TROUT FARM EFFLUENTS CHARACTERIZATION AND.pdf

8/10/2019 TROUT FARM EFFLUENTS CHARACTERIZATION AND.pdf

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Envi ronmen ta l Po l lu t ion Vol. 95, No. 3, pp. 379-387, 1997 1997 Elsevier Science Ltd

All r ights reserved. Printed in Grea t Bri tain0269-7491/97 17.00+0.00

E L S E V I E R

T R O U T F A R M E F F L U E N T S : C H A R A C T E R I Z AT I O N A N DI M PA C T O N T H E R E C E I V I N G S T R E A M S

R u i B o a v e n t u r a , a* A n a M . P e d r o , b J o ~ o C o i m b r a b & E d u a r d o L e n c a s t r e c

aDepartamento de Engenharia Qulrnica, Faculdade de Engenharia, 4099, Porto Codex, Portugalblnstituto de Ci~ncias Biomddicas Abel Salazar, 4000, Porto, Portugal

cCentro Aquicola do Rio Ave, 4480 Vila do Conde, Portugal

(Received 9 January 1996; accepted 17 September 1996)

AbstractEffluents from three rainbow trout ( O n c o r h y n c u smykiss ) farms located in Northern Portugal were char-acterized and their impact on the receiving streams wasevaluated. Mean fish productions in the studied ish farmswere 15, 55 and 500 t of trout per year, respectively. Thefeeding water was abstracted from Fornelo, Inha andCoura Rivers, at flow rates ranging from 1.2 (15 t year -1fish fa rm ) to 4.8 litre s -1 per ton annual fish production(500 t year -1 fish farm ).

As the water flows through the farms, net variations inthe chemical characteristics were observed. a meanreduction in the dissolved oxygen (DO) concentrationbetween 0.7 and 2.4 mg litre-l; mean increases between1.9 and 3.2 mg CaC03 litre -I for total alkalinity,

between 0.9 and 14 mg litre -1 for BODs, between 0.27and 1.46 mg litre -1 fo r ammonia nitrogen (N H4 -N ),between 0.060 and 0.579 mg litre -1 for soluble phos-phorus (PO4-P) and less than 16 mg litre -~ fo r suspen-ded solids; variations in the pH value and nitrate nitrogenconcentration were not statis tically significant (p< 0.05). At the 500 t yea r- l fish farm it was alsopossibleto detect net increases of total hardness (3.2 mgCaCOs lit re-1), electric conductivity (19 mS cm -1) andpermanganate value (3.6 mg02 litre-1) . At the otherfarms net variations in these parameters were notsignificant.

Net mass flow variations reported to the annual fishproduction are presented. The DO mass fl ow decreased,on average, between 255 and 549 g t -1 of fi sh per day.The mean daily BODs increase ranged from 353 to1510 g t -1 off ish . The corresponding ranges fo r the otherparameters were 105-157 g t -1 fo r NH4-N, 24-62 g t -1for PO4-P, 348-1035 g CaCOs t -1 fo r total alkalinityand 224 x 106-506 x 106 t-1 for mesophilic bacteria. Dailynet variations of suspended solids, total hardness, electricconductivity and permanganate value were below1753 g t -t , 342 g CaCOz t -1, 2081 mS c m t - 1 and392 gO2 t -1, respectively.

Longitudinal concentration profiles fo r the most rele-

vant parameters show the impact of the effluent dischar-ges on the physico-chemical and bacteriological river

*To whom correspondence should be addressed.

379

water quality downstream from the trout farms. Analyz-ing the situations from a purely chemical point of view,the pol luted stretches were 3, 5 and 12 km long down-stream from the effluent discharges, respectively. Themicrobiological contamination extended over longer dis-tances. 1997 Elsevier Science Lt d

Keywords: Fish farm, t rout farm, eff luent , water pol lu-t ion, environmental impact , pol lut ion load.

I N T R O D U C T I O N

The q uan t i f ica t ion o f env ironmenta l impac t s o f in ten-s ive aquacu l tu re p rov ide in fo rmat ion fo r dec i s ion-

makers in l i cens ing new f i sh fa rms . The expans ion o faquacu l tu re in a f ive r bas in mus t be l imi ted by thenecess ity o f min imiz ing env i ronmenta l degrada t ion .In tens ive aquacu l tu re a ffec t s wa te r qua l i ty in manyways , inc lud ing (G owenet al., 1990) hy pernutr i f icat ion,ben th ic enr ichment , o rgan ic mat te r inc rease and bac-ter ia l changes. Th e effects are som etimes qui te severeand inc lude (Wa r re r-Hansen , 1982) an inc rease in BO Ds,a dec rease in DO concen t ra t ion , a N and P enr ichmentlead ing to an acce le ra t ion o f a lgae and p lan t s g rowth ,s o m e c h a ng e s o f th e b o t t o m f a u n a a n d o f th e a m o u n tand com pos i t ion o f sed iments , the p resence o f chemica l sand d rugs used aga ins t pa ras i t e s and pa thogen ic bac -ter ia and, e ventual ly, the produ ct ion of foul smel l.

A mon i to r ing p rogram me was deve loped in 1989 inDenmark to quan t i fy and reduce N and P load ings tot h e a q u a t i c e n v i ro n m e n t ( K r o n v a n get al., 1993; Iversen,1995) . Discharges f rom f ish farms have been ident i f iedas a major po l lu t ion sou rce to in land w a te r s d ra in ing tothe No rth Sea. Ni t rogen ha s been considered as the l imit-ing nutr ient except in lakes , where P l imits pr imary pro-duct ion. La nd use affects the P concentrat ion o f the surfacewaters which increases on agr icul tural areas . Neverthe-less , d ischarges f rom sew age t reatm ent plan ts andfishfa rm e ffluent s p roduc e m uch h igher concen t ra t ions .

To ta l N and P load ings measured over a 12-monthper iod f rom a Nor the rn I re land t rou t f a rm were repor-ted as 124.2 kg and 25.6 kg, respect ively (Foy & Rosel l ,1991a) . Day- to -day va r ia t ions were obse rved and a


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380 R. Boaventuraet al.

relationship between temperature and N and P lossrates was investigated. No evidence of such relationshipwas detected for P and only a weak one was apparentfor N.

The distribution of the total N and P contentsbetween soluble and particulate fractions was alsostudied by the same authors (Foy & Rosell, 1991b).

Soluble P has been then separated in the reactive andunreactive species and soluble N in the ammoniacal,nitrite+nitrate and organic fractions. Particulate Prepresents about 30% o f the total P, whereas particulateN only represents about 8.9%. Loss rates weredetermined for all species and it was concluded thattemperature had no effect on particulate P and N lossrates.

The impact of an intensive trout culture on a riverdepends on the size of the farm, the fanning practices,the nature and volume of the wastes produced, thedilution and self-purification capacities of the waterbody and the physico-chemical and bacteriologicalcharacteristics of the river water upstream from thedischarge.

Impacts of a particular farm may be classified asinternal, local or regional (Silvert, 1992). Inter nalimpacts are those of a fish farm on itself and theimmediate environment. Local impacts extend generallyto a kilometer downstream from the discharge and canaffect nearby farms. The effects on the entire waterbody, with space scales of many kilometers, are con-sidered as regional impacts.

This study characterizes intensive rainbow troutOncorhyncus mykiss)farming effluents, both from the

point of view of their physico-chemical and bacterio-logical composition and flow rate, and investigates themagnitude and extent of river water quality changesassociated to the metabolites, faeces and non-ingested

food derived from particular trout farms. Three farmsof different capacity and located in Northern Portugalwere selected for that purpose (Fig. 1). Different smallrivers supply the feeding water to the farms and receivetheir effluents.

MATERIALS AND METHODS

Descr ip t ion o f fa rmsThe smallest farm (A) is located at Bustelo, Amarante,on the right bank o f the Fornelo River, a small tributaryof the Ovelha River. Part of the river water is divertedthrough f arm, a situation which is common to the threestudy farms. Fornelo River is a mountain stream run-ning in a forested narrow valley with an average slope ofabout 2.5% at the study reach. There are no domestic,agricultural or industrial pollution sources other t han thetrout farm discharge. Fornelo River has a dry periodflow of 0.1-0.2 m 3 s -l and farm A has a productionyield of about 15 t year -1 and a dry feed consumptionbetween 50 and 100 kg day -1. The feed protein contentwas between 38% (in Winter) and 42% (in Summer).F o r water quality monitoring purposes four samplingstations were considered along the river: station A1,immediately upstream from the trout farm discharge,and stations A2, A3 and A4, 200, 2200 and 4000 mbelow the farm, respectively.

The medium capacity farm (B) is located at Rebor-delo, with an annual trout production of 55 tons and afeed consumpt ion o f 400-500 kg day -1, with a proteincontent as in farm A. The Inha River, with a dry period

flow of 0.2-0.3 m 3 s -l , constitutes the water source andthe effluent receiving stream. Like the Fornelo River,this tributary of the Douro River also crosses a forestedarea and runs through a narrow, steep valley (about 2%

USl

V I NC S T [

VER B S I N

kin

Fig. 1. Location of the study trout farms (A, B and C) in Northern Portugal.


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rou t f a r m e ffluen ts 381

slope at the study reach). Water flows with high velocityand there are no anthropogenic pollution sourcesdownstream and upstream from the trout farm. Waterquality was determined at five sampling stations alongthe river: station B1, above the farm effluent discharge,and stations B2, B3, B4 and B5, 200, 700, 3000 and4000 m downstre am from the discharge, respectively.

The largest farm (C) is on the left bank of the CouraRiver, at Paredes de Coura, and produces about 500tons of rainbow trout annually with a maximum feedconsumptio n between 3500 and 4000 kg day -l, alsowith the above mentioned protein content, since allfarms were supplied by the same company. The CouraRiver has a d ry period flow of 1 m 3 s -l , about five foldhigher than those of the Fornelo and Inha Rivers. Mostof the drainage area downstream from the trout farm isagricultural land and the river flows through a wide,slightly sloping (about 1.3% at the study reach) valley.There are no industrial effluents or domestic sewagedirectly discharged into the stream but feedlot wastesdownstream the farm may contaminate the water. Sixsampling stations were located along the river: stationC1, just upstream from the farm effluent discharge, andstations C2, C3, C4, C5 and C6, 250, 1000, 6250, 8000and 12 000 m below the discharge, respectively.

The physical, chemical and microbiological charac-terizations of the feeding water and trou t farm effluentswere carried out between April 1988 and February1989, through seven campaigns for farms A and B andeight campaigns for Farm C. Generally, samples weretaken between 10 and 12 a.m. The impact on the waterquality of the receiving streams was evaluated between

August 1988 and February 1989, after an initial effluentcharacterization period to estimate their relativeimportance. During this period, the flow rates rangedfrom 0.1 to 0.4 m 3 s -1 in the Fornelo River, 0.2 to0.4 m 3 s -l in the Inh a River an d 1.0 to 2.0 m 3 s -l in theCoura River. Minimum flow rates occurred betweenAugust to October.

River f low ratesRiver flow rates were calculated from the cross-sectionalarea and longitudinal velocity data at one or two sam-piing sites in each campaign. The cross-sectional areawas first determined by depth measurements and thendivided into vertical sections where river vel ocity wasmeasured using a current meter. Total flow was com-puted summing the flow increments for all the verticalsections. Along the river stretches where the aquacultureimpacts were evaluated there are no significant tribu-taries or water withdrawals, so the calculated flow ratesmay be assumed as constant at all the sampling sites.

Analytical methodsMeasured parameters in feed waters and effluentsincluded temperature, total suspended solids, turbidity,pH, total alkalinity, total hardness, electric conduc-

tivity, DO, BOD5, permanganate value, ammonia (asNH4-N), nitrates (as NO3-N), phosphates (as POa-P)and mesophilic aerobic bacteria. Temperature, conduc-

tivity, pH and D O were measured in situusing a Horib aU-7 water quality checker. The other analyses wereperformed according to Standard Methods (1985),taking two replicates per sample. The water and theeffluents were analyzed during the same period and themeans and 95% confidence limits of the results werecalculated.

The mean variations in the chemical and bacterio-logical river water composition induced by fish farmswere evaluated by analyzing water samples takenupstr eam (reference levels) and downs tream fr om theeffluent discharges. The selected sampling stations ineach river and the corresponding distances were thosereported before. Characterization included determi-nation of electric conductivity, turbidity, BODs, DO,NH4-N, soluble P and mesophilic bacteria. The adop-ted analytical methods were the same as for the inletwater and effluents.

RESULTS AND DISCUSSION

Feeding water and effluentsThe mean water flow rates through the farms andthe associated 95% confidence limits, during theperiod of the field observations, are 722 2 litre s -1,25031 litre s -1 and 624 33 litre s -1 at f arms A, Band C, respectively. Taking into account the annualproductions, the specific mean water flow rate is mini-mum at farm C (1.2 litre s -l per to n ann ual fish pro-duction) and maximum at farm A (4.8 litre s -1 per tonannual fish production). The corresponding value forfarm B is 4.5 litre s -1 per ton annual producti on. Thesevalues are small when compared with about5 litre s-i t -1 in Denmark, 10 litre s -i t -1 in Finland and35 litre s- it -1 in Ita ly (Alabaster, 1982). Also in aNorthern Ireland trout farm (Foy & Rosell, 1991a) theaverage water consumption over a 12-month period was6.8 litre s -1 per ton annual production. For fish farmssupplied with river water it has been proposed (Ala-baster, 1982) tha t a dir ect relationship exists betweenthe maximum annual production and the dry weatherflow. Whereas for UK and Denmark average produc-tions are 40 and 200 ton per m 3 s-I dry weather flow,

respectively, in farm C the pr oduc tion is abou t 500 tonper m 3 s -1 dry weather flow (Coura River).The physico-chemical and bacteriological compo-

sition of the river water, determined at each farm inlet,is presented in Table 1.

Effluent flow rates at each farm outlet are assumed tobe the same as those o f the feeding water. Table 2 showsthe means and the 95% confidence limits of the analy-tical results. The relationship between the inlet andoutlet data sets for the different parameters was investi-gated. Then the correlation coefficient was calculated todetermine whether the two data sets from all samplesare related, that is, whether larger values of one set are

associated with large or small values of the other.A paired two-sample comparison-of-means t-test was

used (Table 3) for comparing the means o f the input and


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382 R B o a v e n t u r a e t a l .

o u t p u t d a t a s e ts . A t t h e 9 5 % s i g n if i c a nc e l ev e l , t h e n u l lh y p o t h e s i s ( e q u a l m e a n s ) i s a c c e p t e d f o r t h e f o l l o w i n gp a r a m e t e r s : p H , t o t a l h a r d n e s s ( f a r m A ) , t u r b id i t y a n dt o t a l s u s p e n d e d s o l id s ( f a r m A ) , t e m p e r a t u r e a n d e l e c -t r ic c o n d u c t i v it y ( f a r m s A a n d B ), p e r m a n g a n a t e v a l u e( f a r m s A a n d B ) , n i t r a t e s a n d m e s o p h i l i c b a c t e r i a . I nf a r m C t h e m e a n v a l u e s f o r a l l p a r a m e t e r s b u t p H a n d

n i t r a t e s a r e s t a t is t i c a l ly d i f f e r e n t f r o m t h o s e o f t h e f e e d -i n g w a t e r. Ta b l e 4 s h o w s t h e m e a n c a l c u l a t e d v a r i a t i o n sb e t w e e n t h e i n l e t a n d o u t l e t o f e a c h f a r m ( c o n c e n t r a -t i o n s a n d m a s s f l o w s ) f o r t h o s e p a r a m e t e r s p r e s e n t i n gs t a t i s t i c a l l y s i g n i f i c a n t c o n c e n t r a t i o n c h a n g e s . M a s sf l o w v a r i a t i o n s i n c o n s t i t u e n t s o f w a t e r , r e p o r t e d t o t h ea n n u a l p r o d u c t i o n o f f is h i n e a c h f a r m , a r e p r e s e n t e d i nTa b l e 5 .

T h e m e s o p h i l i c b a c t e r i a d e n s i t y i n t h e e f f l u e n t s f l u c -t u a t e s g r e a t l y t h r o u g h o u t t h e y e a r , w h i c h is i n d i c a t e d b y

l a rg e c o n f i d e n c e l i m i t s w h e n c o m p a r e d w i t h t h e m e a n s .T h e h i g h e s t c o n c e n t r a t io n s a r e o b s e r v e d a t f a r m C , a n df a r m s A a n d B p r e s e n t s m a l le r v a l ue s o f th e s a m e o r d e ro f m a g n i t u d e ( T a b l e 2 ) .

E x c e p t f o r t o t a l h a r d n e s s a n d e l e c t r i c c o n d u c t i v i t yw h i c h a r e m u c h h i g h e r a t f a r m B , t h e c o m p o s i t i o n o ft h e f e e d i n g w a t e r i s s i m i l a r a t t h e t h r e e f i s h f a r m s .

A l t h o u g h s o m e s e a s o n a l v a r i a t i o n s w e r e d e t e c t e d , t h e9 5 % c o n f i d e n c e i n t e r v a l s u s u a l l y k e e p s m a l l v a l u e s f o rm o s t p a r a m e t e r s ( Ta b l e 1 ).

Ta k i n g i n t o a c c o u n t t h e d a t a f r o m t h e t h r e e f i s hf a r m s , n o s i g n i f i c a n t c o r r e l a t i o n s w e r e s y s t e m a t i c a l l yo b s e r v e d b e t w e e n t h e i n l e t a n d o u t l e t d a t a s e t s . I ns p i t e o f s o m e h i g h c o r r e l a t i o n c o e f f i ci e n ts s p o r a d i -c a l l y o b t a i n e d i n a p a r t i c u l a r f i sh fa r m , d a t a s e t s a r eo n l y fa i r ly c o r re l a t e d ( r < 0 . 8 ) o r u n c o r r e l a t e d w h e nc o n s i d e r i n g a l l f i s h f a r m s . T h e p h y s i c o - c h e m i c a l a n d

Table 1. River water composition (mean and 95% confidence limits) at each farm inlet

Pa rame te r Fa rm A F a rm B Fa rm C

n a M ean con - n a Mean con - n afidence limits fidence limits

Mean c on -fidence limits

Tempera ture , Cp H bTot al alkalini ty, mg CaC O3 l i tre - lTota l hardness , m g CaCO3 l i t re -1Turb id it y, N TUTota l suspended so lids , mg l i t re - iCond uctivi ty (20C), m S c m -1Dissolved oxygen, mg l it re - lBODs, mg l it re - IPerm angana te va lue , mg 02 l it re - INH4-N, mg l i t r e -NOa-N , mg l i t r e -NO2-N , mg l i tr e - IPO4-P, mg l i t re -Mesophi l ic bac ter ia , co lonies m l - I

7 13 .22.5 7 12 .12.2 87 6 .50 .5 7 6 .10 . 5 87 5 .7~1.2 6 5 .11 .0 77 8 .0~1.3 5 18 .32.1 77 1 .30.1 7 1 .70.5 86 1 .3~1.4 5 0 .6 0.2 86 4 3 1 3 6 7 6 1 5 76 10.80.8 6 11.11.1 87 1 .1~0.3 6 1 .30.2 76 0 .9 0.2 6 0 .70.1 67 0 .040.02 6 0 .060.04 85 1 .10.3 5 2 .2 0.4 77


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T r o u t f a r m e f f ? u e n ts 383

bacter iological composi t ion of the eff luents is mainlydetermined by farming pract ices . The net product ion ofNO3-N close to zero that was observed in the threefarms has been al ready reported in other s tudies(Bergheim e t a l . 1984; Solb6, 1982). The DO con-centra t i on decreased f rom the inle t to the out le t , but

the remainin g parameters present higher concen tra t ion sat the farm outlets. Addi tion ally , the recorded differ-ences at farm C were always higher than those at farmsA and B. Comparing the performance a t these las t twofarms a discrepancy seems to occur conc erning DO,NH4- N and POa-P concen t ra t ions . H igher va r i a t ions

Table 3 . Com par ison of the in le t and out le t parameter means us ing a pa ired two-sample t - tes t (5% s igni f icance level )

Parameter Farm A Farm B Farm C

Ia tcb t a tcb la tc b

Tem perat ure , C 1.656 1.943 1.329 1.943 2.341pH 0.980 1.943 1.549 1.943 0.719Tot al alkal init y, mg CaCO3 litre -1 5.857 1.943 3.850 2.015 4.482Tot al hardn ess, mg CaCO3 lit re- I 0.111 1.943 2.163 2.132 5.852Tur bidi ty , NT U 1.882 1.943 5.716 1.943 9.410Tot al suspe nded solids, mg litre-~ 0.696 2.015 2.295 2.132 6.099Con duc tiv ity (20C), mS cm -1 1.704 2.015 1.684 2.015 13.684Dissolve d oxyg en, mg litre 1 5.121 2.015 3.722 2.015 8.761BODs, mg litre -1 4.054 1.943 3.243 2.015 3.456

Perm ang ana te value, mg 02 litre -1 1.784 2.015 1.464 2.015 2.296NH4- N, mg litre -1 5.020 1.943 5.153 2.015 11.007NOa -N, mg lit re- 1 1.633 2.132 1.089 2.132 0.132PO4-P, mg litre -~ 5.357 2.015 3.961 2.132 9.620Mesophil ic bacteri a, colonie s ml i 1.977 2.015 1.980 2.015 4.168

1.8951.8951.9431.9431.8951.8951.9431.8951.943

2.0151.8951.9431.8952.353

a Calculated t.b Critical one-tail t.

Table 4 . Net var ia t ion (mean 95% conf idence l imi ts ) in cons t i tuents of water pass ing through t rout - farms


Tota l alkali nity, mg CaCO3 lit re- 1 1.9 + 0.6 2.6 + 1.3 3.2 + 1.4kg CaCO3 day -~ 11.7 + 3.9 56.9 + 29.0 174 76

Total hardness, mg CaCO3 litre -~ -- -- 3.2 + 1.1kg CaCO3 day -~ - - - - 171 + 57

Suspe nded solids, mg litre -1 - - 1.8 + 1.5 16 + 5kg day -1 - - 38.9-4- 33.2 877 282

Conduct ivity (20C), mS cm -l -- -- 19 4-3S cm-ld ay -I -- -- 1040+ 149

Dissolved oxygen, mg iitre -l a 1.3 + 0.5 0.7 + 0.3 2.4 + 0.5kg day -~a 8.2 +3.2 14.1 +7. 4 128+29

BOD 5, mg li tr e- ~ 0.9 + 0.4 1.8 + 1.1 14 -4- 8kg day -~ 5.3 + 2.6 37.84- 22.9 755 -4- 428

Permanganate value, mg 0 2 lit re- i __ - - 3.6 3.1kg 02 day J -- -- 196+ 167

NH 4- N, mg litre-~ 0.38 + 0.15 0.27 + 0.10 1.46 0.26kg day -~ 2.36 0.9 2 5.762 .19 78.5 14.0

PO4-P, mg li tr e- 1 0.086 0.032 0.060 0.030 0.579 O. 118g day-J 533.6 195.2 1297.0 641.8 31227 + 6362

a _ decrease.

Table 5 . Dai ly ne t m ass f low var iat ions in cons t i tuents of water per ton o f fi sh (annual product ion)


Total alkalinity, g CaCO3 t -~ day -1Total hardness, g CaCO 3 t -~ day -~Total suspended solids, g t -1 day -1Condu ctivit y (20C), mS cm -1 t -~ day -1Dissolved oxygen, g t-1 day-iBODs, g t -1 d ay-1Permanganate value, g 02 t-~ d ay-

NH4-N, g t -1 day -lPO4-P, g t - i day - 1Mesophi lic bacte ria, 106 coloni es t -1 day -1

783 1035 348-- -- 342- - 707 1753- - -- 2081549 255 256353 688 1510-- -- 392

157 105 15736 24 62

224 300 506


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384 R Boaventura et al.

a re obse rved a t f a rm A, where the annua l f i sh p ro -duc t ion i s lower. Never the less , va r i a tions o f DO andN H 4 - N m a s sf lows a re , a s wou ld be expec ted , h igher a tfarm B.

Total a lkal ini ty increased from the inle t to the out le to f each fa rm and va r ia t ions range be tween 1 .9 +0 .6( fa rm A) and 3 .2+ -1 .4m g CaCO3 l it re -1 ( fa rm C) .

These va lues cor respond to mean da i ly p roduc t ions o f783 and 348 g CaC O3 t -1 o f annu al prod uct io n of fish ,respect ively. Farm B dai ly product ion is three t imeshigher: 1035 gCaCO3 t -1 off is h .

Net increase of to ta l hardness is ins ignif icant a t farmA. At fa rms B and C , however, ha rdness inc reased , ona v er ag e , 1 . 7 + 1 .5 a n d 3 . 2 a : l . l m g C a C O s l i tr e - 1,which equa tes to a ne t da i ly p roduc t ion o f 342 and559 g CaC O3 t - l , respect ively. Similarly, the e lectr icconduct ivi ty increase is only s ignif icant in farm C. Therepo rted me an increme nt is 19+-3 mS cm -~, c orre-spond ing to a dai ly increase of 2081 mS cm -l t -~ of fish .

The DO concen t ra t ion dec rease be tween thewa ter in le t and out le t avera ged 2.4+-0.5 mg l i tre -1 a tfa rm C, bu t the reduc t ion was lower a t f a rms B(0 .7+ 0 .3 mg l it re - l ) and A (1 .3+ 0 .5 mg l i t re - l ) .Even a t f a rm C, however, the e ff luen t DO concen t ra t ionmaintained a high value, varying from 7.5 to8.5 mg l i tre -~. Co rresp ond ing resul ts are repo rted e lse-where (Alabas te r, 1982 ; Berghe im & Se lmer-Olsen ,1978; Solbr, 1982) . The DO dai ly mass f low decreasereported to the annual f ish product ion is qui te s imilar a tfa rms B and C bu t abou t twice as h igh a t f a rm A. Theanomaly i s p robab ly re la ted to d i ffe rences in the t ankconf igura t ion and a lower reae ra t ion ra te when wa te rf lows th rough success ive t anks a t f a rm A.

Wate r f rom the Inha and Forne lo r ive r s had BOD5rang ing be tween 0 .4 and 1 .6 mg li t r e - l ; in the C ouraRiver i t was som ewha t h igher, no t exceed ing , however,the va lue o f 2 .7 mg l i t r e - t The h ighes t BOD5 va lueswere found a t f a rm C e ff luen t (be tween 6 .8 and39.4 mg l i t re- l ) , wi th an increase of 14 +8 mg l it re -~be tween wa te r in le t and ou t l e t . At f a rms A and Bthe inc reases were , on average , 0 .9+0 .4 and1 .8+1 .1 mg li tr e - l , r e spect ive ly. The da i ly o u tpu t o fo rgan ic mat te r dur ing the obse rva t ion pe r iod , expressedin t e rms o f BOD s, was ab ou t 353 g t -~ annua l p roduc-

t ion o f fi sh a t f a rm A , 6 8 8 g t -1 a t f a rm B and1510 g t -1 a t fa rm C.T h e m a x i m u m i n c r e a s e i n t h e N H 4 - N c o n c e n t r a t i o n

was obse rved a t f a rm C (1 .46+0 .26 mg l i t r e -~) , co r re -spond ing to a da i ly inc rement o f 157 g t -1 annua l t rou tp roduc t ion . At f a rms A and B da i ly inc rements were157 an d 105 g t -1 annu al prod uct ion , respect ively.These va lues a re re la t ive ly low com pared wi th the usua lrepo rted ran ge of 100 -1500 g t -1 (Alaba ster, 1982) .

The h ighes t inc rease o f PO4-P was a l so a t f a rm C(0 .579+0 .118 mg l i tr e -1 ) and the lower one a t f a rm B(0.060 + 0.030 mg l i tre-1) . The calculated dai ly net ma ssf low fo r PO 4-P var ied f rom 24 ( fa rm B) to 62 g t -1

annua l p roduc t ion ( fa rm C) . These va lues a re nea r thelower l imi t o f the range (30-300 g t - ld ay -1 ) u sua l lyfound in t rout farm eff luents (Alabaster, 1982) . The

annua l o u tpu t s (8 .8 -22 .6 kg t - ly ea r - l ) a re compa rab leto the mean P re lease (16 kg t - lyea r -1 ) f rom U K fa rmsproduc ing main ly ra inbow t rou t (So lb r, 1982) . Annua lload ing ra te f rom a Nor the rn I re land fa rm (Foy &Rosel l , 1991a) was 25.6 kg tota l P pe r ton o f ra inb owt rou t p roduced . Assuming tha t so lub le reac t ive P,accord ing to the same au thors , i s 60 .0% of to ta l P, the

annu al ou tpu t is 15.4 kg t - l , a value c lose to thatob ta ined in UK fa rms .

To ta l suspended so l ids mean concen t ra t ions ran-g e d b e t w e e n 1 . 8 + l . 5 m g l i t r e - ~ a t f a r m B a n d16 +5 mg l i tr e -~ a t f a rm C. These l eve l s a re c ons ide r-ably low when compared with typical s i tuat ions in f ishfarms (Sumari , 1982) . The dai ly contr ibut ion to thesuspended so l ids p roduc t ion was abou t 707 and1753 g t -~ o f annua l p roduc t ion a t f a rms B an d C ,respect ively. As previously reported, the suspendedsol ids net di fference a t farm A was not s ignif icant . Theto ta l annua l amount p roduced a t f a rm B i s be low

the range (1 .3-11 kg t -1) m ent ion ed in the l i tera ture(Alabaster, 1982).In t e rms o f da ily mean inc rement , m esoph i li c bac te r i a

concen t ra t ion va r ied be tween 224106 ( fa rm A) and506 106 ( fa rm C) co lon ies pe r ton o f annua l p roduc t ionof fish. The cor respond ing va lue a t f a rm B was30 0 106 t -1.

I m p a c t o n r i v e r w a t e r qualityThe m ean va lues o f the phys ica l, chemica l and bac te r-iological parameters in the sampling s ta t ions upstreamand do wns t ream f rom t rou t f a rms a re shown in Tab le 6.The long i tud ina l concen t ra t ion p ro f i l e s (mean+95%confidence l imits) for BODs, DO,NH4-N, PO4-Pand ae rob ic mesoph i l ic bac te r i a a re p resen ted in F igs 2and 3. The electr ic conduct ivi ty increases , a t a s ta t is t i -cal ly s ignif icant level , only a t farm C. Also turbidi tyseems to inc rease on ly in s t a t ions C2 and C3 , bu t thehigh dispers ion o f the individual resul ts ma kes imp os-s ible a safe conclusion. So, the corresponding longi-tud ina l p ro f i l e s were d rawn on ly fo r the Coura River(Fig. 2(c) and Fig. 2(d)) . The f i rs t point corresponds tothe feeding water in all the figures.

Eff luent BOD 5 was 2 .1+ 0.6 , 3 .2+-1.1 and 15.7+10.0 mg l i tre -1 a t farms A, B and C, respect ively. After

d i lu t ion by r ive r wa te r, max imum BOD5 amountedt o 1 . 7 + 0 . 2 m g l i t r e - l i n th e F o r n e l o R i v e r, 2 . 5 +0.6 mg l i t re -1 in the Inha Rive r and 5.6+ 2.1 mg l i tre -1in the Coura River (Fig. 2(a)) . The running water se l f -pur i f icat ion capaci ty rapidly reduced the BOD level . Inthe Forne lo and Inha Rivers , BOD5 re tu rned to thefeeding w ater values (1 .1 +0 .3 and 1.3+-0.3 mg l i t re -1 ,respec t ive ly ), abo u t 2 and 3 km down s t ream f rom theeff luent discharges (0 .94-0.4 and 1.3+ 0.3 mg l i tre -~,respect ively) . In the Fornelo River, however, therewas a s l igh t inc rease be tween s t a t ions A3 and A4 , bu teven a t s t a t ion A4 the 95% conf idence range i s lowerthan a t s t a t ion A2 , where the maximum va lues were

recorded . A m ore m arked e ffec t was obse rve d inthe C oura River (maxim um of 5 .6+ 2 .1 m g l it re -~ a ts t a t ion C2) and BOD5 on ly recovers a t s t a t ion C6 ,


7/9

rout far m effluents 385

abo ut 12 km below the farm discharge (1 . 9+0. 6 mgli t re - ] , not s ta t iscal ly different f rom the feeding waterBODs, 1 .6 +0. 4 mg l i t re-] ) . This pa rameter exceededthe maximum recommended level (EEC, 1978) for

salmo nid waters (3 mg l i t re -~) in the 1-km reach down -

stream from the effluent discharge.Effluents from the fish farms were well oxygenated

(9 .6+0 .3 , 10 .5+0 .9 and 8 .5+0 .6 mgO2 li tr e -1 a t f a rms

T a b l e 6 . M e a n v a r i a ti o n s in f iv e r w a te r c o m p o s i t i o n

Station BOD5 DO mg litre -1 NH 4- N PO4-P Mes. bacteria Conductivity Turbiditymg litre -~ mg litre -~ mg litre -] colonies m1-1 /zS crn -1 NTU

Farm AA1 1.1 10.8 0.04 0.011 174 43 1.3A2 1.7 10.7 0.27 0.096 496 47 1.8A3 0.9 11.1 0.11 0.026 440 50 - -A4 1.0 10.3 0.04 0.025 356 60 1.3

Farm BBI 1.3 11.1 0.04 0.005 79 85 2.4B2 2.5 9.7 0.20 0.090 3069 87 2.7B3 2.2 10.7 0.16 0.099 1725 87 2.8B4 1.3 11.2 0.09 0.078 395 84 2.8B5 1.2 10.8 0.10 0.075 154 83 2.7

Farm CC1 1.6 10.7 0.06 0.005 191 40 2.6C2 5.6 9.6 0.89 0.468 13 360 52 4.7C3 5.6 10.0 0.95 0.459 52 064 52 4.7C4 2.5 10.8 0.32 0.217 16 517 49 2.5C5 2.0 11.3 0.18 0.192 2777 47 2.4C6 1.9 11.0 0.11 0.183 5509 45 2.1

a Indiv idual values, mean of 2 values and m ean o f 3 values for Farms A, B and C, respectively.

10~(a)

9

8 :

7

-4 5

o 4

3 1

---.o--- FarmA FarmB FarmC

-1000 1000 3000 5000 7000 9000 I1000 13000Distance (m)

6o

55

so

'~ 45

u 405

m 35

(c)

a FarmC

3 0 , , , I , . . i . . i , . , i . . - i . . . i , . .

-1000 1000 3000 5000 7000 9000 11000 13000Distance (m)

b )

i

9 - - -a . - - Far inA* FarmB

8 Fa rm

-1000 1000 3000 5 ~ 7 ~ 9000 11000Distance (m)

130OO

(d)

~ FannC

0 . . . t . . . i , . . i , . . 1 . . , i . , i , . .

-1000 1000 3000 5000 7000 9000 11000 13000Distance (m)

Fig. 2. BOD5 (a) and DO (b) mean variations (mean + 95% confidence limits) in the Forne lo (Farm A- 15 t yea r-l) , Inha (Fa rmB -- 55 t year -1) a nd Cou ra (Far m C - - 500 t year -I) Rivers, upstream and downstrea m from effluent discharges; electric

conducti vity (c) and tu rbidity (d) variations (mean 4- 95% confidence limits) in the Cou ra river.


8/9

386 R Boaventura et al.

A, B and C, respectively). Nevertheless, the organicmatter biodegradation caused slight decreases immedi-ately downs tream from the effluent discharges (Fig. 2(b)).The lowest concentrations were observed in the CouraRiver (station C2) but they decreased, on average, from10.74-0.8 mg litre -1 upstream from the discharge to9.6 4- 1.1 mg lit re-I immedia tely downst ream from the

discharge. No injurious effects to aquatic life would beanticipated at these concentrations.

The NH4 -N concentration in the effluents from farmsA, B and C was 0.464-0.17, 0.27+0 .08 and 1.624-0.30mg litre -1, respectively. In spite of dilu tion with uncon-tami nated river wa ter (0.044).06 mgN H4- N litre-1),

1.5 I

1.0%

Z

(a)+ FarmA

* Farm B

Farm C

0.5

0.0 ' ' ' ' ' ' ' ' ' . . . . . . .- 1 0 0 0 1 0 0 0 3 0 0 0 5 0 0 0 7 0 0 0 9 0 0 0 1 1 0 0 0 1 3 0 0 0

D i s t a n c e ( m )

0.8

0.7

0.6

~ 0.5t ~ 0 . 4

0.3

Co)

1 0 0 0 0 ~

0.21 i

0.1

0 .0 . . - . . - . . . . . .

1000 3000 5000 7000


n F a r m A

F a r m B

F a r m C

. . . . . . .

9 1 1 0 0 0 1 3 0 0 0

o o o o o

e.

o I 0 0 0 0o

o o o

= tO

10

c )

. . . . . . . . . . . .

J lO 1 0 0 0 3 0 0 0 5 0 0 0 7 0 0 0 9 0 0 0 I I 0 0 0 1 3 0 0 0

+ FarmA

, Farm B

Farm C


Fig. 3. NH4-N (a) , PO4-P (b) and mesophilic bacteria (c)variations (mean95% confidence limits) in the Fornelo(Farm A -- 15 t year-l), Inha (Farm B -- 55 t year -1) andCoura (Farm C -- 500 t year -1) Rivers, upstream and down-

stream from effluent discharges.

high NH4- N levels occurred downstream from the threetrout farms (Fig. 3(a)). Maximum concentrations of0.27+0 .09, 0.20+ 0.07 and 0.95+0.11 m g litre -1 wereobserved in the Fornelo, Inha and Coura Rivers,respectively. Even though the NH4-N con tent decreaseddownstream, moder ately high concentrations (twice thefeeding water concentration) were still present at the

end of the Inha and Coura River study reaches(0.10+0.04 and 0. 11+ 0. 05m gl it re -1, respectively).The maximum concentrations exceeded the recommen-ded limit for salmonid waters (0.04 mgNH4 litre -1) butthey remain below the maximum allowable level(1 mg litre -1) (EEC, 1978) even in the Cour a River.However, NH4 -N was already above the recommendedlevel upstream from farms B and C.

The longitudinal concentration profiles for POa-P(Fig. 3(b)) show a pattern similar to that observed forammoniacal nitrogen. The PO4-P content markedlyincreased downstream from the trout farms, which hasbeen reported by other authors (Carr & Goulder, 1990),with maximum values immediately downstream fromthe farms of 0.099 + 0.050 mg litre-1 in the For nelo andInha Rivers and 0.468 +0.172 m g litre -1 in the Cour aRiver. The POa-P content in the Fornelo River rapidlynearly reached the feeding water level but only a slightdecrease was observed in the Inha river and concentra-tions stayed above 0.075 mg litr e-l; in the Coura Riverconcentr ations almost decreased to 0.2 mg litre -l alonga 6 km reach but remained constant downstream. Con-centrations up to 1 mg litre-1 can be considered as notimpairing aquatic life (EEC, 1978).

A characteristic effect of trout farm effluents on river

water is an increase in the number of microorganisms,especially bacteria (Sumari, 1982). The effluent aerobicmesophilic bacteria densities during the study periodshowed great variation at each farm, making the 95%confidence levels very large. The cal culat ed values were662651, 8424-751 and 4723+ 1691 m1-1 at f arms A,B and C, respectively. Figure 3(c) illustrates changes inthe mesophilic aerobic bacteria abundance downstreamfrom the discharges. Maxi mum measured densities were4964-455 m1-1 in the Forne lo River ( station A2),30694-3493 ml -I in the Inha River (st ation B2) and52 064+88 171 m1-1 in the Coura River (station C3).Mean bacterial levels in the Coura River decreased to2780 bacteria m1-1 at station C5, even though the meanvalue (5500 bacteria m1-1) increased again at stationC6, 12 km downs tream from the farm. The mean valueat this station is the result of an abnormally high value(10 000 bacteria ml-1) obtained in one trip. Eliminatingthis value, the bacterial density decreased downstreamfrom station C3. Farms A and B induced a more limitedeffect and the mean bacterial densities returned to thefeeding water mean values 4 km downstream from thedischarge.

The effect of effluent discharges on water conductivityand turbidity was evaluated in all rivers but only detec-

ted in the Coura River (Fig. 2(c) and Fig. 2(d)). Theeffluent conducti vity at farm C was 58 4- 3/zS cm -1 andthe maximum registered in the river (station C2) was


9/9

Trout fa rm eff luents 387

5 2 + 2 / z S c m - l , s li g ht ly h ig h e r t h a n t h e f e e d i ng w a t e rc o n d u c t i v i t y (3 9 + 2 / x S c m - 1 ) . T h e c o r r e s p o n d i n g e ff lu -e n t t u r b i d i t y w a s 11 3 N T U a n d t h e h i g h e r v a l u e s i nthe r ive r occur red a t s t a t ion s C2 and C3 (4 .7 1 .6 and4 .7 2 .2 NT U, r e spec t ive ly ) .

T h e p o l l u t i o n l o a d p e r u n i t o f d r y w e a t h e r f l o w m a yb e a u s e f u l p a r a m e t e r f o r t h e i m p a c t a s s e s s m e n t o f d i s-

c h a rg e s o n t h e r e c e i v i n g s t r e a m s w h e n w a t e r t e m p e r a -tu re , specie s , f eed ing and f a rm ing p rac t i ces do no t va rym u c h a s i n N o r t h e r n P o r t u g a l . A s s u m i n g t h a t t h ep o l l u t i o n l o a d i n t h e c r i t i c a l d r y w e a t h e r c o n d i t i o n s i sp r o p o r t i o n a l t o t h e a n n u a l f i s h p r o d u c t i o n , t h e r a t ioP : D W F ( a n n u a l f is h p r o d u c t i o n , t : d r y w e a t h e r f l o w,m 3 s - I ) s h o u l d b e a n a p p r o x i m a t e p a r a m e t e r f o r c a l cu -l a t i n g t h e e f f e c t e d d o w n s t r e a m d i s t a n c e f r o m s i m i l a rt r o u t f a r m s , i n a g i v e n r e g io n . C o n s i d e r i n g o n l y t h eana ly t i ca l chemica l da t a , i t i s poss ib l e to e s t ima te ( f romFigs 2 and 3 ) the e ff ec ted d i s t ances o f abo u t 3 km fo rP : D W F = 7 5 ( f a r m A ) , o f a b o u t 5 k m f o rP : D W F = 183( f a r m B ) a n d o f a b o u t 1 2 k m f o rP :D W F = 500 ( farmC) . So , t he r e l a t ionsh ip be tween the l eng th o f po l lu t eds t r e t ch (Lp, k m ) a n d t h e r a t i oP : D W F i s g iven by:

Lp = 1.264 + 0.0214 (P :D W F ) r2 ~_ 1

C O N C L U S I O N S

I n t h e p r e s e n t s t u d y, w a t e r c o n s u m p t i o n i n t r o u t f a r m sw a s f o u n d t o b e b e l o w t y p i c al v a lu e s i n o t h e r c o u n t r i e s( m a x i m u m o f 4 . 8 l it r e s - 1 t - l a n n u a l t r o u t p r o d u c t i o n ,a g a i n s t a b o u t 5 l it r e s- ~ t - I i n D e n m a r k , 7 li tr e s - I t - l

i n N o r t h e r n I r e l a n d , 1 0 l it re s - i t - ~ i n F i n l a n d a n d35 li t re s - I t -1 in I t a ly ) . On the o th e r ha nd , an nua l f i shp r o d u c t i o n s p e r d r y w e a t h e r r i v e r f l o w s a r e h i g h e r.L o a d i n g s o f t o t a l a l k a l i n i t y, t o t a l h a r d n e s s , B O D s ,N H 4 - N , P O 4 - P, t o t a l s u s p e n d e d s o li d s a n d m e s o p h i l i cb a c t e r i a i n c r e a s e d i n g e n e r a l a s w a t e r p a s s e d t h r o u g ht h e f a r m s . D O c o n c e n t r a t i o n s h o w e d a s l i g h t d e c r e as e( m a x i m u m d e c r e as e o f 2 . 4 m g l it re - 1 a t f a r m C , b u ta lways kep t above 7 .5 mg li t r e -1 ) and n i t r a t e s wereu n c h a n g e d . N H 4 - N d a i l y i n c r e m e n t s w e re b e t w e e n 1 05a n d 1 57 g t - l a n n u a l p r o d u c t i o n , a t t h e l o w e n d o fm o s t f a r m s ( 1 0 0 -1 5 0 0 g t - l ) . P O 4 - P w a s b e t w e e n 2 4and 62 g t -1 , c lose to the lower l imi t o f t he typ ica l t angei n t r o u t f a r m s : 3 0 - 3 0 0 g t - l . N e v e r t h e le s s , t h e i m p a c t o fthese two cons t i tuen t s on the r ece iv ing s t r eams i s s t i l ld e t e c t a b le a t t h e e n d o f t h e s t u d y r e a c h e s o f f a r m s Ba n d C , a l t h o u g h t h e c o n c e n t r a t i o n s a r e b e l o w t h e m a x -i m u m r e c o m m e n d e d l e v e l f o r s a l m o n i d w a t e r s . O rg a n i cm a t t e r h a s a s l ig h t i m p a c t o n r i ve r s, e i t h e r i n t e r m s o fB O D 5 o r D O . M a x i m u m B O D 5 w a s o b s e r v e d i n t h eCo ura r ive r, t he r ece iv ing s t r eam o f the l a rges t f ish f a rme f fl u en t , b u t c o n c e n t r a t i o n o n l y i n c r e a se d b y a f a c t o r o f4 r e g a r d i n g t h e u p s t r e a m v a l u e . T h e a s s o c i a t e d D Odep le t ion doe sn ' t exceed 1 .1 mg l i tr e -1 . Bac te r i a l co n-t a m i n a t i o n d o w n s t r e a m f r o m t h e t r o u t f a r m s i s a n

i m p o r t a n t f e a t u r e b u t o n l y f o r th e l a rg e st t r o u t f a r m t h ei m p a c t e x t e n d s o v e r a l l t h e s t u d y r e a c h . C o n s i d e r i n go n l y t h e e f f ec t o n t h e c h e m i c a l c o m p o s i t i o n o f t h e

rece iv ing s t r eam, a l i nea r r e l a t ionsh ip be tween the r a t ioP : D W F ( a n n u a l f i s h p r o d u c t i o n , t : d r y w e a t h e r f l o w,m 3 s - l ) a n d t h e e x t e n s i o n o f t h e p o l l u t e d s t r e tc h w a so b t a i n e d .

R E F E R E N C E S

Alabaster, J. S. (1982). Survey on F ish -Fa rm Effluents in someEI FA C Countries. In:Report o f the EIF AC Workshop onFish-Farm Effluents,ed. J. S. Alabaster. Silkeborg, Den-mark , 26-28 May 1981, Europ ean In land Fisheries, Advi-sory Commission, Technical Paper No. 41, Food andAgriculture O rganization of the United Na tions, Ro me,pp. 5-15.

Bergheim, A., Hustveit, H., Kittelsen, A. & Selmer-Olsen, A.R. (1984). Estimating pollution loadings from Norwegianfish farms. II. Investigations 1980-1981.Aquaculture, 36,157-168.

Bergheim, A. & Selmer-Olsen, A. R. (1978). River pollutionfrom a large trout farm in Norway.Aquaculture, 14, 267-270.

Carr, O. J. & Goulder, R. (1990). Fish-farm effluents in rivers.II . Effects on inorganic nutrients, algae and the macro phyteRanunculus penicillatus. W ar. Res., 24,639-647.

Europ ean E conomic Co mm unity (1978). Council Directive of18 July on the quality of fresh w aters needing p rotection orimprovement in order to support fish life, 78/659/EEC,Brussels.

Foy, R. H. & Rosell , R. (1991). Loadings of nitrogen andphosp horus from a No rthern Ireland fish farm.Aquaculture,96, 17-30.

Foy, R . H. & R osell , R. (1991). Fraction ation of phosp horusand nitrogen loadings from a Northern Ireland fish farm.Aquaculture,96, 31-42.

Gowen, R. J. , Rosenthal, H., M/ikinen, T. & Ezzi, I . (1990).

Env ironmental impacts of aqu aculture activities. In:Aqua-culture Europe 89 - Business Joins Science,eds N. de Pauw& R. Billard. Europ ean A quaculture Society, Spec. Publ.No. 12, Bredene, Belgium, pp. 257-283.

Iversen, T. M. 0995). Fish farming in Denmark: environ-mental impact of regulative legislation.Wat. Sci. Tech.,31(10), 73-84.

Kronvang, B., /Ertebjerg, G., Grant, R., Kristensen, P. , Hov-mand, M. & Kirkegaard, J . (1993). Nationwide monitoringof nutrients and their ecological effects: state o f the Danishaquatic environment.Ambio,22, 176-187.

Silvert, W. (1992). Assessing environm ental impacts o n finfishaquaculture in m arine waters.Aquaculture,107, 67-79.

Solbr, J . F. de L. G. (1982). Fish-Farm Effluents: A UnitedKingdom Survey. In:Report of the EIFAC Workshop on

Fish-Farm E ffluents,ed. J. S. Alabaster. Silkeborg, Den ma rk,26-28 M ay 1981, Europ ean Inlan d Fisheries, AdvisoryCommission, Technical Paper No. 41, Food and Ag ricultureOrganization of the United N ations, R ome, pp. 29-55.

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Sumari, O. (1982). A Report on Fish-Farm Effluents in Fin-land. In: Report of the EIFAC Workshop on Fish-FarmEffluents,ed. J. S. Alabaster. Silkeborg, Denmark, 26-28M ay 1981, Europ ean Inlan d Fisheries, Advisory Com -mission, Technical Paper No. 41, Food and AgricultureOrganization of the United Nation s, Rom e, pp. 21-27.

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