Aquaculture, 24 (1981) 63-75 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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CULTIVATION OF THE SYDNEY ROCK OYSTER CRASSOSTREA COMMERCIALIS (IREDALE AND ROUGHLEY) IN PRAWN FARMING PONDS

GREG B. MAGUIRE, BAUGHAN WISELY and MARGARET E. SKEEL

N.S. W. State Fisheries, Brackish Water Fish Culture Research Station, Salamander Bay, N.S. W. 2301 (Australia)

(Accepted 4 November 1980)

ABSTRACT

Maguire, G.B., Wisely, B. and Skeel, M.E., 1981. Cultivation of the Sydney rock oyster Crassostrea commercialis (Iredale and Roughley) in prawn farming ponds. Aquaculture, 24: 63-75.

Sydney rock oysters were grown on continuously submerged trays within estuarine ponds stocked with school prawns (Metapenaeus macleayi). The shell growth rates of these oysters were similar to those obtained on an intertidal lease. In 0.11 ha ponds stocked with oysters at a low density (1560 oysters per pond on three 1.7 m2 trays), very high oyster condition index values were obtained and at a high density (38 200 oysters per pond on fifty 2.5 mz trays), oysters containing meats that were in very poor condition when stocked rapidly reached marketable condition. Oyster mortality was, however, a serious problem in one trial. The ponds showed promise as spat collection sites and aho

produced excellent broodstock for artificial, oyster larval rearing trials.

INTRODUCTION

Polyculture, the simultaneous cultivation of two or more organisms, is often used to increase the yield from an aquaculture site. Prawns have been commercially farmed with milkfish (Chanos chanos) in the Philippines (Caces-Borja and Rasalan, 1968) and with the red alga Gruciluria or milkfish in Taiwan (Chen, 1976). Experimental polyculture of prawns and various species of fish has been attempted by Raman (1968), Liao (1977) and Tatum and Trimble (1978). However, most prawn farming projects have been based on monoculture (see Wickins; 1976).

Oysters have potential for polyculture with prawns because they are sus- pension feeders and are therefore unlikely to compete with prawns for food. Artificial ponds have frequently been used for oyster farming but mostly as sites for collecting spat and improving oyster condition (Carriker, 1959). In more recent studies (Hughes-Games, 1977; King, 1977) rapid oyster shell growth has also been obtained in artificial ponds.

0044-8486/81/0000-0000/$02.50 o 1981 Elsevier Scientific Publishing Company

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This study investigated the potential for farming Sydney rock oysters (Crussostrea commercialis) in school prawn (Metapenaeus macleayi) cultiva- tion ponds and arose from the observation that oysters which settled within prawn farming ponds grew rapidly. The work of Wisely et al. (1979 a, b, c, d and e) had also shown that small, continuously submerged oysters could be grown to a valuable size within 3-4 months. McBride and Maguire (1979) noted that school prawns could be successfully fattened in ponds within this period. During the study the value of ponds as sites for improving oyster condition was also investigated.

MATERIALS AND METHODS

Pond design

The polyculture trials were carried out in ponds at the Brackish Water Fish Culture Research Station, Port Stephens, N.S.W. (32”42’S, 152”12’E). The 1.0 m deep, 0.11 ha, tidally flushed, earthen ponds have been described by Maguire and Bell (1981).

Trial 1

This trial (October 1978-January 1979) involved two ponds each con- taining three plastic mesh (25 mm) oyster trays (1.7 m2 per tray) supported 0.8 m above the pond bottoms by fixed timber frames. Air from a single compressor (maximum flow 65 1 per minute) was delivered through 13 mm plastic pipe to two outlets beneath each tray. In both ponds half of the pond volume was replaced during each week of the 12 week trial.

The single layer trays were each stocked with 20 kg (306 oysters per m’) of second grade oysters (Table I). One pond was fertilized with 5 kg of urea and 2 kg of superphosphate every 2 weeks whilst the other was not fertilized. The fertilized pond contained 15 (3.3 m2) netting enclosures (Maguire and Bell, 1981) stocked with school prawns. A maximum of 1.2 kg per week of a pelleted chicken grower diet (23 % protein) was added to the fertilized pond as a prawn diet.

TABLE I

The values and sizes for various commercial grades of Sydney rock oysters (after Wisely et al., 1979 e)

Grade

Plate Second Large spat Small spat

Whole oyster Wholesale value, 1980 weight (g) ($A per 60 kg bag)

> 40 130 29-40 70 22-29 40 18-22 30

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Trial 2

In this trial (February 1979-June 1979) 50 trays (25 m2 per tray) of oysters in one pond (A) were suspended from floating pontoons similar to those used by Wisely et al. (1979 b). The plastic or galvanized wire oyster trays were supported by 200 mm steel mesh that was attached to 6 m floats made of 90 mm PVC pipe. Each pontoon held two trays which were con- tinuously submerged to a depth of 0.22 ‘m. Each tray was stocked with 25 kg (306 oysters per m2) of second grade oysters.

Pond A was also stocked with 55.4 kg of school prawns. Food was provided for the prawns by casting up to 27.2 kg per week of chicken grower pellets between the trays. Fertilizer was added at the same rate as in Trial 1 and the aeration system was similar except that there were two compressors and 80 air outlets for Pond A. For the first 4 weeks the Trial 1 water exchange rate was used but this was trebled for the next 4 weeks, Because fortnightly washing of the oysters failed to prevent a buildup of decaying material on the oysters, they were removed from Pond A after 8 weeks, hosed and transferred to a similarly fertilized pond (B). This was not aerated, only occasionally flushed and not stocked with prawns. The oysters grown on the 50 trays in Pond A and then in Pond B are referred to as the high density pond oysters. Spare oysters not required for the Pond A stocking were grown on three trays in Pond B for the first 8 weeks in a similar manner to those in Trial 1 and are referred to as the low density pond oysters.

Twelve wire trays stocked with oysters at a similar density to the Pond A trays were set up as controls on an intertidal estuarine tray lease similar to those described by Korringa (1976). These trays were covered with 55 mm mesh wire to exclude predators and with 4 mm mesh shade cloth. They are referred to as the control lease oysters.

Sampling methods

In Trial 1, 20 oysters were taken from each tray in both ponds every 2 weeks. For the first 8 weeks of Trial 2, fortnightly samples of 40 oysters per 2.5 m2 tray were taken from four trays in Pond A whilst 30 oysters per 1.8 m2 tray were taken from three control lease trays. After 8 weeks the sampling interval used was every 3 weeks. Where possible, individual trays were only sampled once during Trial 2.

Before being weighed oysters were immersed in sea water (So/oo >28°/oo) for a minimum of 15 min, scrubbed, cleaned of spat and dried with paper towelling. Whole oyster weight and volume readings were obtained by measuring the weight and volume changes after 20-40 oysters were placed in a preweighed, 5 1 cylinder partially filled with sea water. After oyster meats were removed, shell weights and volumes were similarly obtained and the meats superficially dried with paper towelling. Meat volumes and weights were obtained using a preweighed, 250 ml measuring cylinder partially filled

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with sea water. An oyster condition index which measured the percentage of the inner volume of the shell occupied by oyster meat was used (see Galstoff, 1964). It was:

Condition index (%) = Av. oyster meat volume X 100

Av. whole oyster volume - Av. oyster shell volume

Environmental monitoring

Daily measurements of rainfall, pond salinity and pond temperature range were made.

Statistical methods

All comparisons were made using Student’s t tests.

RESULTS

Trial 1

Fig. 1 shows the oyster growth, mortality and mudworm infection rate results for oysters grown in two ponds in Trial 1. Rainfall and pond temper- atures and salinities are also included.

After 6 weeks the meats from oysters in the fertilized pond (Fig. la) were significantly heavier than those from the unfertilized pond (P < 0.05) and were in extremely good condition (Condition index + S.E.% = 72.4 + 2.3). After this peak was attained a 2-4 week decline in condition ensued during which time a partial spawning may have occurred. Subsequent recovery in condition was very rapid in both ponds during weeks 10-12.

There was no significant difference in cumulative oyster mortality (P > 0.05) between the two ponds (Fig. lb). Mortality increased during weeks lo-12 as did the incidence of mudworm blisters caused by the presence of the spionid polychaete Polydora websteri (Fig. lc). Mudworm infection rates were similar in both ponds.

Oyster growth (Trial 2)

Figs. 2 and 3 contain meat and shell growth, mortality and mudworm infection rate results for the three groups of oysters grown in Trial 2. Rain- fall and pond temperatures and salinities are also shown. The oysters used in Trial 2 were in very poor condition when the trial began. The high density pond oysters reached marketable condition (approximately 45% Condition index) within 2 weeks but did not continue to fatten rapidly even after being transferred to Pond B (Fig. 2a, b). They appeared to partially spawn during weeks 11-14 after heavy rain depressed pond salinity (Fig. 3d). They did, however, fatten better than the intertidal control lease oysters. Thus

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NO” DEC JAN 207

lo-

0 ,

2 4 6 6 10 12

I .

0 2 4 6 8 10 12

0 2 4 6 6 10 12

TIME (weeks).

Symbols: 1 mean ? S. E.

a,b- from fertilized pond. w--from unfertilized pond.

CM infection rate for live oysters. o---o for dead oysters (samples pooled),

d c+--c average maximum temp. n---o average minimum temp.

*warmest temp. mcoolest temp.

e W average pond bottom salinity. III total rainfall.

Fig. 1. Growth, mortality and mudworm infection rates for oysters grown in a fertilized and an unfertilized prawn farming pond during Trial 1.

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O 2 4 6 a 10 12 14 FEB MPlR P.PR !“?A”

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0 2 4 6 8 10 12 14

32 1

26 ---____

I

$ v, g24 c

G 2320 I I I I I I I I

0 2 4 6 a

v&+&.;,i-;,:-:;i-::--

30 , I I I I I I I 0 2 4 6 6 10 12 14

TIME (weeks).

Symbols: 3 mean? S.E. t time at which oysters were transferred from pond D to pond A.

a - d -oysters grown in pond D and later pond A (high density).

=--=control oysters grown on an intertidal lease. 0 oysters grown at low densities in pond A.

Fig. 2. Oyster growth in prawn farming ponds and on a commercial, intertidal oyster lease during Trial 2.

69

0 2 4 6 6 10 12 14 17 FEB MAR PlPR MAY

201

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_-._ b . .

..__ *. -*_____-_______l

0 , I I I I I I 1 0 2 4 6 6 10 12 14

0 2 4 6 a 10 12 14

0 2 4 6 6 10 12 14

TIME (weeks).

Symbols: f +

mean -SE.

* time at which oysters were transferred from pond D to pond A.

e- oysters grown in pond D and transferred to pond A(high density).

u control oysters on en intertidal lease.

be-r infection rate for live oysters grown in pond D and transferred to pond A.

* - 4 infection rate for live control oysters from intertidal lease.

zzr live mudworms found in blisters. c-Aaverage maximum temp. LF--o average minimum temp.

l warmest pond temp. m coolest pond temp.

d- average pond bottom salinity. I-j total rainfall.

Fig. 3. Pond water temperature and salinity and oyster mortality and mudworm infection rates in prawn farming ponds and on a commercial, intertidal oyster lease during Trial 2.

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during weeks 2-14 the high density pond oysters had significantly heavier meats and higher Condition index values (P < 0.01) than the control lease oysters. In contrast to the other two groups, low density pond oysters be- came very fat and after 8 weeks had significantly heavier meats and higher Condition index values (P < 0.01) than the high density pond oysters (Con- dition index + SE. % = 60.4 + 0.9 and 47.6 + 2.2 respectively). Some low density pond oysters were used as broodstock for oyster larval rearing trials and they spawned readily.

Comparisons of the shell and total weights (Fig. 2c, d) of high density pond and control lease oysters over weeks 2-14 and high and low density pond oysters after 8 weeks revealed no significant differences (P > 0.05). The high density pond oysters showed an overall increase of 20.1% (8.0% per month) in average shell weight and 19.0% (7.6% per month) in average total weight. However, some of this increase was due to remnants of spat oysters that settled on the oysters during the trial.

Oyster mortality and mudworm infection (Trial 2)

Oyster mortality was high during Trial 2 (Fig. 3a). Few mortality readings were taken for control lease oysters as many were displaced from the trays by persistent wave action during the study (Table II). After 2 weeks 15.9% of high density pond oysters died compared with 9.7% of control lease oysters. The control lease mortality result was the number of oysters found dead expressed as a percentage of the number stocked. This is an underes- timate as some of the oysters displaced from the trays would probably have been dead. This error would be increased if dead oysters were more prone to being displaced than live oysters.

During weeks 2-8 mortality amongst the high density pond oysters was 12.8% (8.6% per month) but was much lower after the oysters were transfer- red to Pond B. For weeks 8-17 it was 10.6% (4.7% per month). The apparent mortality rate for weeks O-17 (Table II) was higher amongst the high density

TABLE II

.Percentages of oysters that died or were displaced from trays by wave action when grown for 17 weeks at high densities in prawn farming ponds or on a commercial, intertidal oyster lease during Trial 2

Oyster group % of oysters % of oysters % of oysters Total

displaced found dead found alive from trays after 17 weeks after 17 weeks

High density pond oysters 0 34.5 65.5 100.0

Control lease oysters 26.0 14.5 59.5 100.0

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pond oysters (34.5%) than the’control lease oysters (14.5%) but this latter result was again an underestimate. Some of the apparent causes of mortality are shown in Table III.

Although many empty mudworm blisters were recorded no live mudworms were found in the control lease oysters. A few live mudworms were recorded in the week 11 and 14 samples from the high density pond oysters (Fig. 3b).

TABLE III

Causes of death for oysters grown at high densities in prawn farming ponds or on a commercial, intertidal lease during Trial 2 (expressed as a % of the total number of dead oysters)

Oyster

group

Hole in Broken shell Mudworm shell (culling or (large (culling) predation) blister)

Unknown Total

High density pond oysters 10.0 0 5.2 84.8 100.0

Control lease oysters 10.3 8.0 3.7 78.0 100.0

Oyster colour (Trial 1 and 2)

During Trial 1 the gills of the oysters in the fertilized pond developed a dark green colouration which remained for most of the trial whilst oysters in the unfertilized pond had only pale green gills. This colouration did not occur during Trial 2 but in Pond B a dense bloom of a flagellate, resembling a ChZoreZZa sp., made the oysters stomach contents green and also tinted the pond water retained within the shell cavity.

Pond spatfall (Trial 1 and 2)

Oysters held in ponds during both trials became covered with small Sydney rock oyster spat. After 8 weeks of Trial 2 the high density Pond A oysters had an average of 9.2 f 1.6 (S.E.) spat attached to each oyster. These spat had grown rapidly to a shell length of l-2 cm. The estimated total number of spat which had attached to the Pond A oysters after 8 weeks was approxim- ately 349 000.

DISCUSSION

The major purpose of this study was to see if the rapid oyster growth rates, obtained by Wisely et al. (1979 b) with oysters continuously submerged beneath floating pontoons in estuaries, could be achieved in prawn farming ponds.

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In Trial 2, oysters suspended in ponds only grew at similar rates to those on a commercial, intertidal lease. In contrast, Wisely et al. (1979b) found that oysters similarly suspended from pontoons in the Port Stephens estuary grew at three times the rate of oysters on a similar lease. In Trial 1, second grade oysters grown in a fertilized pond for 12 weeks did reach a size (average oyster weight + S.E. = 43.0 + 0.7 g) at which they could be sold as plates (Table I). In Trial 2, however, the second grade oysters did not reach plate grade. The slow oyster growth was probably due to slow shell growth as the oyster meats were in good condition.

The most likely explanation for this slow shell growth was that the flow rate of water past the oysters was inadequate. In this study it was considered that the aeration used would cause sufficient circulation and thus tidal flush- ing was restricted to retain nutrients within the pond. Walne (1972) showed that a high water flow rate increased the filtration and growth rates of Crussostrea gigas whilst Malouf and Breese (1977) noted that oysters required a water flow that was inversely .related to its food content. Hughes-Games (1977) obtained rapid C. gigas growth rates in fish ponds with a much higher and more continuous water exchange rate than that used in the present study. King (1977) also obtained rapid C. gigas growth rates but in a non-tidal pond in which growth was related to the amount of water circulation caused by wind action. The Port Stephens ponds were much smaller than the 100 ha pond used by King (1977) and were thus far more sheltered by pond walls. Better Sydney rock oyster growth rates may be obtained in larger ponds which allow better wind circulation or in ponds with a high water exchange rate.

Oysters held in ponds which contained only a small number of trays developed very good meat condition. Although there was no replication of ponds, the results for Trial 1 suggest that pond fertilization aided meat growth. Carriker (1959) discussed several studies which suggested that fertilization improved oyster growth. Oysters grown at a high density in Trial 2 reached marketable condition but did not become as fat as oysters grown at a low density during both trials. This was unfortunate as very fat seconds may be marketed as plates when the latter are scarce. However, the fattening observed in ponds suggests that they warrant further investigation as fattening sites. The number of oysters stocked in the high density pond trial was equivalent to 33.8 oysters per m2 of pond surface area. Anderson (1977) noted that in French fattening ponds (claires), oyster densities of 3-15 per m2 were used. Thus the oyster stocking density used in Trial 2 may have been too high to allow very high condition levels to be reached.

For profitable fattening operations high survival rates are necessary. In Trial 1 the average survival rate for oysters in both ponds over 12 weeks was high (83.4%) and much of the mortality occurred during the final 2 weeks. Although the mudworm infection rate increased during this period the increased mortality was not due to mudworms as few of the dead oysters had mudworm blisters on their shells (Fig. lc). However, during this period

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a maximum pond water temperature of 33°C was recorded (Fig. Id). Such a high water temperature was only recorded once by Wolf and Collins (1979) in a long term study of N.S.W. commercial oyster lease water temperatures. Roughley (1922), Lipovsky and Chew (1972) and Baab et al. (1973) all attrib- uted oyster mortality to high water temperatures.

During Trial 2, mortality amongst high density pond oysters was high (Table II). Mudworm, predation and physical damage during culling were not the major causes of mortality (Table III). During the first 2 weeks of Trial 2, mortality was high in both high density and control lease oysters and thus much of this mortality was probably not caused by pond conditions. During weeks 2-S the observed mortality may have been due to a thick accumulation of decaying organic matter which developed on the upper surface of the high density pond oysters. Hughes-Games (1977) encountered a similar problem and attributed the fouling to “silt and faeces”. After being transferred to Pond B, where no such fouling occurred, the mortality rate was much lower. It should be noted, however, that pond water temperatures declined through- out Trial 2 (Fig. 3~). Lunz (1955) and Elam (1972) both found that disease was a major cause of oyster mortality in estuarine ponds. During this study the only oysters that were suspected of being diseased were two collected from the control lease trays.

Mudworm infection can seriously affect the health and marketability of oysters. In Trial 1 the oysters were already infected with live mudworms before being stocked and a serious level of infection was found after 10 weeks. Lunz (1955) also found serious Polydora infestations in oysters from estuarine ponds. In contrast no live mudworms were apparent when the Trial 2 oysters were stocked and very few were found throughout the 17 weeks of the trial although at times there were large numbers of empty blisters.

Oyster meat colour may also affect the marketability of oysters. The green gill condition observed in the fertilized pond in Trial 1 has been previously recorded in Sydney rock oysters by Wolf (1965). A similar colouration, considered desirable by consumers, occurs in oysters from the French claires and is caused by the diatom Nauicula ostrearia (Anderson, 1977).

The oysters grown in ponds during this study became covered with oyster spat. Experience has shown that attached spat may be killed by leaving the oysters out of water for a few days. However, the spat may be of value and Matthiessen (1972) has described a commercial spat collection operation in an estuarine pond. The attached spat also grew rapidly and the ponds may have value as a nursery for artificially reared oyster seed. Finally, the ponds when stocked with oysters at a low density were a very good source of broodstock for artificial rearing trials.

CONCLUSION

Rapid oyster shell growth rates were not obtained in the polyculture trials. In any future trials it may be necessary to use larger ponds with better

74

wind circulation or to artificially increase the rate of water flow past the oysters. Oyster mortality was a serious problem in the large scale polyculture trial and much of this may have been due to accumulation of decaying material on the oysters. As this fouling only occurred in the heavily fed pond stocked with prawns the prospects for farming oysters and prawns together in ponds are not good unless different pond management techniques can be devised. Should very frequent washing of oysters be necessary economic returns from ponds could be seriously diminished.

The ponds did, however, show promise as fattening sites for Sydney rock oysters and such sites would be valuable if they allowed fattening of oysters during colder months when oyster condition is usually poor. The value of the ponds as spat collection or nursery areas, or as a source of broodstock for oyster larval rearing, warrants further investigation.

ACKNOWLEDGEMENTS

The authors wish to thank J. Holliday, B. Reid, W. Fox and other N.S.W. State Fisheries staff for their assistance. We are grateful to Dr. S.W. Jeffrey for the flagellate identification and to Mr. R. Holcombe for his help and use of his oyster lease.

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Wisely, B., Holliday, J.E. and Reid, B.L., 1979a. Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis = Saccostrea cucullata) I. Growth of vertical clumps of oysters (‘ren’). Aquaculture, 16: 127-140.

Wisely. B., Holliday, J.E. and Reid, B.L., 1979 b. Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis = Saccostrea cucullata) II. Pontoon tray cultivation. Aquaculture, 16: 141-146.

Wisely, B., Holliday, J.E. and Reid, B.L., 1979 c. Experimental deepwater culture of the Sydney rock oyster (Crussostrea commercialis) III. Raft cultivation of trayed oysters. Aquaculture, 17: 25-32.

Wisely, B., Holliday, J.E. and Reid, B.L., 1979 d. Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis) IV. Pilot production of raft oysters. Aquaculture, 17: 77-83.

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Wolf, P.H., 1965. Green-gilled oysters need not be a loss. Fisherman, l(12): 16. Wolf, P.H. and Collins, A.J., 1979. Summary of daily temperature and salinity records

for major oyster-bearing estuaries of New South Wales 1966-1973. N.S.W. State Fisheries Tech. Rep., 107 pp.