AquacuUure and Fisheries Management 1993, 24, 529-543

Use of effluent water from fish-ponds as a food sourcefor the Pacific oyster, Crassostrea gigas Thunberg

M. SHPIGEL National Center for Mariculture, Israel Oceanographic and LimnologicalResearch, Eilat, Israel

J. LEE & B. SOOHOO Biology Department, City College of New York, New York, NewYork, USA

R. FRIDMAN & H. GORDIN National Center for Mariculture, Israel Oceanographicand Limnological Research, Eilat, Israel

Abstract. Growth rates, condition indices and diet composition of the Japanese oyster,Crassostrea gigas Thunberg, were studied in two types of ponds which form part of afish-bivalve integrated culture system. Although abiotic parameters (e.g. temperature,salinity, pH, ammonia, particulate inorganic matter) were similar in the two pond types, oysterssupplied with water from a sedimentation pond grew significantly faster and showed bettercondition indices than the oysters supplied with water from the PVC-lined ponds. It is suggestedthat the main reasons for the better performance of the oysters supplied with water fromsedimentation pond water are: higher algal diversity, additional nutritious Food consisting ofattached benthic diatoms and stable algal concentration.

Introduction

Intensive research being carried out at the National Center for Mariculture (NCM), IsraelOceanographic and Limnological Research (IOLR), Eilat, Israel is aimed at development oftechnology for the culture of fish, mainly gilthead seabream, Sparus aurata L. The purpose ofthis research and development is to introduce a new industry utilizing sea water as the basicmeans of production in this desert area. Thus, new sources of income and jobs will begenerated in a region where only limited agricultural potential exists.

Intensive culture of fish results in the production of high levels of metabolites. These,together with naturally occurring microalgae and high solar radiation, result in high algalproduction (3-5-5gOm-/day, Motzkin, Cohen, Gordin & Padan 1982). Large quantities ofwater rich with algae and nutrients discharged into the sea pose a threat to the environmentthrough eutrophication. Such a threat is even more severe in a coral reef environment andmay degrade the recreational facilities of the Gulf of Aqaba.

Filter-feeding bivalves can be used to improve water quality and may provide a partialsolution to the problem by removing excess phytoplankton in maricuiture waste water beforeit is discharged into the sea. A preliminary study on oyster culture was initiated at NCM in1975 (Hughes-Games 1977). This study demonstrated the feasibility of growing oysters infish-pond effluents. As a result, a fish-bivalve integrated system was designed and built atNCM in 1985 to test the hypothesis of water treatment by bivalves. Effluent water from

Correspondence: Dr M. Shpigel, National Center for Mariculture, Israel Oceanographic and LimnologicalResearch, PO Box 1212, Eilat, Israel.

529

530 M. Shpigel tia\.

FISH/BIVALVE POLYCULTURE SYSTEM

Schematic diagram of the fish-bivalve integrated system.

intensive PVC-lined fish-ponds drained into an earthen sedimentation pond. From thesedimentation pond, water was pumped into bivalve tanks where the bivalves filteredphytoplankton from the water before discharge into the sea (Fig. 1). Preliminary studiesshowed that newly set oyster spat grew better in the water from the sedimentation pond (to60g in 14 months) compared to those in the water of the PVC-lined pond (to 14g in 14months) (Shpigel, unpublished).

The results of these studies led us to search for an explanation for the difference inperformance in the two type of ponds. Subsequently, a project was carried out: (1) tocompare biotic and abiotic parameters of the sedimentation and the PVC-lined ponds; (2) tocompare growth rates, condition indices and survival of C. gigas in water from these differentsources; and (3) to explain the difference in oyster performance.

Materials and methods

Experimental system

In the integrated system at NCM, unfiltered sea water is pumped into three octagonalPVC-lined fish-ponds of lOOm^ x 1 m depth from an inlet located 300 m offshore at a depth of20m. The flow rate provides a daily water exchange of 30-50% of the pond volumes. Eachpond is stocked with 500-770 kg of fish, fed a pelleted diet containing 40% protein. Effluentfrom the three ponds drains into a 250 m^ sedimentation pond with one complete daily waterchange by the addition of fresh unfiltered sea water.

Growth rates and survival of C. gigas within three units of the integrated system (Fig. 1)were compared. In the first unit, A (PVC-lined pond), water from a single fish-pondrecirculates through one oyster tank. The second unit. B (sedimentation pond), receivedmixed effluents coming from three fish-ponds, drained to an earthen sedimentation pond.

Effluent water as food for Pacific oyster 531

The third unit, C, received mixed effluents from three fish-ponds prior to discharge into thesedimentation pond.

The oysters in each unit were kept in a 600-1 cylindrical tank with a steep conical bottom.Water entered from the upper side of the tank and flowed out from a central vertical standpipe. This design allowed faecal and pseudofaecal material from the oyster to sink to thebottom of the tank. The flow rate to each experimental tank was 1200 ± 2001/h. The tankswere flushed once daily to remove sediment.

Experimental protocol

The experiment lasted for 60 days, from 5 January to 4 March 1988. The oysters were 4-5months old, hatchery-reared C gigas with a mean live weight of 4-80 ± l-40g. They weremaintained at a density of 50g/l and held on plastic mesh trays. This density allowed for anexcess of phytoplankton so that growth was never food limited (Shpigel, unpublished).

Relative daily growth rates {K) were calculated from the live weight of fifty taggedoysters, using the equation:

K = {In Wf - In

where W, = the initial mean live weight (g) of the oysters, Wf=i\\e final mean live weight (g),and T = the duration of the experiment in days.

Condition index (C/) was determined by examining 18 randomly selected oysters fromeach treatment at the beginning and end of each experiment. The meat was separated fromthe shell and both were dried to a constant weight at 80°C. Condition index was calculated by:

Cl={WJW,)x 100,

where Wm = mean dry meat weight and W^ = mean dry sheil weight, both in grams.

Abiotic parameters

Temperature, oxygen, salinity, pH and ammonia (NH4-N) levels were measured daily (at1500h). Algal concentration, particuiate organic nitrogen (PON) and particutate organicmatter (POM) were measured every 3 days. Oxygen levels were measured with a YSI model57 oxygen-meter and pH with a Hanna HI 8424 pH-meter. Ammonia (NH4-N) was measuredwith a Technicon-2 autoanalyser (Technicon, Basingstoke, Hants, UK) after Krom, Porter &Gordin (1985). Samples for PON analysis were collected on GF/C glass fibre filters and driedat 80°C for 24 h before analysing with a Perkin-Elmer model 240 CHN analyser (Perkin-Elmer, Norwalk, CT, USA). Preliminary study showed that the size of particulate matter(PM) measured with a Coulter Counter (Model ZB) (Coulter, Dunstable, Beds, UK) rangedfrom 2 to 32 microns with an average of 8 microns spherical diameter. PM samples werecollected on GF/C glass fibre filters, washed and dried in an oven at 80°C for 48 h and thenashed in a furnace at 450°C for 6h.

Algal identification

Samples of water from the inlet supphes to the three units were used to detennine algalconcentration and identification. Algal concentrations and some identification were

532 M. Shpigel

determined using a light microscope and haemacytomeier. Other algae identifications andmicrographs were made with scanning electron microscopies (SEM). Algal samples for SEMwere prepared as follows: samples of water ( 2 x 1 1 per sample) were taken every week(February-October 1989) from the ponds, common effluent pipes, and the oyster tankattached to the sedimentation pond. Stomach contents of oysters were also sampled. Thesamples arrived at City College of New York, preserved in a mixture of formalin (4%) andsea water. After the samples settled, the fluid was removed by pipette and replaced withdistilled water. The suspension of cells was centdfuged and the supernatant decanted. Thecells were resuspended in hydrogen peroxide (30%) for 0-5-2h until all colour was bleached.Samples with large amounts of organic matter (e.g. oyster gut) were warmed in a water bathto accelerate their oxidation. After completion of oxidation the samples were centrifugedagain, the supernatant decanted, and replaced with distilled water. Samples were decantedthrough a membrane filter (Nucleopore, 0-45 um, Gelman Sci., Ness-Ziona, Israel). Thefilter funnel was rinsed several times to wash any adhering cells on to the filter, and then it wasremoved and brushed, so that the next sample would not become contaminated withspecimens from the previous one. The individual filters were removed from the apparatus,placed into planchettes, and then transferred to a vacuum desiccator jar which itself wasplaced in a warm (60°C) oven. After the filters were dried they were mounted on stubs withthe aid of double-stick tape (3M brand). The stubs were returned to the vacuum desiccator jarso that the volatile solvents were removed at this stage. After an overnight treatment, stubswere sputter-coated (Poloron model 5150, CT, USA) with lOnm Au, before being examinedin one of two scanning electron microscopes (Zeiss DSM 940 and 950, Oberkochen,Germany). The stubs were scanned horizontally to the cut edge of the filter so that there wereno overlaps of fields, an important consideration since we were making quantitativeobservations. Statistical analysis was done by fitting the data to least squares regression to testthe significance of the sites sampled relative to each other with respect to the genera ofdiatoms observed. An analysis of variance was utilized to determine if the calculatedregression coefficients departed from zero by more than chance.

Results

Growth rate and condition indices

Growth rate of oysters in the sedimentation pond was 1-56% per day (4-89 + 1- lOg to 12-52 ±2-26g in 60 days) and significantly faster (P < 0-01; F= 12-1, n = 25) than those in the mixedwater (unit C), 1-24% per day (4-97 ± l-84g to 10-48 ± 2-82 g). Growth rate of oysters in unitC was significantly faster {P < 0-01; F = 9-35, n = 25) compared with the results obtained inthe PVC-Iined pond unit, 0-32% per day (4-85 ± MOg to 5-91 ± l-75g; Fig. 2).

Condition indices of oysters from the sedimentation pond were significantly higher 12-34± 1-75 (P < 0-05; F = 7-66) compared with 9-14 ± l l l g in the PVC ponds. No significantdifferences were found between condition indices of the oysters in unit C (11-12 ± 2-11) andthose of the other water sources (Table 1). No mortality was observed during the 60 days ofthe experiment.

Abiotic parameters in the experimental units

Temperature, salinity, pH, ammonia, oxygen and particulate inorganic matter (PIM) levelsin the three units observed were similar (Table 1). Temperature averaged O^C in January

Effluent water as food for Pacific oyster 533

12

•=10

PVC POND

L

10 20 30DAYS

40 50 60

figure 2. Growth rates of Crassostrea gigas in the sedimentation pond, in the PVC-lined pond and in unit C. (Oneside error bars = standard deviation.)

and ITC in March with a diurnal fluctuation of 4 ± T'C. Salinity was fairly constant at 40 ±lppt, and pH ranged from 7-9 to 8-7 with a positive correlation with microalgalconcentration- Dissolved oxygen levels fluctuated between 8-5 and 10-1 mg/1 (80-135%saturation). Ammonia levels ranged from 5 to 60 micromole in the sedimentation pond, andfrom 5 to 95 micromole in the PVC-lined ponds, with a negative correlation wilh microalgalconcentrations. PIM in the sedimentation pond averaged 65 ± 18mg/l. In the PVC-linedpond and in unit C, PIM levels averaged 59 ± 15 and 59 ± 11 mg/1 respectively. In both pondtypes 80-90% of the PIM consisted mainly of particles smaller than 5 microns.

Algal dynamics and distribution

A total of 52 distinct taxa of diatoms were identified {Table 2). Species belonging to eightgenera. Amphora, Asterionella, Cyclotella, Fragillaria, Navicula, Nitzschia, Pleurosigma and

Table 1. Summary of the abiotics and biotics parameters in three units of the integrated system during the experiment(± = SD)

Temperature ("C)pHOxygen (mg/1)Ammonia (Umole)Algal concentration (cells/ml)PIM (mg/I)POM (mg/I)PON (mg/I)ON Ratio (Algae)Oyster growth rate (% per day)Condition index

PVC pond

12-5-18-27-8-8-76-8-10-1

5-950-2-60 X 10̂

59 ± 15134 ± 50

2-78 ± 0-46-11 ±0-84

0-329-14 ± M

Sedimentation pond

13-0-1707-9-«-47-5-10-1

5-602-0-4-2 X 10*

65 ± 1861 ± U

:-01 ± 0-27-05 ± 0-65

1-5612-34 ± 1-8

Unite

12-0-18-278-8-468-9-9

5-870-15-4-1 X 10*

59 ± 1270 ±7

1-06 ±0-47-04 ±0-1

1-24n-21 ±2-1

534 M. Shpigel etal.

Table 2. Diatoms found in the integrated system

Amphora bigibba GrunowAmphora caroUniana GiffcnAmphora calarinaria CholnokyAmphora costala Wm. SmithAmphora hamata Heiden and KolbeAmphora ovalis var. pediculus (Kutzing) Van HeurckAmphora pediculus (Kutzing) GrunowAmphora proteus GregoryAmphora tenuissima HustedtAmphora veneta Kutzing

Cocconeis disculoides HustedtCocconeis placenlula var. euglypta (Ehr.) CleveCocconeis diminula Pantocsek

Coscmodiscus nitidus var. Gregory

Cycloiella bodanica EulensteinCyclotella michiganiana SkvortzovCyclotella slelUgera Cleve & Grunow

Cymbella pusilta Grunow

Dimeregramma minor var. minor (Greg) Ralfa

Fragilaria pinaia Ehrenberg

Fragilaria ulaia var. ulala (C.A.Ag.) Lange-Bertalot

Hanlzschia virgala (Roper) Grunow

Melosira sulcala var. coionaia Grunow

Navicula confervacea (Kutzing) GrunowNavicula delognei Van HeurckNavicula diserta HustedtNavicula directa Wm. SmithNavicula forcipata var. GrevilleNavicula gracilis EhrenbergNavicula gregarina DonkinNavicula hagelsteinii HusledtNavicula lacustris GregoryNavicula lindae Sullivan & ReimerNavicula pennata SchmidtNavicula placyventris MeisterNavicula spp.Navicula zostereti Grunow

Nitzschia cotxstricta (Kutzing) RalfsNitzschia frustulum (Kulzing) GrunowNitzschia kutzingiana HilseNitzschia lanceolaia Wm. SmithNitzschia laevis HustedtNilzschia longissima (De Brebisson) GrunowNiizschia marginulata GrunowNitzschia patea (Kutzing) Wm. SmithNitzschia palea var. debilis (Kutzing) GrunowNilzschia punctata (Wm. Smith) GrunowNilzschia sublinearis Hustedt

Pleurosigma salinarum var. GrunowPleurosigma rhombeum GrunowPleurosigma roslratum Hustedt

Stephanodiscus parvus Grunow

Synedra accounted for >95% of the diatoms in the samples (Figs 3-5). Factorial analysisshowed that diatom species composition was significantly different among sampling sites{P = O-(XX)1). Algal composition of the stomach contents of the oysters closely matched thatof the pond water In which the oysters were grown (Fig. 6).

PVC-Uned ponds

In the PVC-lined ponds, microalgal blooms consisting of one dominant species comprisedusually planktonic species (67%) such as Chlorella sp., Tetraselmis sp., Chaetoceros sp. andLithodesmium sp. The rest, 17% and 16% on average, were benthic diatoms belonging to thepennate genera Nitzschia, Navicula, Cocconeis, Pleurosigma and microflagellatesrespectively (Fig. 6). Cell wall fragments and individual scales of Chlorella sp. andTetraselmis turned up in the diatom preparations but Chaetoceros frustules were rare. Thiswas expected because Chaetoceros frustules do not preserve well in formalin and arecompletely digested by the hydrogen peroxide treatment we used. Cyclotella striata andCyclotella sp. (Figs 3 and 4) were quite abundant but Melosira suculata var coronata wasabundant only in one sample.

Effluent water as food for Pacific oyster 535

Figure 3. Scanning electron micrographs of diatoms in the integrated system: (a) Cyclotella bodanica; (b) Cyclolellastelligera; (c) Cyclotella michiganiana; (d) Stephanodiscusparvus; (e) Navicula sp.; (f) NUzschia marginulaia. Scalebars: (a-d) 2jim; (e) 5(j,m; (f) 10(i.m.

536 M. Shpigel

Figure 4. (a & c) Cyclotella bodanica', (b) Stephanodiscus parvus; (d) Cycloiella michiganiana: {e & f) Naviculadiserta. Scale bars: fa & c) 5|xni; (b & e) 1 M-m; (c, d&f)2ftm.

Effluent water as food for Pacific oyster 537

Figures, (a) Pteurosigma salinarum vcr;(b) Fragilaria ulata var ulata; (c) Nitzschia frustulum; (d) Navicula lindae;(e) Nitzschia laevis; (f) Achnanthes sp.; (g) Amphora catarinaria; (h) Amphora bigibba. Scale bars: (a) 20 .̂m; (b)

; (c) lp.ni; (d)5M.m.

538 M. ShpigeUtal

16.0%21.0%

SEDIMENTATION POND PVC UNED POND

SO.0%

SEDtMEKTATION POND PVC UNED PONDSTOMACH CONTENTS STOIAACH CONTENTS

• BENTHIC DIATOIMS H PLAN KTONIC ALGAE Q MICRO FLAGELLATES

Figure 6. Proportion of benthic diatoms, planktonic algae and microfiageliates in the pond, PVC-lincd ponds andstomach contents of Crassostrea gigas.

Total phytoplankton abundance ranged between 2 x 10'* and 6 x 10^cells/ml. POM was134 ± 50mg/l. PON was 2-78 ± 0-78mg/1 and the carbon to nitrogen ratio was 6-11 + 0-84(Table 1).

Sedimentation pond

In the sedimentation pond algal populations consisted of mixtures of the planktonicmicroalgae drained from the fish-ponds and algal populations developed in the pond itselfwhich consisted mainly of benthic diatoms, attached to the sediment. These benthic diatomscomposed 67% of the microalgal population. In the oyster tank attached to the sedimentationpond Navicula deserta and Pleurosigma salinarum were the most numerous species. Naviculadeserta was the dominant organism in the samples, comprising 36-5% of the total samples.

The stomachs of the oysters contained mainly diatoms (80%) represented by Naviculaspp., Nitzschia spp. and Amphora spp. (Fig. 6). Only 21% and 12% were planktonic algae{Melosira, Cyclotella and Actinocyclus) and microflagellates respectively.

Algal concentrations ranged between 2 x 10̂ to 4 X 10^ceils/ml. POM averaged 60-9 ±llmg/I (Fig. 7). PON levels were averaged 1-01 ± 0-2mg/l, and the carbon to nitrogen ratiowas 7-05 ±0-65 (Table 1).

Unit C

Algal population and POM levels in unit C consisted of a mixture of effluents from three PVCponds. Algal concentration ranged between 15 x la* and 40 x 10"*cell/ml and POM levelsaveraged 70 ± 7. PON levels were 1-06 ± 0-4mg/l, and the carbon to nitrogen ratio were 7-04±0-99 (Table 1).

Algal fluctuations did not occur in unit C, or in the sedimentation pond, because of thecompensating effect of the three PVC-hned ponds. The difference in POM concentrations

Effluent water as food for Pacific oyster 539

200 r

10

Figure 7. Particulate organic matter profile in the sedimentation pond and PVC-lined pond.

between unit C and the sedimentation pond can be attributed to the replenishment of freshsea water and/or sedimentation processes in the pond.

Discussion

We suggest that the reasons for better performance of the oysters in the sedimentation pondwater are higher algal diversity, additional nutritious food consisting of attached benthicdiatoms, and stable algal concentration.

In our study we found that greater algal diversity was well correlated with higher growthindices. In the PVC-lined ponds algal blooms consisted mainly of one dominant speciesfollowed by small quantities of other species. In Unit C, the water consisted of a mix of algaefrom the three PVC-lined ponds. In the sedimentation pond, the algal population consistedof a mixture of planktonic species from the PVC-lined ponds and a significant contribution ofdiverse benthic diatoms attached to PIM. The higher CI and growth rates demonstrated bythe animals in the sedimentation pond vs unit C and for unit C vs the PVC-lined pond clearlyshow that a more diverse diet improves the performance of the oysters.

The positive effect of a mixed algal diet on bivalve growth rates is well known (Walne1970, 1974; Langton & McKay 1974; Epifanio 1979). Epifanio (1979) demonstrated highergrowth rates with mixed algal diets than with diets based on one species. In his study he foundthat although the chemical content, i.e. percent protein, lipid and carbohydrate, might havebeen the same, the nutritive value of the diets was different. He speculated thatgrowth-promoting micronutrient and/or digestibility may account for differences in nutritivevalue of one alga species over another but was at a loss to explain the 'synergistic nutritionalinteractions among algae in diets yielding high growth'. The explanation for this effect wasfound to be with the nutrient balance which was more efficiently obtained in mixed diet(Webb & Chu 1982; Dunbar 1991).

The best overall performance of the oysters was seen in the sedimentation pond wherebenthic diatoms attached to the PIM made up the major portion of their diet. It was found

540 M.ShpigeUtal.

that 80% of the total stomach contents of the oysters from the sedimentation pond wereattached to benthic diatoms, which suggested that the main food source of those oysters wasprovided by PIM. It is well known that relatively low concentrations of PIM in the sestonwhen fed in combination with an algal diet enhance growth rates of shellfish. This effect hasbeen attributed to stimulating clearance and ingestion rate (Foster-Smith 1975; Winter 1976),increasing absorption efficiency by making nutritious dissolved organic matter and/orbacteria digestible (Murken 1976; Bricelj 1984; Urban & Langdon 1984), and utilization oforganic sediments (Bricelj 1984). In the integrated systems, all three units exhibited the samePIM levels, and the differences in growth rates among the units cannot be attributed to thesuspended sediment/7er .ye. The only difference between the units is the higher diversity andquantity of benthic diatoms attached to the PIM in the sedimentation pond. Benthic diatoms(such as Amphora sp., Fragilaria sp., Navicula sp., Cocconeis sp. and Nitzchia sp.) are knownas a natural food for oysters in many regions in the world, e.g. the Arcachon Basin, France(Chertiennot-Dinet & Guillocheau 1987; Gouleau 1988), the Marennes-OIeron Basin,France (Heral, DesIous-PaoH & Razet 1984), Georgia, USA (Stevens, 1984). Heral etal.(1984) pointed out that in nature, adult oysters preferred phytobenthos, especially diatomsliving in the sediment-water interface, as a main part of their diet.

In total we recognized 52 distinct taxa of diatoms (Table 2). AU these taxa with theexception of one genus have been found in the benthos of the nearby Gulf of Aqaba (Lee etal. 1988). Species belonging to eight genera. Amphora, Asterionella, Cyclotella, Fragillaria,Navicula, Nitzschia, Pleurosigma, and Synedra accounted for >95% of the diatoms in thesamples in the integrated system all of which were present in the sedimentation pond. Manyof the genera that have been found to be common to the Gulf of Aqaba were absent or rare inour samples (Lee, Erez, Ter-Kuile, Lagziel & Burgos 1988). Little attention has been givento the physiological ecology of tropical and semi-tropical littoral marine diatoms so thatinterpretation of the absence or presence of particular diatom taxa is not possible at present.We can speculate that the ponds are fed nutrient-rich water which selects species which canthrive in such enriched conditions. It is interesting to note that the genus Cyclotella found inour samples has heretofore only been observed in northern temperate oligotrophicfreshwater lakes. There is no doubt about the identification because Dr E. Theriot, whoidentified it, is an acknowledged expert on this genus.

The contribution of a stable algal concentration to the performance of the oysters can beseen when CI and growth rate in unit C are compared wilh the PVC-lined pond (Fig. 2),where POM reached extremely high levels and fluctuated widely (Fig. 7). Filter feeding ofbivalves has been shown to be a function of cell concentration. Bivalves regulate theirpumping so as to maintain the optimal filtration rate (Winter 1978). Bivalves show a rapid risein pumping rate to a plateau level with increasing algal concentration. In higher concentra-tions, filtration rate, ingestion rate and absorption efficiency are drastically reduced, withexcess material being rejected as pseudofaeces (Winter 1978). In low concentration, forexample, during bloom crashes, oysters are unable to filter sufficient food and starvationoccurs. In both cases, growth will be arrested. Throughout the year (Fig. 8), and during theexperiment, algal fluctuations did not occur either in unit C or in the sedimentation pond.POM levels during the experiment averaged 50 ± 7mg/l and 70 ± 11 mg/1 in thesedimentation pond and unit C respectively. In the PVC pond, POM levels were extremelyhigh and averaged 135± mg/1. It is suggested that such extremely high POM concentration

oT—

X

ECO_ 1—1LUo

o

1.6-

1.4-

1.2-

1,0-

0.8-

0.6-

0.4-

O 0.2-

Efftuent water as food for Paciflc oyster 541

• PVC pondE pond

I—I—r-~T—I—\—I—I—I—I rDEC FEB APR JUN AUG OCT DEC

MONTH

Figure 8. Average microalgal concentration in the sedimentation pond and in the PVC-lined pond during 1988 (afterShpigel & Fridman 1990).

has reduced absorption efficiency and has probably slowed down oyster growth. The abovecombined with starvation during bloom crashes, contributed to the poor performance of theoysters in the PVC-lined pond water.

In summary, oysters grown in effluent waters of a semi-closed intensive fish culture systemdemonstrated different levels of performance although abiotic parameters were the same inall water sources. The higher growth rate observed for oysters grown in mixed water comingfrom several PVC-lined fish-ponds over that for oysters in water from one PVC-linedfish-pond may be explained by both; the mixed algal diet and stable POM available from theformer. Oysters supplied with water from an earthen sedimentation pond grew significantlyfaster and showed better condition indices than oysters grown in water from the PVC-linedponds. Here a high diversity of mixed diet, consisting mainly of benthic diatoms attached tothe PIM as well as stable algae concentration, provided an excellent food supply for theoysters.

Further studies are required to isolate and determine experimentally those algae specieswhich are responsible for the better growth rates and the proportion of each alga necessary inthe diet. In addition, the contribution of the benthic diatoms in the sediment-water interfacein terjns of energetics of the oysters should be studied further.

Acknowledgements

We are grateful to Drs Charles Reimed and Edward Theriot, Academy of Natural Sciences ofPhiladelphia, for very fruitful taxonomic and physiological discussion of the results of thisstudy. Mr Soohoo was a City College Scholar during this study. We also thank Bruce Hopkinsfor his comments and suggestions. Ms Marta from CCNY Salazar assisted us withphotography. A. Davidson and I. Cohen from IOLR assisted us with sampling and analysis.The study was supported by the Ministry of Energy and Infrastructure as part of themariculture research and development grant provided by the Israeli government. The editingof the anonymous reviewer is greatly appreciated. Some costs were born by a grant from theH.B. Cantor Foundation and PSGCuny Grant 6-6601.

542 M. Shpigel et al.

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