Shorter immersion times increase yields of the blacklip pearl oyster, Pinctada margaritifera...

Ž .Aquaculture 187 2000 299–313www.elsevier.nlrlocateraqua-online

Shorter immersion times increase yields of theblacklip pearl oyster, Pinctada margaritifera

ž /Linne. , from spat collectors in Solomon Islands

Kim J. Friedman a,b,), Johann D. Bell a

a ICLARM Coastal Aquaculture Centre, PO Box 438, Honiara, Solomon Islandsb School of Marine Biology and Aquaculture, James Cook UniÕersity, TownsÕille, Queensland 4811, Australia

Received 22 July 1999; received in revised form 11 December 1999; accepted 7 January 2000

Abstract

Ž .In Solomon Islands, ‘‘spat’’ juveniles of the blacklip pearl oyster, Pinctada margaritifera,suffer high levels of mortality while on collectors, presumably due to predation by ranellidgastropods, xanthid and portunid crabs, and flatworms that also settle to the collectors from theplankton. This study tested a new strategy for reducing the mortality of spat on collectors. Theexisting practice of leaving spat on collectors for 6 months was compared with removal of spatafter 3, 4 or 5 months, followed by the rearing of the spat in panel nets to facilitate removal ofpredators. In one of two field experiments, there were significantly fewer spat on collectors aftersix months than after 3–4 months. A similar, but non-significant, trend occurred for the other fieldexperiment. A tagging experiment showed that survival of spat left on collectors between months4 and 6 was 58%. By contrast, spat removed from collectors after 3 and 4 months at a mean size

Ž .of )15 mm dorso-ventral measurement DVM , and reared in panel nets, had mean rates ofsurvival of 82 and 93%, respectively. Provided spat are a mean size of )15 mm DVM whenremoved from collectors, and there is adequate husbandry of spat in intermediate culture systems,greater numbers of blacklip pearl oysters can be produced in the western Pacific by using a3–4-month cycle for collecting spat. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Pinctada margaritifera; Spat collectors; Predation; Survival; Immersion

) Corresponding author. Fisheries Western Australia, West Coast Drive, PO Box 20, North Beach, WesternAustralia 6020, Australia. Tel.: q61-8-9246 8425; fax: q61-8-9447-3062.

Ž .E-mail address: [email protected] K.J. Friedman .

0044-8486r00r$ - see front matter q2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0044-8486 00 00311-2

( )K.J. Friedman, J.D. BellrAquaculture 187 2000 299–313300

1. Introduction

It is now apparent that commercially viable numbers of spat of the blacklip pearlŽ .oyster Pinctada margaritifera can be collected at some sites in the open reef systems

Žof the western Pacific Friedman, 1998; Friedman and Bell, 1996, 1999; Friedman et al.,.1998 One of the features of spat collection in such systems, however, is the high

proportion of dead individuals on the collectors at harvest. For example, Friedman et al.Ž .1998 found that 42% of spat present on collectors immersed for 6 months were dead.Much of this mortality appears to be due to predation by the ranellid gastropods,Cymatium spp., portunid crabs and flatworms, which recruit to collectors from the

Žplankton together with the pearl oyster spat Dayton et al., 1989; Newman et al., 1993;.Friedman and Bell, 1996 . The thin shells of P. margaritifera spat and juveniles make

Ž .them vulnerable to predation Gervis and Sims, 1992; Friedman and Southgate, 1999 ,Ž .and predators such as Cymatium spp. grow rapidly after settlement 0.8–0.9 mm a day ,

Ž .and have high consumption rates of tropical bivalves Govan, 1995 .ŽAs predation pressures are likely to increase over time collectors recruit more

.predators and individual predators increase in size , we postulated that one of the mainreasons for the high proportion of dead spat on collectors recorded by Friedman et al.Ž .1998 was the 6-month duration of immersion. To test this hypothesis, we compared thenumbers of live spat on collectors after immersion times ranging from 3 to 6 months.We also tested whether greater numbers of live spat could be produced for farmingoperations by removing spat from collectors at 3, 4 and 5 months and rearing them inintermediate culture, compared to leaving them on collectors for six months.

In the course of these experiments, it became apparent that the numbers of spat on acollector at a particular time reflected two processes:the number of cohorts that settled,and the mortality of spat. To partition these processes, we conducted another experimentin which spat present on collectors after four months were tagged. This enabled us todistinguish the mortality of existing spat from the number of newly settled spat duringthe ensuing period of immersion.

2. Methods

2.1. Effects of immersion time of collectors on harÕest of spat

Two experiments were conducted to examine the effects of immersing collectors fordifferent periods on the abundance of live spat on collectors. Both experiments werecarried out between December 1995 and June 1997 in Noro and Gizo in the Western

Ž .Province of Solomon Islands Fig. 1 .In both experiments, spat collectors were suspended from 100 m longlines made of

12 mm polypropylene rope. Longlines were placed at a depth of 3–4 m as recommendedŽ . Ž . Ž .by Shirai 1970 , Cabral et al. 1985 and Sims 1993 . This was achieved by attaching

Ž .the longline to buoyed dropper lines anchor lines of 10 mm polypropylene rope withŽ .one end attached to cement blocks on the substratum at depths of 28 to 45 m . The spat

( )K.J. Friedman, J.D. BellrAquaculture 187 2000 299–313 301

Fig. 1. The two regions where P. margaritifera spat were collected in the Western Province of SolomonIslands. Solomon Islands is situated South East of Papua New Guinea.

Ž .collectors used were similar to those described by Coeroli et al. 1984 and Hauti et al.Ž . Ž .1987 . They were made of black plastic 55% shademesh Ogura Trading, Japan andconsisted of either a 1.6 m2 sheet of shademesh or four, 0.2 m2 pieces of shademesh. Inboth designs, the shademesh was folded ‘concertina’ style, and tied onto a 1.15 m

Ž .monofilament line see Friedman et al., 1998, for details .In the first experiment, we examined variability in abundance of live spat on sets of

collectors immersed at the same time, but harvested sequentially after 3 to 6 months.Ž . 2Forty four sets of 10 of the 1.6 m shademesh ‘‘sheet’’ collectors were deployed in the

Ž .second week of December 1995 at five sites in Gizo and four sites in Noro Fig. 1 . Tenof the collectors were then harvested at random from each of the nine longlines after

Ž .103, 133, 163, 194 days hereafter referred to as 3, 4, 5 and 6 months .Spat harvested from collectors at 3, 4 and 5 months were grown-out in panel nets

Ž .Gervis and Sims, 1992 measuring 45 cm=75 cm. The mesh size of the panel nets wasvaried depending on the size of spat. Individuals -10 mm dorso-ventral measurementŽ .DVM, Nicholls, 1931 were held in nets with 2=5 mm mesh, those measuring 10–20mm DVM were held behind 6 mm mesh, and oysters )20 mm DVM were retainedwith mesh of 12 mm. Each panel net contained 20–25 oysters.

All panel nets were deployed in the lagoon adjacent to ICLARM’s Nusa Tupe fieldstation at Gizo. Panel nets were held at a depth of 9 m and cleaned by divers fortnightlyto remove algal growth and predators. When the last batch of collectors was harvestedafter 6 months, the mortality and growth of juvenile oysters in the panel nets wasmeasured.

The second experiment was similar to the first one, but differed in three ways. First,the smaller ‘four piece’ collectors were used instead of the larger 1.6 m2 ‘‘sheet’’


collectors. Second, collectors were immersed at four sites at Gizo and five sites at NoroŽ .for marginally longer periods 104, 135, 165 and 196 days . Thirdly, the spat were not

placed in panel nets for grow-out after removal from collectors.In both experiments, we measured the number of live and dead spat removed from

Ž .each collector, and the DVM of each spat to the nearest mm . The abundances ofpotential predators of juvenile pearl oysters were also recorded. These were Cymatiumspp.; portunid crabs, including Thalamita quadridens; xanthid crabs, including Gail-lardiellus orientalis; and flatworms. Counts of crabs were combined into 10 mm sizeclasses based on carapace width.

2.2. Assessing mortality of spat and the extent of subsequent settlement

For this experiment, 20 ‘four piece’ shademesh collectors were deployed at theŽ .beginning of October 1996, at four sites in Gizo lagoon. After 118 days 4 months ,

Ž .divers enclosed the collectors in fine-meshed 1=1 mm bags and brought them toshore, where the collectors were teased apart carefully so that spat and predatorsassociated with them could be measured. Coloured threads were then glued to one valve

Ž .of each spat using ‘superglue’ Loctite 454 gel without removing the oyster from itsposition on the shademesh. A total of 134 spat were tagged in this way. Each collectorwas re-assembled to its original shape, enclosed in a fine-meshed bag and returned to thelongline. The mesh bag was removed after 24 h. This procedure ensured that allpredators that settled on a collector, except fish, were not lost during re-deployment andremained associated with the collector throughout the experiment.

Ž .After 184 days 6 months , collectors were harvested from all four sites. For eachŽ .collector, we counted the number of tagged live and dead spat, the number of smaller

untagged spat and the abundance of each predatory species. We also measured the sizeof potential predators and spat.

2.3. Analysis of data

To examine spatial and temporal variation in abundance of spat and predators in theŽ .first two experiments, we used a 2-way ANOVA site=month . Each variable was

analysed separately for Gizo and Noro. To test for variation in the size of live spat andpredators among months, we pooled data across sites within regions for each month andused a 1-way ANOVA. Separate analyses were done for Gizo and Noro.

In the experiment to measure survival of spat, and the extent of additional settlementbetween months 4 and 6, we used a repeated measures ANOVA for the two sampletimes at the four sites. A t-test was used to test for variation in sizes of Cymatium spp.between the two sampling times.

Where ‘‘site’’ was used in an ANOVA model, it was treated as a random factor. Allother factors were treated as fixed factors. Because ‘‘settlement’’ was often patchy in

Ž .space and time, not all data sets had homogeneous variances Cochran’s test ; data wereŽ .transformed to log xq1 in such cases. When variances remained heterogeneous after10

transformation, we used ANOVA but set As0.01 to compensate for the increased


Ž .likelihood of a Type 1 error Underwood, 1981 . Significant differences among meanswere identified using Tukey’s HSD test.

3. Results

3.1. Effects of immersion time on the harÕest of spat

3.1.1. Experiment oneBetween December 1995 and June 1996 a total of 1461 P. margaritifera spat were

Ž .collected over four harvests means4.1 per collector"0.18 S.E., ns360 . In general,there was a progressive decline in both the total number of spat, and the number of live

Ž .spat, harvested from collectors after 3, 4, 5 and 6 months at both Gizo and Noro Fig. 2 .

Ž . Ž . Ž .Fig. 2. Total blank and live solid abundance "S.E. of P. margaritifera spat harvested from collectorsŽ .after 3, 4, 5 and 6 months at Gizo and Noro in Experiment One. The mean size DVM"S.E. of live spat

Ž .harvested each month ––v –– is also shown.

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Table 1Ž . Ž .Results of the Two-way ANOVAs for effects of site random factor and time fixed factor on the number of live P. margaritifera spat on collectors immersed for 3,

Ž .4, 5 and 6 months at Gizo and Noro. Analyses were carried out on data transformed to log xq1 , but real means are shown at the bottom of each table. Means10Ž . Ž ."S.E. underlined do not differ significantly As0.05 Tukey’s HSD


At both regions, the number of live spat harvested after 6 months was significantlyŽ .lower than the number collected after 3 months Table 1 . This trend was consistent at

all sites within each region, i.e., there was no significant interaction between site andŽ .month in the analysis Table 1 . There were, however, significant differences in the

Ž .numbers of live spat collected at each site in both regions Table 1 .The mean size of P. margaritifera spat did not always increase from one month to

Ž .the next Fig. 2 , indicating that spat continued to settle on collectors between the fourthand sixth month of the experiment. Length frequency data showed that the reason whymean size of spat did not increase substantially between month 4 and month 6 was that

Ž . Ž .small recently settled spat continued to be found on collectors at these times Fig. 3 .

Ž .Fig. 3. Size-frequency distributions DVM for spat of P. margaritifera removed from collectors after 3, 4, 5and 6 months at Gizo and Noro during Experiment One. Note 3 month graphs have had the Y-axis truncated inorder to accommodate the large abundance of small size recruits. The X-axis comprises of animals from within3 mm size classes except for the first bar which includes spat 1–7 mm in size.


Table 2Variation in mean abundance of predators on collectors after 3, 4, 5 and 6 months at all sites combined forGizo and Noro in Experiments One and Two. Means with the same letter do not differ significantly by

Ž .Tukey’s HSD test performed following a 2-way ANOVA site=month for each variable from each region ineach experiment.

Ž . Ž .Experiment One 1995r1996 Experiment Two 1996r1997

Gizo Noro Gizo Noro

Months 3 4 5 6 3 4 5 6 3 4 5 6 3 4 5 6

Cymatium spp. 2.1 1.6 1.5 1.5 2.4 1.4 1.2 0.7 0.9 1.3 1.9 2.4 1.2 1.4 1.2 1.1a a a a a a a a a ab ab b a a a a

Crabs 2.3 1.6 3.6 3.3 1.9 3.6 2.4 3.1 1.3 1.6 2.2 3.8 3.5 5.4 5.0 6.2) ) ) )ab a b b a a a a a a b b a a a a

Flatworms 2.3 1.8 1.7 1.7 1.1 1.8 1.0 1.0 0.5 0.3 0.2 0.4 1.2 0.8 0.6 0.6a a a a a a a a a a a a a b b b

) Ž .Indicates that data were heterogeneous after transformation log xq1 .10

The abundance of predators varied among sites, but seldom among months. Forexample, large numbers of Cymatium spp. were found after three months at one site in

Ž .Noro means5.2 per collector"1.38 S.E., ns10 , however, abundances were notŽ .significantly different among sites over the duration of the experiment Table 2 . At

Ž .Fig. 4. Mean shell size mm"S.E. of Cymatium spp. associated with collectors deployed for 3, 4, 5 and 6Ž . Ž .months at Gizo solid and Noro blank in Experiments One and Two.

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Table 3Summary details and results of experiments to assess and survival of spat of P. margaritifera removed from collectors and grown-out in panel nets

Type of spat Duration of grow-out n Number Spat Mean initial Mean growth Meanof panel per size DVM increment Survival

Ž . Ž .nets net mm "S.E. DVM mm % "S.E.Ž ."S.E.

Ž . Ž . Ž .3-month harvest 29r3r96 to 29r6r96 240 12 20 16.8 0.4 19.7 0.4 81.7 3.10Ž .3 months

Ž . Ž . Ž .4-month harvest 29r4r96 to 29r6r96 200 8 25 19.6 0.5 1.6 0.4 93.5 1.30Ž .2 months

Ž . Ž . Ž .5-month harvest 29r5r96 to 29r6r96 300 12 25 25.0 0.6 4.6 0.2 97.3 0.01Ž .1 month

Ž . Ž . Ž .Small shell sizes 29r3r96 to 29r6r96 336 6 56 7.3 0.2 16.0 1.0 53.0 1.93Ž .-10 mm 3 months


Gizo, crabs were found in significantly greater abundance as immersion time ofcollectors increased; there were significantly more crabs on collectors after 6 months

Ž .than after four months Table 2 . There were no significant differences in abundance ofŽ .flatworms among harvests Table 2 .

Ž .The mean size of Cymatium spp. was significantly F s13.617, P-0.0013, 325Ž .larger at 4, 5 and 6 months than at 3 months at Gizo Fig. 4 , but there was no

Ž . Ž .significant difference Ps0.09 in size among months at Noro Fig. 4 . Numbers ofŽ .crabs in larger size classes )20 mm also increased as a function of immersion time:

Ž29 crabs )20 mm carapace width were found on collectors harvested at 3 months 12. Ž .Gizo, 17 Noro compared to 42 crabs )20 mm at 6 months 25 Gizo, 27 Noro .

Ž . Ž . Ž .Fig. 5. Total blank and live solid abundance "S.E. of P. margaritifera spat harvested from collectors atŽ .3, 4, 5 or 6 months from Gizo and Noro in Experiment Two. The mean size DVM"S.E. of live spat

Ž .harvested each month ––v –– is also shown.


Survival rates of P. margaritifera spat removed from collectors and grown-out inŽ .panel nets Table 3 were greater than for spat remaining on collectors. For example,

spat removed from collectors at a mean size of 17 mm DVM and placed in panel netshad a mean survival rate of ;82% three months later, and those removed after 4months at a mean size of ;20 mm DVM had a mean survival rate of ;93% two

Ž .months later Table 3 . By contrast, the number of spat on collectors after 6 months atGizo and Noro averaged only 52.1% and 23.6% of those present at 3 months,respectively.

3.1.2. Experiment twoIn this experiment, 266 P. margaritifera spat were collected during the four harvests

Ž .means0.74 per collector"0.06 S.E., ns360 . There was an increase in the totalnumber of spat and number of live spat per collector between months 3 and 4, followed

Ž .by declines in abundance in months 5 and 6 Fig. 5 . Differences in the number of liveŽ .spat among immersion times were marginally significant F s3.858, Ps0.0501 at3,144

Ž . Ž .Gizo and non-significant Ps0.18 at Noro Table 1 .Ž .Analysis of the mean size of P. margaritifera spat Fig. 5 provides further evidence

for a relatively large recruitment of spat between months 3 and 5, i.e., the mean size ofŽ .spat did not vary in line with average growth rates see Table 3 between months 3 and

Ž .5. This recruitment occurred at Gizo and Noro Fig. 5 , despite the distance separatingŽ .the two regions 35–45 km . Recruitment of spat had evidently finished by month 6 as

Ž .the average size of spat was significantly greater F s4.618, P-0.01 at 6 months3, 98

than at 3, 4, or 5 months at Gizo. A similar trend occurred at Noro but it was notŽ .significant F s1.713, Ps0.18 .3, 46

As for Experiment One, there were few significant differences in abundance ofpredators over time. At Gizo, abundances of Cymatium spp. were significantly greaterafter 6 months than after 3 months, and crabs were found in significantly greater

Table 4Ž . ŽResults of the repeated measures Two-way ANOVA for the effects of month fixed factor and site random

. Ž .factor on abundance of live P. margaritifera spat in the tagging experiment. Data transformed to log xq110

were used in the ANOVA, but real means are shown in the table. Pairs of means with an asterisk differŽ .significantly P -0.05 Tukey’s HSD

Source of variation df MS F P

Month 1 0.623 10.806 0.046Sites 3 0.120 0.961 0.416Month=Sites 3 0.058 4.475 0.006Residual 76

Sites One Two Three Four

( )Means "S.E.Ž . Ž . Ž . Ž .After 4 months 1.1 "0.28 1.9 "0.30 1.9 "0.53 1.7 "0.36Ž . Ž . Ž . Ž .After 6 months 0.5 "0.20 0.7 "0.23 1.4 "0.45 1.2 "0.27

) )


Ž .numbers after 6 months than after 3 and 4 months Table 2 . Numbers of flatwormsŽ .were significantly greater after 3 months than after 4, 5 and 6 months at Noro Table 2 .

Ž .The mean size of Cymatium spp. was significantly greater F s10.54, P-0.0013, 501Ž .after 6 months than after 3 months Fig. 4 . As in Experiment One, crabs )20 mm

Ž . Ž . Ž . .Fig. 6. Mean abundance "S.E. of a Cymatium spp., b crabs and c flatworms after spat collectors hadŽ . Ž . ) Žbeen immersed for 4 months blank and 6 months solid at four sites in Gizo. denotes pairs of means four

.and 6 months that differ significantly by Tukey’s HSD test.


Ž .carapace width were found in greater numbers after 6 months 15 Gizo, 22 Noro thanŽ .after 3 months 7 Gizo, 7 Noro .

3.2. Assessing mortality of spat and the extent of subsequent settlement

Ž .Seventy eight 58% of the 134 P. margaritifera spat tagged at 4 months were alivewhen collectors were harvested at 6 months. Analysis of variation in numbers of livetagged spat among sites showed that changes in mean abundance of spat per collector

Ž .between 4 and 6 months were significantly different at only two of four sites Table 4 .When collectors were removed after six months, only 20 dead tagged shells were

recovered. Another 36 shells could not be found, presumably because they weredestroyed by predators or fell from the collectors. Spat which survived until collectors

Ž .were removed at 6 months had a mean size of 20.74 mm DVM"0.92 S.E. ns78 at4 months when the experiment was started. Thus, spat that survived were on average 7mm larger at 4 months than those that did not. Spat that remained alive over the last twomonths of the experiment had a mean growth increment of 12.05 mm DVM"0.70 S.E.,Ž . Ž .ns78 during that period ranges2–25 mm .

Ž .Collectors harvested in the sixth month held a total of 90 new untagged spat, ofŽ . Ž .which 76 84.4 % were alive at harvest. These spat had a mean shell size DVM of

6.85 mm"0.60 S.E.Fewer Cymatium spp. were found on collectors than in Experiments One and Two.

Nevertheless, abundance of Cymatium spp. increased significantly from month 4 to 6Ž .Fig. 6 . Due to new settlement, the mean size of Cymatium spp. was significantly

Ž . Ž .smaller t s2.14, P-0.05 at 6 months means14.93 mm"0.96 S.E., ns69 than85Ž .at 4 months means19.67 mm"2.33 S.E., ns18 . Abundance of crabs increased

Ž .significantly between months 4 and 6 at only one of the four sites Fig. 6 ; however,crabs with a carapace width )20 mm increased from 5 to 43 individuals over the2-month period. By contrast, the abundance of flatworms decreased significantly at one

Ž .of the four sites between months 4 and 6 Fig. 6 .

4. Discussion

This study provides convincing evidence that production of spat of blacklip pearloysters from habitats like those in the Western Province of Solomon Islands can beimproved by deploying collectors on two 3–4 month cycles, rather than for a singlesix-month cycle. Over a period of six months, the twin processes of spat continuing tosettle, and mortality of previously collected spat, resulted in fewer spat than thealternative strategy of removing collectors after 3–4 months and growing-out the spat inintermediate culture. To be specific, survival of spat left on collectors between months 4and 6 was 58%, whereas survival of spat removed from collectors at 4 months andgrown in panel nets for two months was 93%. In addition, there is no penalty inremoving collectors after 3–4 months; they can be cleaned of spat and fouling biota and

Ž .redeployed immediately Friedman, 1998 .


The circumstances that might mitigate against adoption of a 3–4 month collectionŽ .cycle include: 1 marginal improvement in survival of spat in intermediate culture, such

that the increased yield of spat does not cover the additional costs of labour to make twodeployments instead of one, nor the cost of the longer grow-out period in intermediate

Ž .culture; and 2 very late settlement of spat, resulting in small sizes of spat on collectorsafter 3–4 months.

Our data show that, provided spat have a mean size of )15 mm DVM, survival ratesof 82–93% can be expected from intermediate culture. Such rates should outweigh theadditional costs outlined above, especially when spat are not particularly abundant andthe relative value of each spat is high. The ‘‘late’’ settlement of spat during a 3 – 4month collection cycle is likely to be a greater problem due to inter- and intra-annual

Ž .variation in abundance of P. margaritifera Coeroli et al., 1984; Cabral et al., 1985 .Such variation was evident in this study. For example, few spat settled during Experi-ment Two, and those that did settle evidently did not do so early in the 3–4 month cycle.The most likely explanation for this variation was the adverse effects of cyclones in late1996–early 1997, which created conditions unsuitable for settlement and survival of spatŽ .Friedman and Bell, 1999 . The strong likelihood of variation in availability of spat

Ž .during the main settlement ‘‘season’’ Friedman et al., 1998 does not present a majorproblem. Rather, late settlement of spat during the 3–4 month collection cycle can beovercome by subsampling collectors prior to harvest to confirm that the mean size of

Ž .spat is large enough for intermediate culture i.e. )15 mm DVM .This study provides further circumstantial evidence that much of the mortality of spat

on collectors is due to predation. In particular, collectors removed after 6 months hadrelatively high numbers of large predators, and spat removed from collectors and rearedin panel nets, which were inspected regularly to remove predators, had better survivalthan spat left on collectors. Alternative hypotheses for major mortality of spat on

Ž .collectors are overcrowding Hauti et al., 1987; Preston, 1990 , and fouling by algae andŽ .ascidians Sumpton et al., 1990 . Crowding of spat was unlikely to have been the cause

of mortality in this study as total numbers of spat were low compared to those reportedŽ . Ž .by Hauti et al. 1987 and Preston 1990 . However, experiments are needed to ascertain

the relative importance of fouling and predation on survival of spat of P. margaritifera.In conclusion, this study shows that production of spat of P. margaritifera for pearl

culture from the ‘‘open’’ reef systems in the western Pacific can be improved byremoving collectors after 3–4 months immersion, and protecting spat from predators in

Ž .intermediate culture systems until they are large enough )65 mm DVM to hang onŽ .chaplets Preston, 1990 for grow-out.

Acknowledgements

We thank Gideon Tiroba and Ruth and Barley White Dunne for their assistance withthe fieldwork. This research was conducted as part of the project entitled: ‘‘Develop-ment of Small-Scale Village Farms for Blacklip Pearl Oysters in Solomon Islands UsingWild Spat’’ funded by the Australian Centre for International Agricultural ResearchŽ .ACIAR . ICLARM Contribution No. 1461.


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Shorter immersion times increase yields of the blacklip pearl oyster, Pinctada margaritifera...

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