthe year and result in the m ea-sured patterns.

Ammonia sources

The main source of ammonia infish p ond s is fish excretion. Therate at which fish excrete ammo-nia is directly related to the feed-ing rate and the protein level infeed. As dietary protein is broken

dow n in the bod y, some of the

nitrogen is used to form protein(includ ing m uscle), some is usedfor energy, and some is excretedthrough the gills as amm onia.Thus, protein in feed is the ulti-mate source of most amm onia inpond s wh ere fish are fed.

Another m ain source of amm oniain fish ponds is diffusion from thesediment. Large quantities oforganic matter are prod uced byalgae or add ed to pond s as feed.Fecal solids excreted by fish anddead algae settle to the pond bot-tom, wh ere they decompose. Thedecomposition of this organicmatter prod uces ammonia, whichdiffuses from the sediment intothe water colum n.

Ammonia sinks

There are two main p rocesses that

result in the loss or tran sformationof ammonia. The most importantis the uptake of ammonia by algaeand other plants. Plants use thenitrogen as a nutrient for growth,packaging the nitrogen in anorganic form. Algal photosyn the-sis acts like a sponge for amm o-nia, so anything that increasesoverall algal growth will increaseamm onia up take. Such factorsinclud e sufficient light, warm tem-perature, abund ant nutrient sup -ply, and (to a point) algal density.

The other important process ofammonia transformation in fishpon ds is nitrification. Bacteriaoxidize ammonia in a two-stepprocess, first to n itrite (NO2

-) andthen to nitrate (NO3

-). The m ainfactors that affect nitrification rateare ammonia concentration, tem-perature and dissolved oxygenconcentration. During summer,ammonia concentration is verylow an d so n itrification rates arealso very low. During winter, low

temperature supp resses m icrobialactivity. During sp ring and fall,amm onia concentration and tem-perature are intermediate, condi-tions that favor maximum nitrifi-cation rates. Spring and fall peaksof nitrite concentration are com-monly seen in fish p onds.

Figure 2. The effect of daily fluctuation in pH on u n-ionized amm onia concentra-tion in fish pond s. The top h orizontal line ind icates a total am monia concentrationof 2.5 mg N / L, which is assumed n ot to change d uring the d ay. The two curvedlines indicate daily changes in un-ionized am monia concentration where the maxi-mu m afternoon pH is 9.0 or 9.5. These conditions indicate that fish m ay be exposedto toxic, un-ionized am monia concentrations for brief periods d uring the late after-noon.


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Other processes, such as thevolatilization of ammonia gasfrom the pond surface into the air,are respon sible for a relativelysmall and variable amou nt ofamm onia loss from fish p onds.

When is ammonia most

likely to be a problem?In fish p ond s, it is extremelyunlikely that un-ionized amm oniawou ld accum ulate to a concentra-tion th at w ould become toxicenough to kill fish. However, un-ionized ammonia will occasional-ly accum ulate to levels that causesub-lethal effects.

The following analysis is based onwater quality criteria for ammoniadeveloped by the U.S. Environ-men tal Protection Agen cy (EPA).

The EPA has established threekinds of criteria (one acute an dtwo chronic) for ammonia(expressed as nitrogen), based onthe d uration of exposure. Theacute criterion is a 1-hour averageexposure concentration and is afunction of pH . One chronic crite-rion is the 30-day average concen-tration and is a function of pHand temperature. The other chron-ic criterion is the h ighest 4-dayaverage within the 30-day periodand is calculated as 2.5 times the

30-day chronic criterion. The EPAcriteria h elp determine w henamm onia might be a problem.

During winter

It is generally assumed thatamm onia is not a problem inthe winter because feeding ratesare very low. (Fish are fed ononly the warm est days of winter,usually when the water tempera-ture is higher than 50 F.) How-ever, ammonia concentrationtends to be greater during w inter(2.5 to 4.0 mg/ L, or even h igher)than d uring sum mer (less than0.5 mg / L) (Fig. 3). The relativelylow concentration d uring sum mercan be attributed to intense photo-synthesis by algae, which removesammonia. During winter, algae

take up little amm onia but theamm onia supp ly continues, pri-marily from the decomposition of

organic matter that accumulatedon pond sediment during thegrowing season. In general, themagnitude and duration of highamm onia concentrations d uringthe late fall and winter can berelated to the total amount of feedadded to a pond d uring the pre-ceding growing season.

The 30-day chronic criterionfor amm onia (as nitrogen) inwinter ranges from about 1.5 to3.0 mg/ L, depending on pH.Amm onia concentrations d uringthe winter usually exceed this cri-terion. This may cau se stress infish at a time of year w hen th efish imm une system is supp ressedbecause of low temperature.

After the crash of analgae bloom

Some pon ds h ave very dense algaeblooms d ominated by one or twospecies. For reasons that are n otwell und erstood, these blooms aresubject to spectacular collapse,often called a crash, w here all thealgae sud denly d ie. When thisoccurs, ammonia concentrationincreases rapidly because the mainmechanism for ammon ia removalalgal uptakehas been eliminated.Rapid d ecomposition of d ead algaereduces the dissolved oxygen con-centration and p H and increases

ammon ia and carbon d ioxide con-centrations. After the crash of analgae bloom, ammonia concentra-tion can increase to 6 to 8 mg/ Land the pH can decline to 7.8 to8.0. The 4-day chronic criterion,the app ropriate criterion to ap ply

Figure 3. Approximate ann ual var iation of total am monia concentration in fishpond s. Amm onia concentration is generally lowest du ring sum mer an d highestduring winter.


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following th e crash of an algaebloom, ranges from abou t2.0 mg/ L at pH 8.0 to about3.0 mg/ L at pH 7.8. Therefore,ammonia concentration after thecrash of an algae bloom mayexceed th e 4-day chron ic criterion.

Occasionally during the late

afternoons in late summer orearly fall

Seasonal variation in ammoniaconcentration d epend s on algaldensity and photosynthesis. Whenthese are high, ammonia concen-tration is low. Daily variation inthe concentration of toxic, un -ion-ized amm onia depends onchanges in p H (from p hotosynthe-sis) and , to a mu ch lesser extent,temp eratu re (Fig. 2). In the latesumm er or early fall, ammonia

concentration begins to increasebut d aily changes in p H rem ainlarge. In th ese situations, fish m aybe exposed to am monia concen-trations that exceed the acute cri-terion for a few hou rs each day. Iflate afternoon pH is about 9.0,the acute criterion is abou t 1.5 to2.0 mg/ L total amm onia-nitrogen.Total amm onia-nitrogen concen-trations du ring summ er are typi-cally less than 0.5 mg/ L, so fishare un likely to be stressed if thelate afternoon pH is less than 9.0.

It is difficult to be more preciseabout the risk of ammonia toxicitybecause of deficiencies in themethodology u sed in research.Near ly all ammon ia toxicity testsare cond ucted in systems thatmaintain a constant ammon ia con-centration. These conditions donot reflect the fluctua ting concen-trations of NH3 in ponds.Accordingly, one m ust be carefulwh en ap plying research resultsto production situations. Forexample, in one study, growth ofchannel catfish exposed to a con-stant amm onia concentration of0.52 mg/ L NH 3 was reduced by50% relative to u nexposed fish.How ever, brief (2- to 3-hou r) da ilyexposure to 0.92 mg/ L NH 3 (suchas might occur in pond s) did n otaffect growth and feed conversionratio. The fact that man y fish can

acclimate to repeated exposure tohigh concentrations of un-ionizedammonia is a further complicatingfactor.

Ammonia managementoptions

On rare occasions ammonia con-

centration becomes high enou ghto cause problems. What practicalsteps can be taken if this occur s?The short answer isnot much.

Theoretically, there are severalways to redu ce ammonia concen-tration, but m ost approaches areimpractical for the large pondsused in commercial aquaculture.Following is a discussion of someoptions, their p racticality andtheir effectiveness.

Stop feeding orreduce feeding rate

The primary source of nearly allthe amm onia in fish pond s is theprotein in feed. When feed proteinis completely broken d own(metabolized), ammonia is pro-du ced within the fish and excret-ed throu gh the gills into pondwa ter. Therefore, it seems reason-able to conclud e that amm onialevels in p ond s can be controlledby man ipulating feeding rate orfeed p rotein level. This is true to

some extent, but it depend s onwh ether you w ant to control itover the short-run (days) or thelong-run (weeks or month s).

In the short-run, sharp redu ctionsin feeding rate have little immedi-ate effect on amm onia concentra-tion. The ecological reason for thisis based on the comp lex move-ment of large amounts of nitrogenfrom one of the man y componentsof the pond ecosystem to another.In essence, trying to red uce

amm onia levels by withholdingfeed can be comp ared w ith tryingto stop a fully loaded freight trainrunn ing at top speedit can bedone but it takes a long time.

Producers can reduce the riskover the long-run by adjustingboth feeding rate an d feed p roteinlevel. Limit feed to the amountthat w ill be consum ed. In mid-

summer the maximum dailyfeeding rate shou ld be 100 to125 pound s per acre. By feedingconservatively, the poten tial forhigh ammonia in pond s and therisks associated with sub-lethalexposure (disease, poor feed con-version, slow growth) can be min-imized.

Increase aeration

The toxic form of amm onia (NH 3)is a dissolved gas, so some pro-du cers believe pond aeration isone way to get rid of ammoniabecause it accelerates the diffusionof amm onia gas from pon d w aterto the air. However, research hasdemon strated that aeration is inef-fective at reducing ammonia con-centration because the volume ofwater affected by aerators is quite

small in comp arison w ith the totalpond volume and because theconcentration of ammonia gas inwa ter is typically fairly low (espe-cially in the m orning). Intensiveaeration may actually increaseammonia concentration because itsuspends pond sediments.

Add lime

It has long been thought that lim-ing pond s decreases ammoniaconcentrations. In fact, using lim-ing agents such as hydrated limeor quick lime could actually makea p otentially bad situation mu chworse by causing an abrup t andlarge increase in p H. IncreasingpH shifts ammonia toward theform that is toxic to fish. In ad di-tion, the calcium in lime can reactwith soluble phosphorus, remov-ing it from w ater and making itunavailable to algae.

In pond s w ith similar algal densi-ty, daily fluctuations of pH in low-alkalinity pon d waters are more

extreme than th ose in waters ofsufficient alkalinity (greater th an20 mg/ L as CaCO3; see SRACPublication No. 464). Therefore,liming can moderate extreme pHvalues, particularly those thatoccur d uring late afternoon w henthe fraction of total ammonia thatis in the toxic form is highest.How ever, this techn ique is effec-


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tive only in pond s with low alka-linity. Most fish pon ds have suffi-cient alkalinity. Increasing thealkalinity above 20 mg/ L asCaCO3 will not provide add itionalbenefit. Furthermore, liming doesnot ad dress the root causes ofhigh ammonia concentration; itonly shifts the distribution of

ammonia from the toxic to thenon-toxic form by moderatinghigh pH in the afternoon.

Fertilize with phosphorus

Most of the ammonia excreted byfish is taken up by algae, so any-thing that increases algal growthwill increase ammonia u ptake.This fact is the basis for the ideaof fertilizing pond s with p hospho-rus fertilizer to reduce ammonialevels. However, under normal

pond conditions, algae blooms infish pond s are very dense and therate of algae growth is limited bythe availability of light, not nu tri-ents such as phosph orus or nitro-gen. Therefore, adding ph ospho-rus does nothing to redu ce amm o-nia concentration because algaeare already growing as fast as pos-sible u nd er the prevailing condi-tions.

The highest amm onia concentra-tions in fish ponds occur after thecrash of an algae bloom. Fertili-

zation, particularly with p hospho-rus, m ay accelerate the re-estab-lishment of the bloom, but m ostpond s have p lenty of dissolvedphosphorus (and other nutrients)to support a bloom and do notneed more.

Reduce pond depth

Algal growth (and therefore therate of ammonia up take by algae)in fish p onds is limited by theavailability of light. Anything th at

increases light increases amm oniaup take. Theoretically, dense algaeblooms in shallow pond s willremove amm onia m ore effectivelythan the sam e dense blooms indeeper ponds. On balance, how -ever, there are probably more ben-efits associated w ith deeper pond s(e.g., ease of fish ha rvest, water

conservation, more stable temper-atures, redu ced effect of sedimen -tation on interval between renova-tions).

Increase pond depth

Obviously, deeper ponds containmore water than more shallowponds. Therefore, at a given feed-ing rate, deeper p onds shou ldhave lower am monia concentra-tions because there is more waterto dilute the am monia excreted byfish. In reality, deeper pon ds d onot usually have enough water tosignificantly dilute ammoniawh en compared to the largeamou nts of ammonia in constantflux between various biotic andabiotic comp artments in p ond s.Furthermore, deeper pon ds aremore likely to stratify and the

lower layer of pond water (thehypolimnion) can becomeenriched w ith ammonia anddep leted of dissolved oxygen.When this layer of water mixeswith surface water in a turn-over, severe water quality prob-lems may result.

Flush the pond with well water

Amm onia can be flushed fromponds, although pu mping thehuge volum e of water required todo so in large commercial ponds

is costly, time-consum ing an dun necessarily w asteful. It is alsodeceptively ineffective as anamm onia managem ent tool. Forexample, assume the am moniaconcentration in a full, 10-acrepond is 1 mg/ L. The ammoniaconcentration after p um ping500 gpm continuously for 3 days(equivalent to abou t 8 inches ofwater) will be 0.90 mg/ L, a d ropof only 0.10 mg/ L.

Instead of simply runn ing water

through a p ond as in the examp leabove, now assume that about 8inches of water is discharged fromthe pond before refilling with wellwa ter. In th is case, the d ecline inammonia concentration will beslightly greater (to 0.83 mg/ L),but even this decrease is notenough in an em ergency situation,

particularly w hen th e extra timeneeded to d rain the water beforerefilling is considered. The differ-ence in the two flushing scenariosis related to the blending of pon dwater with pu mped water beforedischarge in th e first case.

Just as paddlewheel aeration cre-ates a zone of su fficient d issolved

oxygen concentration, p um pinggroundwater creates a zone of rel-atively low ammonia concentra-tion ad jacent to the w ater inflow.The effectiveness of this practice isquestionable because it does notadd ress the root cause of the prob-lem and wastes w ater. Flushingpon ds is not on ly ineffective, buthighly undesirable because ofconcerns about releasing pondeffluents into the environment.

Add bacterial amendments

Common aquatic bacteria are anessential part of the constantcycling of ammonia in a pondecosystem. Some peop le believethat amm onia accum ulates inpond s because the wrong kind orinsufficient n um bers of bacteriaare present. If this were true,add ing concentrated formulationsof bacteria w ould ad dress theproblem. However, research withmany brand s of bacterial amend-ments has consistently given the

same resu lt: Water qu ality is unaf-fected by the addition of thesesupplements.

Standard pond management cre-ates very favorable cond itions forbacterial growth . Bacterial growthand activity is limited more by theavailability of oxygen and by tem-perature than by the number ofbacterial cells. Also, the mostabund ant type of bacteria in m anyamendments (and in pond w aterand sediment) is responsible forthe decomposition of organic mat-ter. Therefore, if bacterial am end -ments accelerate the decomposi-tion of organic matter, ammoniaconcentration would actuallyincrease, not decrease.


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Another kind of bacteria inamend ments oxidizes ammonia tonitrate. Adding th em w ill notreduce the amm onia concentrationrapidly because the bacteria m ustgrow for several weeks beforethere is a large enough popu lationto affect amm onia level.

Add a source of organic carbon

If the dissolved oxygen concentra-tion is adequ ate, adding a sourceof organic carbon , such aschopp ed h ay, to intensive fishpond s can redu ce amm onia con-centration. Many bacteria in fishpond s are starved for organiccarbon, d espite the add ition oflarge amounts of feed. Organicmatter in fish pond s (dead algaecells, fish fecal solids, uneatenfeed) does not contain the opti-

mum ratio of nutrients for bacteri-al grow th. There is more thanenough nitrogen for bacterialgrowth so the excess is released tothe pond water.

Add ing organic matter with a highconcentration of carbon relative tonitrogen prom otes the fixation orimmobilization of the am moniadissolved in w ater. Incorporatingammonia into bacterial cells pack-ages the nitrogen into a p articulateform that is not toxic to fish. Thedow n side of this app roach is that

it is hard to ap ply large amounts oforganic matter to large ponds andthe effect on ammonia concentra-tion is not rapid . Furthermore, aer-ation will have to be increased toaddress the demand for oxygen bylarge quantities of decomposingorganic matter.

Add ion exchange materials

Certain naturally occurring mate-rials, called zeolites, can adsorbammonia from water. These are

practical to use in aquaria or othersmall-scale, intensive fish-holdingsystems, but impractical for large-volume fish pond s.

Some shrimp farmers in SoutheastAsia have tried m aking monthly

applications of zeolite at 200 to400 poun ds p er acre. How ever,research h as dem onstrated thatthis practice is ineffective atreducing am monia concentrationin pond s and it has now beenabandoned.

Add acid

In theory, adding acid (such ashydrochloric acid) to water willreduce pH. This can shift theamm onia equilibrium to favor thenon-toxic form. H owever, a largeamount of acid is necessary toreduce the pH in well-bufferedponds and it would have to bemixed rapidly throughout thepond to prevent hot spots thatcould kill fish. Fur therm ore,adding acid would destroy muchof the buffering capacity (alkalini-

ty) of the pon d before any changein pH could occur. Once theammonia concentration is low-ered, treated p onds m ight requireliming to restore the bufferingcapacity. Working with strongminera l acids is a safety hazardfor farm workers and for fish.

How often shouldammonia be measured?

From the foregoing discussion,you might assume that measuring

amm onia in pond s is unnecessary.After all, research ha s ind icatedthat brief daily exposure toammonia concentrations far high-er than those measured in com-mercial pond s does not affect fishgrowth. And , on the rare occa-sions when ammonia doesbecome a problem, there is noth-ing you can d o about it. However,there are som e special circum -stances wh en it is worthw hile tomonitor am monia levels.

In the South, am monia concentra-

tions in m ost pond s usually startincreasing in September and peakabou t mid -October, abou t 5 to 6weeks after the last stretch of highfeeding rates. Then, abou t 2 to 4

weeks later, nitrite concentrationspeak. This is a general pattern . Itdoes not app ly to all ponds, andammonia or nitrite problems canoccur with variable intensity atany tim e, especially betweenSeptember and March.

Thus, the magnitude of the amm o-nia elevation in the early fall canindicate the severity of the nitritespike that w ill follow. Salt can p ro-tect fish against nitrite toxicosis(see SRAC Publication No. 462). Ifenough salt is add ed to pond s toachieve chloride levels of 100 to 150mg/ L, there is no reason to mea-sure amm onia even as a predictorof high nitrite concentrations.

Amm onia should be measuredevery other day after the crash ofan algae bloom and weekly in thecooler months of the year to iden-

tify pond s that may have a p oten-tial problem with nitrite. Otherthan those times, it is probably notnecessary to measure am monia infish p onds.

To summarize, fish producersshould n ot be alarmed if ammoniaconcentration becomes elevated,although a high am monia leveloften indicates that n itrite concen-trations may soon rise. In thiscase, farmers should focus on pro-tecting fish from n itrite poisoningby add ing salt, rather than on try-

ing to manage the ammon ia prob-lem. Extra vigilance after an algaecrash is also probably warran ted.Usually, the concentration ofammonia will fall again once thebloom becomes re-established .

Because there is little that can bedon e to correct problems w ithamm onia once they occur, the keyto ammonia management is to usefish culture practices that m inimizethe likelihood of such p roblems.This means stocking fish at a rea-

sonable den sity, harvesting asoften as practical to keep th estand ing crop from being too large,and using good feeding practicesthat maximize the proportion ofthe feed consumed by fish.


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Additional reading

Boyd, C. E. and C. S. Tucker. 1998.Pond Aquaculture Water Q ualityManagement. Kluwer AcademicPublishers, Norwell, Massachus-etts.

Cole, B. A. an d C. E. Boyd. 1986.Feeding rate, water qu ality, and

channel catfish production inponds. Progressive Fish-Culturist81:25-29.

Emerson, K., R. C. Russo, R. E.Lund and R.V. Thur ston. 1975.Aqueous am monia equilibriumcalculations: effect of pH and tem-perature.Journal of the FisheriesResearch Board of Canada 32:2379-2383.

Gross, A., C. E. Boyd and C. W.Wood. 2000. Nitrogen transform a-tions and balance in channel cat-fish pon ds.AquaculturalEngineering 24:1-14.

Ha rgreaves, J. A. 1997. A simula-tion model of ammonia dyn amicsin commercial catfish ponds in thesoutheastern United States. Aquacultural Engineering16:27-43.

Hargreaves, J. A. 1998. Nitrogenbiogeochemistry of aquacultureponds.Aquaculture 166:181-212.

Tucker, C. S. 1996. The ecology ofchannel catfish culture ponds innorthwest Mississippi.Reviews inFisheries Science 4:1-55.

Tucker, C. S. and M. van derPloeg. 1993. Seasonal changes inwater quality in commercial chan-nel catfish pon ds in Mississippi.Journal of the World AquacultureSociety 24:473-481.

Tucker, L., C. E. Boyd and E. W.McCoy. 1979. Effects of feedingrate on water quality, productionof chan nel catfish, and economicreturns. Transactions of the A meri-can Fisheries Society 108:389-396.

U.S. Environm ental ProtectionAgency. 1999. 1999 update ofambient water quality criteria forammonia. EPA-822-R-99-014.U.S. Environm ental ProtectionAgency, Office of Water,Washington , D.C.


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The work reported in this publication was supported in part by the Southern Regional Aquaculture Centerthrough Grant No. 2002-38500-11085 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.

Paper1_Managing_Ammonia_in_Fish_Ponds_2004

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