Environmental Implications of Intensive Marine Aquaculture in Earthen Ponds

Environmental Implications of IntensiveMarine Aquaculture in Earthen PondsANTONIO TOVAR, CARLOS MORENO*, MANUEL P. M�ANUEL-VEZ and MANUEL GARC�IA-VARGASFaculty of Sea Sciences, Department of Analytical Chemistry, University of C�adiz, Puerto Real, 11510 C�adiz, Spain

In recent decades, the importance of marine aquaculturehas grown substantially in most countries. Following aperiod of uncontrolled activities, the concern for the en-vironmental implications of intensive mariculture has in-creased notably, and environmental impact is often takeninto account when aquaculture activities are established.Among the most important pollutant e�ects of aquacul-ture are the outputs of dissolved nutrients, suspendedsolids and organic matter. In the present study, we havedetermined the loadings of dissolved nutrients (ammoni-um, nitrite, nitrate and phosphate), total suspended solids(TSS) and organic matter (particulate organic matter(POM) and biochemical oxygen demand (BOD5)) in thee�uent of a marine ®sh farm devoted to the intensiveculture of gilthead seabream (Sparus aurata) in earthenponds. Samples of seawater were taken monthly during atwo-year period (April 1997±March 1999) in the two in-¯ows and the out¯ow of the ®sh farm. The environmentalimpact of marine aquaculture was established by esti-mating the total amount of each compound dischargedinto the receiving waters as a direct consequence of theculture activities. Thus, 9104.57 kg TSS, 843.20 kgPOM, 235.40 kg BOD, 36.41 kg N±NH�4 , 4.95 kg N±NOÿ2 , 6.73 kg N±NOÿ3 and 2.57 kg P±PO3ÿ

4 , dissolvedin the seawater, were estimated to be discharged to theenvironment for each tonne of ®sh cultured. Ó 2000Elsevier Science Ltd. All rights reserved.

Keywords: pollution; seawater; nutrients; suspendedsolids; organic matter; environmental impact; marineaquaculture; Sparus aurata.

Gilthead seabream (Sparus aurata) is widely consideredas an extremely suitable species to be used in aquacul-ture and is successfully cultured in many parts of theworld, due mainly to the high yields and the speciesÕconsiderable commercial value (Girin and Harache,1976). Spain is one of the most important producingcountries, the Bay of C�adiz (SW of Spain) being a focusfor the intensive mariculture of S. aurata in earthenponds. The total area devoted to aquaculture in 1994 in

the Bay of C�adiz was 2919 ha including extensive, semi-intensive and intensive farming systems (M�arquez et al.,1996). The farms largely consist of old salt-pans modi-®ed for ®sh farming purposes.

This development generates pro®t and income, but italso bears risks of negative environmental impact, suchas pollution, landscape modi®cation or biodiversitychange. In general, it has been accepted that mariculturehas a comparatively low impact on the environment.Nevertheless, wastewater discharged from intensivemariculture into coastal water may lead to deteriorationin water quality resulting from depletion of dissolvedoxygen, hypernutri®cation and discharge of organicmatter. The environmental impact of marine ®sh-farm-ing depends strongly on species, culture method,stocking density, food composition, feeding techniquesand hydrography of the site. To date, there have beenonly a limited number of studies on the environmentalimpacts of culture of gilthead seabream (Barnab�e, 1990;Barbato et al., 1996; Reggiani, 1996; Papoutsoglouet al., 1996).

Attention has previously been focused on nutrientloadings, which can boost primary production and leadto eutrophication as, for example, shrimp farmingpractices in the inner Gulf of Thailand (Suvapepun,1995). Depending on the species and culture techniques,up to 85% of phosphorus and 52±95% of nitrogen inputinto a marine ®sh culture system as feed may be lost intothe environment through feed wastage, ®sh excretion,faeces production and respiration (Wu, 1995). Althoughconcentrations of total nitrogen and phosphorus areusually low, their impact on the environment cannot beignored because of the high volumes of water utilizedduring ®sh farming.

Several studies have estimated the total nitrogen andphosphorous discharges into receiving waters for dif-ferent species (M�akinen, 1991; Beveridge et al., 1991;Ackefors and Enell, 1994). Due to the improvementsexperimented in the quality of foods and feeding tech-niques, lower values have been obtained on the loadestimations performed in recent years. Thus, an averageof 9.2 kg N and 0.57 kg P, for each ton of channel cat®sh(Ictalurus punctatus) was estimated to be discharged(Schwartz and Boyd, 1994), while for juvenile atlanticsalmon (Salmo salar L.) cultured in freshwater, loadings

Marine Pollution Bulletin Vol. 40, No. 11, pp. 981±988, 2000

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*Corresponding author. Fax: +34-956-016040.E-mail address: [email protected] (C. Moreno).

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of 71 kg N and 10.9±11.1 kg P per tonne have beenreported (Kelly et al., 1996). In a semi-intensive shrimp(P. vannamei) farm in earthen ponds, the estimatedlosses of N and P were 28.6 and 4.6 kg, respectively, pertonne of product (P�aez-Osuna et al., 1997). In a pre-liminary study performed on gilthead seabream (S. au-rata) cultured in cages, around 190 kg N and 28 kg Pwere estimated to be introduced in the environment foreach tonne of ®sh produced (Barbato et al., 1996).

The discharges of suspended solids and organic mat-ter as a consequence of aquaculture activities have alsobeen studied. Average values of 110±520 kg TSS and145±720 kg BOD7 were reported as loadings from Finishfarms, per tonne of ®sh cultured (M�akinen, 1991).Beveridge et al. reviewed several studies where theloadings from freshwater salmonid culture were re-ported. Thus, values of TSS and BOD discharges werein the ranges of 474±4015 and 285±990 kg, respectively,per tonne of ®sh cultured (Beveridge et al., 1991). Sim-ilar values were obtained by Kelly et al., for the samespecies, reporting a discharge of 327±337 kg TSS and422±485 kg BOD5 (Kelly et al., 1996).

P�aez-Osuna et al., reported that 1302 kg TSS and 469kg POM were discharged during the culture of 1822 kgof shrimp in earthen ponds (715 kg TSS and 257 kgPOM per tonne) (P�aez-Osuna et al., 1997). In a studyperformed on a farm devoted to the culture of rainbowtrout (Oncorphyncus mykiss), net mass ¯ow variationswere calculated. Thus, the daily increase of BOD5 was353±1510 g tÿ1, while daily net variations of TSS werebelow 1753 g tÿ1 (129±551 kg BOD5 and 640 kg TSS perton) (Boaventura et al., 1997).

Only one work was found on the study of the dis-charges of TSS and POM matter from the culture ofseabream in earthen ponds. As a consequence of theproduction of 3205 kg of ®sh, very large amounts of TSS(25 557 kg) and POM (3236 kg) were reported to beobtained (7974 kg TSS and 1010 kg POM per tonne)(Jambrina et al., 1995).

Lack of data relating to waste outputs produced bysome species, as S. aurata, and culture systems, speciallyin earthen ponds, makes the planning and design ofwastewater treatments systems di�cult (SMAFF, 1999).The present paper focuses on the study of the environ-mental impact of a farm devoted to the intensive cultureof S. aurata in earthen ponds, on the quality of theadjacent water. Total suspended solids, BOD5, particu-late organic matter and the dissolved concentrations ofammonium, nitrite, nitrate and phosphate in the in¯owsand out¯ow of the ®sh farm were quanti®ed during atwo-year period, corresponding to a complete culturecycle of the species.

Study Site

The study was carried out at a ®sh farm belonging to thecompany CUPIMAR, S.A., located in the margins ofthe San Pedro river (SW Spain) (see Fig. 1). This river

was a tributary of Guadalete river, but was arti®ciallyblocked 12 km from the mouth. The San Pedro river isnow an arm of the sea with a length of 12 km and awidth ranging between 15 and 30 m. Water renovationfrom the Bay of C�adiz is controlled by tidal cycles; hencethe water of this so-called river is seawater in character.It is located within a protected area (The Natural Parkof the Bay of C�adiz), which promotes especially strictrequirements in the water quality of the river (BOJA,1997). In a previous work, the environmental conditionsof San Pedro river were studied and the existence of twodi�erent zones, with di�erent water quality, was de-scribed (Tovar et al., 2000). The ®sh farm under studywas located in the upper section of the river, which ex-tends from 8 km from the mouth to the end of the riverwhere it is arti®cially blocked, and is characterized bypoorer water quality. Thus, low levels of dissolved ox-ygen and high contents of suspended solids and nutri-ents (especially ammonium) were found in this part ofthe river. The anomalies were suggested to be a conse-quence of, among others, aquaculture activities andpoorer renovation of water by the tides, due to thehydrography of this arm of the sea.

Although three ®sh farms are using the water of theSan Pedro river for their activities, the study farm (A inFig. 1) is su�ciently large to be used to establish therelationships between aquaculture activities and thewater quality of the river. The ®sh farm extends to anarea of about 5 700 000 m2 and, as can be observed inFig. 2, consists of a series of batteries of earthen pondswhere ®sh culture is carried out. The farm is divided intwo di�erent areas. In the ®rst (I in Fig. 2) ®shes arecultured in extensive or semi-intensive regime. The sec-ond area (II in Fig. 2) consisted of four batteries ofearthen ponds where gilthead seabream is cultured un-der an intensive regime. In this zone, the majority of the®shes cultured in the farm are found, representing thegreatest source of pollutants.

The seawater used in the culture process is introducedinto the farm by two pumping stations. The water in-troduced by station number 1 (P1 in Fig. 2) passesthrough the zone I prior to feeding the lower half of the

Fig. 1 Geographical situation of the San Pedro river. A: Fish farmunder study.

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zone of intensive culture (zone II). The water pumped bystation 2 (P2 in Fig. 2) goes directly to the upper half ofzone II. Both sources of e�uent are discharged via asingle channel into the river.

The daily total volume of water introduced into the®sh farm from the San Pedro river ranged from 181 151m3 to 287 795 m3. Flow rate data were provided by thesite manager, and occasionally checked using a PortableVelocity Meter, Model 3872, Sigma (USA). At the exitchannel, the ¯ow rate of the e�uent was controlled bymanipulating the opening of the sluice-gate to renovatethe total volume of water in the farm once a day. Thetotal e�uent volume was estimated as the sum of thevolume of in¯uent plus, when needed, contribution ofthe rainwater.

Materials and Methods

Reagents and solutionsAll the chemical reagents used in this work were of

analytical grade and purchased from Merck (Daarms-tadt, Germany). To evaluate the concentration of po-tential pollutants in seawater, corresponding calibrationplots were constructed of six points each. Except forphosphate, all the calibration solutions were preparedwith synthetic seawater, which was made by dissolvingthe needed amounts of salts to a ®nal composition,in g lÿ1, of: 23.926 (NaCl); 4.008 (Na2SO4); 0.677 (KCl);0.196 (NaHCO3); 0.098 (KBr); 0.026 (H3BO3); 0.003

(NaF); 10.826 (MgCl2.6H2O); 1.518 (CaCl2.2H2O);0.024 (SrCl2.6H2O).

Water samplingThe measurements were carried out from April 1997

to March 1999. During this period, up to 30 samplingexpeditions were performed. On each occasion, threesamples were collected; two from the two pumpingstations, P1 and P2, and the third from the e�uent ofthe farm E (see Fig. 2), to characterize the in¯ows andout¯ow of the ®sh farm, respectively. Two di�erentbottles were used for taking each sample: a 1-l poly-ethylene bottle was used for the sample for quantifyingammonium, BOD5, POM and TSS; and a 1-l glass bottlewas chosen for the sample used to determine nitrites,nitrates and phosphates. As a biocidal agent, 40 mg/lHgCl2 were added to preserve the latter sample.

Determination of properties of the waterOnce taken, samples were transported to the labora-

tory, where they were ®rst ®ltered using a 0.45 m ®lter(N04SP04700, Micron Separations, USA) prior to am-monium, nitrite, nitrate and phosphate contents beinganalysed by standard methods (Grassho� et al., 1983,Rodier, 1990) (see Table 1).

To determine TSS and POM, a 500-ml sample waspassed through a cellulose acetate ®lter (1110647N,Sartorius AG, Goettingen, Germany). Then TSS andPOM were gravimetrically quanti®ed, after heating the®lter at 105°C (Heater Model 2000209, Selecta, Spain)and 525°C (Oven RHF 1400, Carbolite Furnaces, UK),respectively.

BOD5 was determined by using a Model FOC 225 Dmanometric apparatus (Velp Scienti®ca, Usmate, Italy).

Results and Discussion

NutrientsOn the one hand, the environmental impact of marine

aquaculture was evaluated by estimating the dissolvednutrients discharged into the San Pedro river as a con-sequence of the ®sh culture. As mentioned before, threesamples were taken during each sampling expedition,corresponding to the two in¯ows and the out¯ow of the®sh farm. In these three samples the concentrations ofammonium, nitrite, nitrate and phosphate were mea-sured.

TABLE 1

Methodology applied to the quanti®cation of each nutrient.

Nutrient Method

N±NH�4 Spectrophotometry (Indophenol blue)N±NOÿ2 Spectrophotometry (Sulphanilamide/N-(1-Naphthyl)-ethylenediamine dihydrochloride)N±NOÿ3 Spectrophotometry (reduction in a Cd column and nitrite method)P±PO3ÿ

4 Spectrophotometry (Ammonium molybdate/ascorbic acid)

Fig. 2 Representation of the ®sh farm under study. I: Extensive orSemi-intensive culture. II: Intensive Culture. P1: Pumping sta-tion 1. P2: Pumping station 2. E: E�uent.

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The variations observed for each nutrient were ana-lysed. Thus, on the one hand, the temporal variation ofthe concentration of the nutrients studied in the in¯owsand the out¯ow was represented during a two-year pe-riod, which is a whole culture cycle. On the other hand,the direct e�ect of the ®sh culture on the river wasevaluated by performing the mass balances between thetwo in¯ows and the out¯ow. Thus, the total discharge ofeach nutrient was estimated. All the results mentionedabove are discussed as follows:

Figs. 3±6 show the temporal variation of dissolvedphosphate, ammonium, nitrite and nitrate, respectively,in the in¯ows and the out¯ow of the ®sh farm during thetwo-year period. The shape of the curves is an indicationof seasonal variation, which was common to all thenutrients studied. However, the observed seasonal pat-tern is di�erent from the known seasonal pattern forcoastal seawater that is dependent on a series of factorssuch as the presence of phytoplankton, temperature,light intensity or ¯ux of nutrients from sediments(Millero, 1996; Valiela, 1995). Thus, in coastal zones, arapid removal of nutrients occurs in the spring, due tophytoplankton growth, the lowest values being mea-sured in summer. Then, nutrient concentration increasesin fall, and is usually highest in winter when they are nottaken up by producers. This behaviour was not observedin the present work. From Figs. 3±6 the existence of adisplacement of the periods of highest and lowest con-centration was established. So, in the area under study,the increase in nutrient concentrations generally took

place in summer, and highest concentrations are reachedin fall. The amount of nutrients dissolved in seawaterthen decreased during winter, the concentration beingminimum in spring. This displacement can be explainedin terms of ®sh culture activities. Among the factorsmentioned above as responsible for the seasonal varia-tion of nutrients in seawater, the amount released from®sh is generally not taken into account because of itsrelatively small contribution. However, the largeamount of ®shes contained in a ®sh farm devoted tointensive aquaculture determines that this becomes themain causative agent of the concentration of nutrientsdissolved in the seawater used in the culture. In Fig. 7,the growth in mean weight of ®sh in a representativeearthen pond during the period of time studied is shown.The curve follows a seasonal pattern with maximumgrowth rate (and, as a consequence, the maximum ex-cretion and so, the release of nutrients from ®shes) beingobtained during the seasons of summer and fall, coin-ciding with the highest nutrient concentrations recordedin the seawater of the zone. By contrast, the minimumlevels of nutrients dissolved in seawater are obtainedduring spring and winter, when the growth of ®shesdecreases and the release of nutrients decreases as well.

The levels of nutrients in the water samples taken inpumping station number 1 (P1) were always lower thanthose in samples taken from pumping station number 2(P2) (Figs. 3±6). This was attributed to the proximity ofP2 to the e�uent of the ®sh farm. This, together with thepoor renovation of water of the San Pedro river due to

Fig. 3 Temporal variation of P±PO3ÿ4 in the in¯ows and out¯ow of the

®sh farm. : P1, : P2, : E.

Fig. 4 Temporal variation of N±NH�4 in the in¯ows and out¯ow ofthe ®sh farm. : P1, : P2, : E.

Fig. 5 Temporal variation of N±NOÿ2 in the in¯ows and out¯ow ofthe ®sh farm. : P1, : P2, : E.

Fig. 6 Temporal variation of N±NOÿ3 in the in¯ows and out¯ow ofthe ®sh farm. : P1, : P2, : E.

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its hydrography (Tovar et al., 2000) results in the waterbeing introduced into the ®sh farm by P2 being of worsequality than that introduced by P1, which is locatedcloser to the mouth of the river and experiences a betterrenovation of the water by tides.

The only exception observed during the study was fornitrate concentrations (see Fig. 6). In this case, very highconcentration values were recorded in winter, and onlyin the ®rst year of the study. These samples were takenafter a strong period of rains, which caused an incre-ment in the concentration of nitrates, due to run-o�from agricultural land adjacent to the river. Thus, someof the very high values measured during the study mustbe considered as caused by the e�ect of rains and cannotbe attributed to aquaculture activities. In fact, nitratelevels always increased after a rainfall, the magnitude ofthis increase being directly related to the magnitude ofthe rains.

The next objective of this work was to carry out themass balance estimates for each of the four dissolvednutrients in the ®sh farm to quantify the direct e�ect ofaquaculture on the environment. These balances weremade taking into account the volume of seawater in-troduced by each pumping station (V1 and V2) and thevolume of e�uent (VE), together with the correspondingnutrient concentrations ([N]1, [N]2 and [N]E, where Nrepresents each nutrient) measured in the samples. Asmentioned above, the data for V1 and V2 were given bythe site manager and occasionally tested, while the vol-ume of e�uent, VE, was estimated as the sum of twofactors: total volume of seawater introduced in the sys-tem by both pumping stations, V1 and V2, plus volume ofwater introduced by rain, VR (estimated from pluviom-etry and culture surface). Thus, the mass of each nutri-ent, n, was estimated as follows:

n � ��N �E � VE� ÿ ��N �1 � V1 � �N �2 � V2�; �1�

where VE � V1 � V2 � VR:

Because of the disturbance produced in the systemdue to the large amount of nitrates introduced fromadjacent agricultural lands after a period of rain, thenitrate values measured during the rain periods were notconsidered when performing the mass balances.

Thus, the individual nutrient loads were calculateddaily, the results being shown in Fig. 8. From the plotteddata, the total amount of nutrients discharged during aperiod of time can be estimated for this particular case.The total nutrient loadings were calculated in relation tobiomass production. Thus we have determined that36.41 kg N±NH�4 , 4.95 kg N±NOÿ2 , 6.73 kg N±NOÿ3 and2.57 kg P±PO3ÿ

4 were discharged for each ton of ®shcultured. As was expected, there is an important di�er-ence between the amount of ammonium discharged intothe environment and the amounts corresponding to theother three nutrients. This can be explained by the factthat ammonium is the principal nutrient released by®shes (Porter et al., 1987; Echevarr�õa et al., 1993).

Suspended solidsThe methodology applied to estimate the importance

of the loadings of total suspended solids was similar tothat used for nutrients. Thus, ®rst, the temporal varia-tions of TSS were quanti®ed in the in¯ows and out¯owof the farm and are presented in Fig. 9, from which aseasonal dependence can be established, with maximumvalues being obtained during summers while the lowestwere obtained in winter.

Fig. 8 Temporal variation of nutrients discharged into the river. :N±NH�4 , : N±NOÿ2 , : N±NOÿ3 , : P±PO3ÿ

4 .

Fig. 7 Temporal variation of the mean weight of a ®sh.

Fig. 9 Temporal variation of TSS in the in¯ows and out¯ow of the®sh farm. : P1, : P2, : E.

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The proximity of P2 to the e�uent of the farm orig-inates that higher values of suspended solids are mea-sured in this pumping station, in relation to thosemeasured in P1. However, the di�erence between thevalues obtained in both pumping stations was not sohigh as expected. This was attributed to the rapid sedi-mentation of solids once they are poured to the river,due to the relatively slow movements of the water in thispart of the river (Tovar et al., 2000).

The very high values recorded in the curve corre-sponding to the e�uent in Fig. 9 show that, as expected,the discharge of suspended solids represents one of themain e�ects of the aquaculture activities.

Mass balances for TSS were estimated by usingEq. (1), adapted to the new parameter to be quanti®ed.Thus, the daily net discharge of TSS was calculated andrepresented in Fig. 10. From these data, the total soliddischarge was estimated in relation to ®sh production,and then a value of 9104.57 kg of suspended solids wascalculated to be introduced into the river per tonne ofseabream cultured.

Organic matterTo quantify the release of organic matter from

aquaculture, two di�erent parameters were studied:BOD5 and particulate organic matter (estimated asvolatile solids). The temporal variations of these twoparameters are shown in Figs. 11 and 12, which showthe expected pattern, with maximum values in summerand minimum in winter. Some BOD values were almostnegligible, like those recorded from December 98 toMarch 99. The behaviour observed is in coincidence,once more, with the growth rate of the ®shes.

Daily, mass balances of BOD and POM were cal-culated following the same procedure explained above,and based on the use of Eq. (1). The results obtainedare shown in Figs. 13 and 14, respectively. These re-sults cover a 20-month period for BOD, while POMloads were quanti®ed during 15 months. The totalloads were divided by the biomass increment of thecorresponding period, and so, mean values of 235.40kg BOD and 843.20 kg POM were estimated to bedischarged into the receiving water for each tonne of®sh cultured.

Fig. 10 Temporal variation of TSS discharged into the river. Fig. 11 Temporal variation of BOD in the in¯ows and out¯ow of the®sh farm. : P1, : P2, : E.

Fig. 12 Temporal variation of POM in the in¯ows and out¯ow of the®sh farm. : P1, : P2, : E.

Fig. 13 Temporal variation of BOD discharged into the river.

Fig. 14 Temporal variation of POM discharged into the river.

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Conclusions

Because of the use of di�erent species and culturesystems, it is di�cult to evaluate the results from thepresent work with those from other studies. Besides,some of these studies are based on theoretical calcula-tions. Thus, Ackefors and Enell (1990) reviewed thedischarge of nutrients from Swedish marine net-cage ®shfarms, and, with a feed coe�cient of 1.5, estimatedvalues of 2.2 kg of P and 61 kg of N as the total amountsof nutrients discharged in dissolved form into the envi-ronment per ton of ®sh produced. These ®gures areconsidered by the authors as average values for ®shfarms in general and therefore, applicable to furthercalculations in di�erent farms. On the other hand, Kellyet al. (1996) reported values of 1.2±2.1 kg P±PO3ÿ

4 and30±35 kg N±NH�4 from experimental measurementsfrom a freshwater atlantic salmon farm in Scotland. Ascan be observed, even for di�erent species and culturesystems, the values of dissolved nutrients obtained inthis work are very similar to those reported by otherauthors. This fact indicates that the role that species andculture system play in determining total waste loading isespecially important for the particulate fraction, while ithas lower in¯uence on dissolved fraction. This can becon®rmed by analysing the results obtained for bothTSS and POM. In the present work, a very high value ofTSS load was obtained (9104.57 kg/tonne), being inconcordance with the value reported for the same spe-cies and culture system: 7974 kg/tonne (Jambrina et al.,1995). These values are much higher (up to 80 times)than the TSS load reported for other species, which werepreviously discussed, even for shrimp in earthen ponds.Besides, the values of POM obtained (843.20 kg/tonne)were also higher than those reported by other authors,but, in this case, the di�erence was not so high (up to 3±4 times). This fact may indicate that the suspendedsolids contained in the water e�uents of the ®sh farmwere mainly of inorganic nature and probably comingfrom the bottom of the ponds, as a consequence of ®shactivity. Thus, less than 10% of the suspended solidspoured into the river were of organic nature their originbeing in uneaten food and ®sh excretions.

On the other hand, the values obtained in this workfor BOD load (235.40 kg/ton) were in the same order ofmagnitude that the discharges produced for other spe-cies and cultured systems as can be observed in thevalues showed in the introduction of this paper. Thisindicates again that the amounts of products originatedas a direct consequence of aquaculture activities (®shgrowing) are very similar for di�erent species and cul-ture systems, the main di�erence being in the inorganicsolids re-suspended by ®shes from the bottom of theearthen ponds, when used.

Finally, the results of the present study also indicatethat, in general, the water quality of the e�uents fromthe intensive culture of S. aurata shows very large sea-sonal variations, due mainly to the ®sh biology and re-

lated management practices. For this reason, optimalcharacterization of ®sh farm e�uents must be based oncontinuous monitoring, information being limited ifbased on single samplings.

The authors want to thank CUPIMAR, S.A., with special mention toF. Fonlut and A. Vidaurreta, for their invaluable collaboration. Thiswork was supported by the CICYT (Spanish Commission for Researchand Development), project No. PTR 95-0087-OP.

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Documents

Environmental Implications of Intensive Marine Aquaculture in Earthen Ponds