Progress in a Sydney rock oyster, Saccostrea commercialis (Iredale and Roughley), breeding program

Aquaculture Aquaculture 144 ( 1996) 295-302

Progress in a Sydney rock oyster, Saccostrea commercialis ( Iredale and Roughley) , breeding program J. A. Nell aT*, A. K. Sheridan b, I. R. Smith a a NSW Fisheries,Port StephensResearch Centre, Taylors Beach, NSW 2316, Austruliu b Project geneticist, NSW Fisheries Research Institute, Cronulla, NSW 2230, Austrulia

Accepted 20 March 1996

Abstract

Four breeding lines of Sydney rock oysters, Saccostrea commercialis, progeny of parents selected for faster growth for one generation, were compared in a 17-month experiment with two control lines (spat produced from non-selected oysters). Oysters from two out of four selection lines were significantly heavier (P < 0.05) than the controls. The improvements in weight gain for the two selection lines grown loose on trays were 2.9 and 8.5%, and for the two slat lines (grown fixed on PVC slats) they were 0.0 and 5.0% for the first generation of selection. These results provide encouragement for the selective breeding of Sydney rock oysters for weight gain. Keywords: Breeding; Growth; Oysters; Selection

1. Introduction

Production of Sydney rock oysters, Saccostrea commercialis, in New South Wales (NSW) declined by 38% from 7411 t in 1981-82 to 5126 t in 1990-91. The downturn in this industry, which relies on natural spatfall, was partly caused by competition from oyster farmers in Tasmania and New Zealand who produce Pacific oysters, Crassostrea gigas. A fast-growing Sydney rock oyster would help the New South Wales (NSW) industry, which has an annual farm gate value of AUS $30 million, to compete with farmers producing the faster-growing Pacific oysters (Nell, 1993). Success in selection for faster growth in oysters has been achieved in Crassostrea uirginica (Paynter and Dimichele, 1990), Ostrea edulis (Newkirk and Haley, 1983; Toro

* Corresponding author. Tel: (+61-49) 821232. Fax: (+61-49) 821107. 0044-8486/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PU SOO44-8486(96)01328-2

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and Newkirk, 1990) and Osrrea chilensis (Toro and Newkirk, 1991). Thus, a mass selection program (Falconer, 1981) of four selection lines in Port Stephens, NSW, was established at the Port Stephens Research Centre in 1990. Larval and spat rearing techniques required, as a minimum, the use of duplicate 1000-I tanks to ensure production of least 200000 spat ( > 3 mm shell height). This study describes the growth comparison of the second generation (one generation of selection) of Port Stephens selection lines with two groups of control oysters.

2. Materials

and methods

2.1. Base population Oysters for the base population were taken in equal numbers from each of the four major oyster-growing estuaries in NSW, Wallis Lake (32”1O’S, 152”29’E), Port Stephens

Qld

GEORGES RIVER

0

Sydney

Fig. 1. Locations

of broodstock

collection

estuaries.

J.A. Nell et d./Aquaculture Table I Number of maximum

possible base population

297

144 (1996) 295-302

oysters contributing

to the first generation

Mass spawning/fertilisation

Females

Males

1 2 3 4 5 6 7 8

26 46 39 23 22 27 21 18

7 9 11 4 2 6 8 4

(32”45’S, 152”lO’E) and the Hawkesbury (33”3O’S, 151”15’E) and Georges (34”O’S, 15 1”O’E) Rivers (Fig. 1). Care was taken to sample only locally caught oysters, not those transferred from other estuaries (Nell, 19931, to attempt to establish a base population with a broad gene pool. Although in many estuaries of NSW farmers grow oysters caught in more than one estuary (Nell, 1993), farmers were requested to supply us with locally caught broodstock only. In February 1990, a series of eight separate natural mass spawnings/fertilisations with a different group of 100 mixed oysters (equal numbers from each of the four estuaries) was carried out over 2 days. A total of 51 males (ranging from 2 to 11 per fertilisation) and 222 females (ranging from 18 to 46 per fertilisation) contributed to the base population (Table 1). When an oyster began spawning, it was rinsed and allowed to continue to spawn alone in a beaker. For each mass spawning, eggs and sperm were separately pooled prior to fertilisation, which was carried out within 1 h or the commencement of spawning (Table 1). Not all oysters spawned and only those that spawned profusely were used.

2.2. First generation Equal numbers of eggs from each of the eight mass spawnings/fertilisations were used to produce a total stocking density of eight larvae ml-’ in a 20000-I tank. Larvae were settled as ‘single seed’ on ground scallop shell chips and reared in upwellers in an outdoor nursery. During larval and nursery culture, numbers were progressively reduced by discarding the slowest growing animals. When the largest spat reached a shell height of 2 12 mm (range 12-20 mm), they were split into two loose and two slat (spat fixed on PVC slats within trays) selection lines. These two growing methods are described by Smith et al. (1995). Oyster lines were established at each of three intertidal sites in Port Stephens (three trays/lines per site>, where they were grown for 18 months. Initial stocking rates for the loose oysters were approximately 1000 loose spat per tray or 240 slat spat per tray (1.8 m X 0.9 m hardwood tray covered with plastic mesh). The stocking density for the loose oysters was maintained at 50% tray coverage (Holliday et al., 1991) by regularly sieving out and discarding the smallest oysters. However, the stocking density for the slat oysters remained unchanged.

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2.3. Second generation

Spawning, fertilisation, larvae and spat rearing conditions were the same as described above unless stated otherwise. Initial stocking rates for the loose oysters were approximately 1000 loose spat per tray or 280 slat spat per tray. The slat oyster growing method used in the second and subsequent generations is as described by Sheridan et al. (1996). In November 1992, the 24 fastest growing loose oysters (on a whole-tray basis) and slat oysters (on a within-compartment basis of 12 compartments per tray) were selected from each tray on a whole-weight basis, i.e. 216 oysters per line as the broodstock for the second generation. The selected broodstock were then placed on one oyster lease for conditioning and spawning in January 1993, when a series of four separate mass spawnings/fertilisations were carried out for each of the four selection lines and two controls. Mass spawnings/fertilisations were completed for two randomly selected lines or controls per day, i.e. the whole procedure was finished in 3 days. Control or non-selected oysters were taken in equal numbers from the same four estuaries as the original base population. The number of oysters induced to spawn per mass spawning/fertilisation ranged from 3 to 24 females and from 3 to 14 males for the selection line oysters, and from 1 to 26 females and from 1 to 6 males for the controls (Table 2). Larvae for each line were reared in duplicate 1000-l tanks initially stocked at 5 ml-‘. Oysters were induced to spawn rather than ‘strip’-spawned because the quality of ‘strip’-spawned eggs is more variable (Nell et al., 1996).

Table 2 Number of maximum possible first-generation the growth comparison experiment)

oysters contributing

to the second generation

(oysters

Selection line or controls

Mass spawning/fertilisation -.

Female

Male

Loose 1

1 2 3 4

19 13 19 13

7 14 8 11

Loose 2

1 2 3 4

24 19 22 14

4 7 8 6

I

2 3 4

19 20 17 3

9 7 4 11

1 2 3 4

7 19 18 18

7 7 8 3

Slat

1

Slat 2

used in

J.A. Nell et ul./Aquuculture

Table 3 Comparison of Port Stephens from August 1993 to January

second generation 1995

Sydney

144 (1996) 295-302

roclc oyster Saccostreu

Selection line or controls

Whole weight ’ at start (mg)

Whole weight at end (g)

Loose 1 Loose 2 Slat I Slat 2 Control 1 Control 2

42.3 f 0.7 40.7 f 0.7 43. I * 0.7 45.1 *0.8 45.9kO.S 46. I * 0.7

35.3 37.2 34.3 36.0 34.0 34.6

k c abc ab a a

* f * * * k

0.23 0.24 0.21 0.25 0.22 0.22

299

commercialis

’

selection lines

Difference ’ at end (%I

bc a c b c ’

2.9 8.5 0.0 5.0 _ _

’ Data are mean *s.e., means with different letters differ significantly (P < 0.05).* Difference (selection line oysters whole weight)/(mean whole weight of both controls)).

2.4. Growth comparison

is (100X

experiment

A growth comparison experiment was established at three sites (Tilligerry Creek, Little Swan Bay and Karuah River) in Port Stephens (Holliday et al., 1991) in August 1993, and terminated in January 1995 (Table 3). Oysters were fixed on PVC slats (Sheridan et al., 1996) to provide a constant and uniform stocking density. There were four replicates per line per site, each replicate comprising one half-tray with 140 oysters fixed on PVC slats. For each selection line or control oysters were taken from the largest size class, which was the same for each line and control. A final grading of spat of all lines was done on plastic mesh screens with diagonal sizes of 13 and 19 mm. At the start and end of the experiment 20 and 50 oysters per replicate (i.e. half a tray), respectively, were weighed (whole weight). Although two separate growing methods were used for selection (loose and slat), this growth comparison used only slats to reduce the effect of oyster density. 2.5. Statistical

analyses

After confirmation of homogeneity of variance using the Cochran test (Winer et al., 1991), the oyster weight data were analysed (on an individual-oyster basis) by a three

Table 4 ANOVA

’ of oyster whole weights at the end of a 17-month growth comparison

Source of variation Site (A) Line (B) Half tray (Cc A X B)) AXB

Residual Total

ss 18809 4296 3136 523

89172 115936

df

MS

F ratio

PF versus

2 5 54 10 3528 3599

9404.26 859.29 58.08

161.91 16.44 2.30

O.OQO 0.000 0.000

Residual

52.26

0.90

0.540

C(AX

C(AX

B)

AXB B)

25.28

’ Site has three levels, and is orthogonal and random; Line has six levels, and is orthogonal and fixed; Half tray has four levels, is nested in both site and line and is random. There are 50 oysters per half tray.

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factor (site, line and replicate) nested ANOVA (Table 4) (Underwood, 1981) on GMAVS software (Institute of Marine Ecology, University of Sydney, NSW). Mortality data were arcsin x0,5 transformed before ANOVA (on a replicate basis) to meet homogeneity requirements. The relationship between the weights at the start and end of the experiment was tested (on a mean of replicate basis) with an analysis of covariance (Winer et al., 1991) using Statgraphics” version 5 software (STSC Inc., Rockville, MD). Mean values were compared using the SNK test (Sokal and Rohlf, 1981).

3. Results At the start of the experiment, oysters from three out of four selection lines were significantly lighter (P < 0.05) than those from the controls (Table 3). As expected, there were no significant differences (P > 0.05) between sites, nor was there a significant interaction (P > 0.05) between lines and sites for weights at the start (Table 3). At the end of the experiment, oysters of the Loose 2 and Slat 2 selection lines were significantly heavier (P < 0.05) than the controls (Table 3). The line Loose 1 oysters were heavier than controls but not significantly so, and Slat 1 oysters were no different from the controls. The Loose 2 selection line oysters were 8.5% heavier for the first generation of selection than the average of the controls. Oysters from three out of four selection lines were heavier than the controls, and oysters from the worst-performing selection line (Slat 1) were similar in weight to the control oysters. Although there were significant differences (P < 0.05) between sites, there was no significant interaction (P > 0.05) between lines and sites for weights at the end. There was no significant covariate effect (P = 0.61) of the weights at the start on the weights at the end of the experiment. The mortality data were not homogeneous even after an arcsin x0.5 transformation (C = 0.43; P < 0.01) and therefore were not analysed by ANOVA. The average survival for the selection lines and controls was 90 + 0.01% (mean + s.e.; range 89-92%).

4. Discussion Although oyster spat from all selection lines and control groups were graded on the same plastic mesh screens (diagonal sizes of 13 and 19 mm>, there were significant differences (P < 0.05) in whole weights of spat at the start of the experiment. As expected, there were no significant site or site X line effects (P > 0.05) for initial weights before the placement of oysters at the three sites. As there was no significant linear relationship (P = 0.15) between weights at the start and end of the experiment, it was concluded that the oyster weights at the end were not affected by differences in initial spat weights at the start of the experiment. The results of the single first generation of selection for whole weight are encouraging, as oysters from two out of four selection lines were significantly heavier (P < 0.05) than the control oysters. Oysters from Loose 2 and Slat 2 selection lines were 8.5% and 5.0% heavier, respectively, than those from the controls. However, after only one

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generation of selection it is too early to assess which of the two methods should be preferred. The loose method has the advantage of a larger number of oysters (initially 1000 per tray) and higher selection intensity, but with more density variations and regular culling of the smallest surplus oysters. On the other hand, the slat method uses fewer oysters (240-280 per tray) and has a lower selection intensity, but has a constant stocking density. The spat nursery and selection methods (loose and slat) used do not allow the estimation of a selection differential. The effect of culling approximately 90% of small spat in upwellers during the nursery phase in this study is difficult to quantify. It is not known whether the small spat were culled because they were slow growing for environmental or genetic reasons. For practical reasons culling by sieving continued in the loose method until close to the final selection of broodstock on weight. In this study, growths of oysters were compared using the slat in an attempt to reduce the effects of stocking density variations. However, as the industry grows oysters on trays, a growth comparison should be carried out using both growing methods to check for the presence of any genotype X growing method interaction. The results reported in this study are encouraging and in agreement with those obtained for C. virginica (Paynter and Dimichele, 1990), 0. edulis (Newkirk and Haley, 1983; Toro and Newkirk, 1990) and 0. chilensis (Toro and Newkirk, 1991). It is proposed to establish parallel tetraploid (Guo and Allen, 1994) breeding lines for crossing diploids (2n) with tetraploids (4n) to produce 100% triploidy (3n). If successful, experiments will be carried out to determine if the faster growth rates and better meat condition obtained with triploidy (Nell et al., 1994) is additive to the faster growth rates achieved by selective breeding in this study.

Acknowledgements We thank the staff of the Port Stephens Research Centre and the NSW oyster industry for their assistance with this study. We also thank M.J. Anderson, Marine Ecology Laboratories, University of Sydney, for her statistical advice. Thanks are also due to Dr. R.D. Ward, CSIRO Division of Fisheries, Hobart, Tasmania, and W.A. O’Connor and Dr. S.C. Battaglene, Port Stephens Research Centre, for editorial comments.

References Falconer, D.S., 1981. Introduction to Quantitative Genetics. 2nd edn. Longman, London, 340 pp. Guo, X. and Allen, S.K., Jr., 1994. Viable tetraploids in the Pacific oyster (Crussostrea &x Thunberg) produced by inhibiting polar body 1 in eggs from triploids. Mol. Mar. Biol. Biotechnol., 31: 42-50. Holliday, J.E., Maguire, G.B. and Nell, J.A., 1991. Optimum stockin, 0 density for nursery culture of Sydney rock oysters (Saccostrecr commerciulis). Aquaculture, 96: 7- 16. Nell, J.A., 1993. Farming the Sydney rock oyster (Succostrea commerckdis) in Australia. Rev. Fish. Sci., I: 97- 120. Nell, J.A., Cox, E., Smith, 1.R and Maguire, G.B., 1994. Studies in triploid oysters in Australia. I. The farming potential of triploid Sydney rock oysters Succostreu commrrcidis (Iredale and Roughley). Aquaculture, 126: 243-255.

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Nell, J.A., Hand, R.E., Goard, L.J., McAdam, S.P. and Maguire, G.B., 1996. Studies on triploid oysters in Australia. V. Evaluation of cytochalasin B and 6-dimethylaminopurine for triploidy induction in Sydney rock oysters Succostrea commerciulis (Iredale and Roughleyl. Aquaculture Res., 27: lOl- 1 10. Newkirk, G.F. and Haley, L.E., 1983. Selection for growth rate in the European oyster, Ostrea edulis: Response of second generation groups. Aquaculture, 33: 149- 155. Paynter, K.T. and Dimichele, L., 1990. Growth of tray-cultured oysters (Crussostreu uirginicu Gmelin) in Chesapeake Bay. Aquaculture, 87: 289-297. Sheridan, A.K., Smith, I.R. and Nell, J.A., 1996. Reducing the impact of environmental variation in a growth rate improvement program for the Sydney rock oyster Succostrea commerciulis. Aquaculture, 143: 145 154. Smith, I.R., Sheridan, A.K. and Nell, J.A., 1995. Evaluation of growing methods for use in a Sydney rock oyster Succosrreu commercialis (Iredale and Roughley) selective breeding program. Aquaculture, 13 1: 189-195. Sokal, R.R. and Rohlf, F.J., 1981. Biometry. W.H. Freeman, New York, 895 pp. Toro, J.E. and Newkirk, G.F., 1990. Divergent selection for growth rate in the European oyster Ostreu edulis: Response to selection and estimation of genetic parameters. Mar. Ecol. Prog. Ser., 62: 219-227. Toro, J.E. and Newkirk, G.F., 1991. Response to artificial selection and realised heritability estimate for shell height in the Chilean oyster Ostrea chilensis. Aquat. Living Resour., 4: 101-108. Underwood, A.J., 1981. Techniques of analysis of variance in experimental marine biology and ecology. Oceanogr. Mar. Biol. Amm. Rev., 19: 513-605. Winer, B.J., Brown, D.R. and Michels, K.M., 1991. Statistical Principles in Experimental Design. 3rd edn. McGraw-Hill, New York, 1057 pp.