Growth, mortality, recruitment and sex-ratio in wild stocks of silver-lipped pearl oyster Pinctada maxima (Jameson) (mollusca: pteriidae), in western Australia.

Growth, mortality, recruitment and sex-ratio of wild stocks of the silver-lipped pearl oyster Pinctada maxima were studied at sites spanning the geographic extent of the commercial fishery using mark-recapture experiments, recruitment cohort analysis and research surveys of stock abundance and reproductive status. Growth parameters ([L.sub.[infinity]], K) from the von Bertalanffy growth equation were estimated at 210 mm dorso-ventral measurement (DVM) ([+ or -]16 mm SD) and 0.74 at the Lacepede Islands, [L.sub.[infinity]] of 199 mm DVM ([+ or -]6 mm SD) and K of 0.79 on 80 Mile Beach, and [L.sub.[infinity]] of 194 mm ([+ or -]6 mm SD), and K of 0.72 at Exmouth Gulf respectively. Estimates of natural mortality (M) by tagging were very low (0.02-0.03), compared with catch-curve analysis, which estimated M to be between 0.1 in deeper (30-34 m) populations and 0.18 in shallow (9-12 m) populations. Settled P. maxima spat (0 + and 1 + age classes) on adult shell were quantified (e.g., 1,317 spat found on 119,000 shell in 2003) to obtain an annual recruitment index, which showed clear temporal trends in abundance. Over 7 y (1992 to 1995; 2001 to 2003) the annual recruitment index varied from 5.1-8.0 spat per 1,000 shell for the 0 + age class, and 3.5-6.2 spat per 1,000 shell for the 1 + age class. Preliminary predictions of future abundance showed promise, however more work is required on spatial and habitat effects on spat settlement before the potential of the 0 + and 1 + recruitment indices can be realized. We also confirm that Western Australian populations of P. maxima are protandrous hermaphrodites, with a 50:50 sex ratio not achieved until females are 170 mm DVM, which is above the maximum size fished.

KEY WORDS: pearl oyster, Pinctada maxima, growth, mortality, recruitment, sex-ratio

INTRODUCTION

Wild stocks of Pinctada maxima (Jameson) underpin an AU $120 million dollar pearling industry in Western Australia and the fishery has been in operation for over 120 y. From the late 1880s up to the mid 1990s, the industry relied primarily on wild caught shell for pearl production. Since then, a concerted shift towards the culture of P. maxima led to the determination of the reproductive cycle of stocks (Rose et al. 1990), techniques for larval and spat culture (Rose & Baker 1994) and the assessment of growth and mortality in hatchery and nursery culture (Mills 2000, 1997, Taylor et al. 1997, Yukihira et al. 1998). This focus on the cultured animal is not surprising; with the exception of Western Australia, there are no substantial wild stocks left anywhere in the Indo-Pacific. Even the estimation of spat settlement and recruitment of wild stocks has been primarily directed towards the potential of farming wild-caught spat for pearl production (Beer & Southgate 2000, Knuckey 1995). Knuckey (1995) suggested that artificial collectors of natural spat could be used as a stock assessment tool, as is the case in scallop fisheries. However, early trials determined that logistical difficulties (12 m tidal ranges) associated with the deployment of artificial spat collectors precluded their satisfactory application in Western Australia (Joll 1994).

In the 1950s, Takemura and Okutani (1955, 1958) identified the "piggyback spat" phenomenon, whereby new recruits of Pinctada sp settled on the backs of adult oysters and remained attached for at least the first year or two of their life. Given that over half a million oysters are caught each year in the fishery, we hypothesized that "piggyback spat" sampled from the wild caught shell may provide a useful broad scale index of recruitment in this species. The rate of "piggyback" spat found on pearl oysters caught by the commercial fishery was examined for its potential as a method for monitoring and measuring recruitment to the pearl oyster stock. This provides a basis for understanding the annual variation in recruitment and predicting the level of recruitment to the fishery 2-3 y later. The current recruitment index is based on oysters reaching legal minimum size (120 mm shell length) and can include a combination of year classes.

Despite the long-history of exploitation of this pearl shell, there have been no published estimates on key demographic parameters of wild stocks. The aims of this study are therefore, to provide comprehensive estimates of growth, mortality, recruitment and sex-ratio in commercially harvested wild stocks of P. maxima.

MATERIALS AND METHODS

Growth

Growth was assessed with data from different stages of the life history; growth-increment measurements from tag-recapture experiments at the northern (Lacepede Islands), central (80 Mile Beach) and southern (Exmouth Gulf) areas of the commercial fishery (Fig. 1), and cohort analysis of the 0 + and 1 + age classes in successive years. Oysters for the tag-recapture experiment were obtained from commercial and research diving. Prior to measurement and tagging, oysters were held in plastic holding crates hung from the side of the boat or aerated seawater on deck (<4 h) or when available, in purpose built seawater through-flow tanks on pearling boats (up to 12 h). All releases and recaptures were made during a single neap (tide) in June of each year at the 80 Mile Beach site, during a single neap in February/March each year at the Lacepede Channel site and during a single neap in June/July each year at the Exmouth Gulf site. Overall, annual growth increments from 2,700 recaptured animals (Table 1), combined with modal length frequency of 0 + and 1 + cohorts over 8 years were used in the determination of growth of P. maxima.

[FIGURE 1 OMITTED]

Oysters were tagged using plastic shellfish tags (Hallprint, South Australia) measuring approximately 15 x 7 mm (7 x 3 mm for smaller oysters <50 mm DVM). These tags were applied to the left and right valves of each oyster using a cyano-acrylate glue (Selleys or Loctite 454 Gel). Each oyster was tagged using a pair of tags with the same 4 character identifier. The dorso-ventral measurement (DVM) of each oyster was recorded using standard calipers and measuring boards. Other morphometric measurements (anterior-posterior, hinge line, hinge depth and thickness) were taken, but only the DVM results are reported here.

Oysters were released on commercially fished habitat characteristic of each region. At the 80 Mile Beach site and Lacepede Channel, oysters were released in 15-18 m depth; at Exmouth Gulf oysters were released in 5-8 m. Oysters recaptured during research surveys were remeasured in successive years and rereleased in the same area. Any new pearl oysters found during recapture surveys were tagged, measured and released along with the recaptured oysters. During the period of growth monitoring, the pearling industry avoided fishing in the vicinity of tag sites.

Growth was analyzed using a maximum likelihood reformulation of the Von Bertalanffy growth curve for tagged data (Francis 1988). Equations 1 and 2, and Table 2 of Francis (1988) describe the reformulated equations and parameters. Model fitting is conditional on specifying two appropriate lengths described by the data ([alpha] and [beta]), from which mean annual growth ([g.sub.[alpha]], [g.sub.[beta]]) and K and [L.sub.[infinity]] is calculated and the resultant fit examined. The best fit (maximum log-likelihood) for the P. maxima growth model occurred with the following values of [alpha] and [beta] for each region. Lacepede Islands (50, 170 mm DVM), 80 Mile Beach (60, 160 mm DVM) and Exmouth Gulf (65, 180 mm DVM). We also use the variability in growth-increment data to estimate the standard deviation of [L.sub.[infinity]], as described by Francis (1988).

Mortality

Pearl shell fishing in Western Australia is a gauntlet fishery targeting 3 age classes (3-6 y olds) in the 120-170 mm DVM size range, with larger animals unexploited. Natural mortality was determined directly and indirectly for the unexploited portion of the stock (shell >170 mm DVM) by tag and recapture studies conducted on fixed transect lines and by examining stock size-structure and undertaking length converted catch-curve analysis (Pauly 1984).

Mortality: Tagging Data

Pinctada maxima were placed in a predefined grid area and monitored at successive time intervals. Five parallel lines were established at 6-m depth in the Gales Bay area of Exmouth Gulf in July 1998. The lines were secured at the ends with anchors along a north-south direction approximately 10 m apart, and numbered 1-5 from east to west. Short lengths of chain were attached to the rope at regular intervals to assist in anchoring each line to the bottom. Oysters were collected from surrounding stocks, brought to the surface, placed in baskets suspended in the water from the side of the boat and tagged and measured as described in the growth section. The number of shell collected, processed and placed on the bottom each time was minimized (approximately 40-50), to ensure the pearl oysters were stressed as little as possible from the catch and tag procedure. Oysters were then hand placed on the substrate next to one of the five lines on the seabed. A total of 721 oysters were tagged and released between July and August 1998. The size range tagged was 60 mm to 240 mm DVM, with over 75% of shell being greater than 170 mm.

To separate natural mortality from experimental mortality (oysters stressed by the collection and translocation to experimental sites), shell were allowed to recover for 1 mo after the initial tag and release. Experimental mortality was estimated for the 1-mo period, after which the estimation of natural mortality began. It was assumed stressed shell would either have died or recovered to a naturally healthy state after a month.

The intention was to collect mortality data after pearl oysters had been at liberty for 1 and 2 y (August 1999 and 2000). However, during the first assessment of the experiment (August 1999), the destructive effects of a category 5 cyclone (Vance, March, 22 1999), which produced Australia's highest ever recorded wind gust of 273 km/h, became apparent. Experimental lines suffered a similar fate to equipment on nearby commercial farms, where 395 mm of rainfall run-off, high winds, and storm surge had caused widespread movement of oysters and equipment. Overall survival rate of shell was quite high, as a large number of pearl oysters were found alive (n = 221, 48%), however there was no discernable structure left to the experimental layout and results obtained were not deemed representative of a normal year.

Consequently, a repeat experiment was set-up in Exmouth Gulf on the August 8, 1999 to determine survival of shell over a single year period. Seven clumps of 20 tagged shell spaced approx 0.8f m apart, were positioned by divers along four 50-m lines (Fig. 2). The experiment sites were revisited a year later when divers retrieved, recorded and measured the tagged oysters. This sampling layout simulated the natural distribution of shell on the bottom.

[FIGURE 2 OMITTED]

Mortality: Length-converted Catch Curves

Length-converted catch curve analysis was used to estimate natural mortality in the unfished portion of the population above 170 mm. First, age-length keys were generated from the parameters of the von Bertalanffy growth equation, with variations of age at length obtained from the residuals in the tag-recapture model fitting (Haddon 2001). Second, shell lengths from research surveys of the northern (Lacepede Channel and Hama Patch) and 80 Mile Beach populations (5 locations: Patterson Shoal, 10 Mile, 13-15 Mile, 17 Mile, Compass Rose) were allocated to ages using proportions in the age length key. No model was run for oysters in Exmouth Gulf because there was no detailed survey of these stocks to provide length frequency information.

Natural mortality (M) was assessed by determining the negative slope of the regression line from log frequency estimates of numbers at age for those shells above 170 mm (Pauly 1984). Mortality was estimated for 4 locations (Lacepede Islands, 80 Mile Beach Inshore, 80 Mile Beach Offshore, Compass Rose).

Recruitment

Research and industry personnel inspected oysters collected by pearl divers for the presence of settled P. maxima spat, which were separated into two age classes (Fig. 3). After each drift, between 100 and 300 shell were caught and placed in a central pile for cleaning. Twenty oysters at a time were randomly selected from the pile, each one inspected, spat collected and measured, and the process repeated until a representative subsection of the catch had been sampled. Between 18,000 (1991) and 137,000 (2001) adult pearl oysters were examined each year for the presence of spat (Table 2). Currently, this sampling effort evaluates 25% to 30% of oysters caught each year. A comparison of spat abundance between research and industry data was also undertaken.

[FIGURE 3 OMITTED]

Sex Ratio

Rose et al. (1990) and Hancock (1993) quantified the reproductive cycle of wild stocks of P. maxima in Western Australia, and our sampling utilized this knowledge to determine the population sex-ratio. To coincide with the beginning of the predicted spawning season, sex-ratio data were examined from oysters collected from 80 Mile Beach in October 1998 (n = 479). DVM of all specimens was recorded and the valves of each oyster were partially opened to inspect gonad condition and determine gender by visual inspection of gonad color (males, white to cream colored gonad; females, pale yellow to orange colored gonad). Animals in which the gonad was a watery white color or appeared translucent were recorded as indeterminate. Sex-ratio variation by size was examined.

RESULTS

Growth

Pinctada maxima obtains an average maximum theoretical length of between 193 and 210 mm DVM, and a von Bertalanffy growth coefficient (K) of 0.72-0.79 (Table 3). Growth curves and residual analysis of model fits describing growth in P. maxima are shown in Figure 4. Oysters grow faster and attain a higher maximum size in the Lacepede Islands, in comparison with the 80 Mile Beach and Exmouth Gulf stocks.

[FIGURE 4 OMITTED]

A comparison of length frequency of the commercial catch and research surveys for both populations (Lacepede Islands and 80 Mile Beach) confirms the growth curve results, namely a larger size of P. maxima at the Lacepede Islands (Fig. 5).

[FIGURE 5 OMITTED]

Spat cohort data shows growth to be relatively slow in the first 18 mo, with average annual increment between 0+ and 1+ cohorts varying from 28-36 mm, the exception being 1995, which at 42 mm, appears to have been an exceptionally fast growth year (Fig. 6).

[FIGURE 6 OMITTED]

Mortality: Effect of Handling on Tag Retention

Of the 93 shell released in Exmouth Gulf in July 1998, 92 were found 1 mo later in August 1999. From these 92 shell, 2 were dead, 3 had damaged tags and 1 shell had lost both tags. Therefore, total mortality from experimental handling was 2/92 = 2.17%, and tag loss was 4.3%.

Mortality: Tagging Estimates

Of the 200 oysters placed-out for 1 y in August 1999, 195 were retrieved live in August 200, 2 were dead and three were not relocated. The collection rate of 195/200 (97.5%) indicates that adult mortality was low (2.5% p.a.). Thus, overall direct estimates of annual natural mortality of adult pearl shell (6+ years) were 1% to 2%, which equates to M of 0.02.

Mortality: Catch Curve Analysis of Adult Pearl Shell (6 + years)

Natural mortality (M) ranged from 0.18 or 16.5% per year (80 Mile Beach, inshore shallow) to 0.1 (10%) at the Compass Rose deepwater stocks (Fig. 7). Mortality at the Lacepede Islands (0.148) and 80 Mile Beach offshore shallow was intermediate between these. This trend is correlated with depth (i.e., highest mortality in shallower waters and lowest in deep in the 80 Mile Beach stocks).

[FIGURE 7 OMITTED]

Recruitment

Separation of 0 + and 1 + age classes of P. maxima spat were obtained consistently over 9 y of sampling the 80 Mile Beach stocks (Fig. 6), and 0-34 mm spat are considered 0 + age (5-8 mo), with 35-75 mm considered 1+. This enabled clear temporal trends in spat settlement to be quantified (Fig. 8), particularly in the Lacepede Islands (Fig. 8). However, the magnitude of the recruitment index differed between research and industry data collection, particularly for the 0 + age class (Fig. 9). Despite this, there was still a significant positive correlation between industry and research personnel data for the 0 + age class (n = 4; r = 0.95; P < 0.05). For the 1 + age class, there was also a positive correlation between industry and research personnel data (n = 4; r = 0.67; P > 0.05), however it was not significant, mainly because of the small sample size (Fig. 8).

[FIGURES 6, 8-9 OMITTED]

Predicting Future Stock Abundance with Recruitment Data

In Zone 2, there was a high positive correlation between 0 + spat abundance and 1 + spat abundance 1 y later for the industry data (n = 4; r = 0.89), and the research data (n = 3; r = 0.92). Neither was statistically significant because of the small sample sizes, however they are sufficiently high to enable reasonable confidence in their predictive potential, which needs to be confirmed with more years of sampling.

Sex Ratio

Sex-ratio data confirms that P. maxima is a protandrous hermaphrodite (Fig. 10). Size-at-maturity for males was around 110 mm, females were identified from 135 mm onwards and the sex ratio reached 50:50 female to male at approximately 170 mm DVM. From 170-200 mm there was a greater proportion of females in the population (Fig. 10).

[FIGURE 10 OMITTED]

DISCUSSION

Results of this study establish for the first time, the crucial demographic parameters of growth, mortality, recruitment and sex-ratio of wild stocks of P. maxima in Western Australia. Growth was estimated in-situ for the 3 main commercially fished stocks, the Lacepede Islands, 80 Mile Beach, and Exmouth Gulf stocks, and represent the first published results for in-situ assessment of growth of wild stocks of P. maxima. This is in contrast to farmed stocks of P. maxima, where there have been many assessments of growth, beginning with Wada (1953). Our growth results confirm that the wild fishery in Western Australia is a gauntlet fishery, targeting between 2 and 3 age classes, depending on location, and that average growth in the initial years from 0 + to 1 + is relatively slow, at around 30 mm per year, although it can vary considerably between years. A secondary spawning peak of P. maxima in March to April (Rose et al. 1990) may affect the integrity of the 1 + cohort data to some degree, however there is enough consistency between years in spat size frequency to be confident that 30 mm per year is an accurate average for 20-40 mm P. maxima spat on the 80 Mile Beach. This is slower than growth rates achieved by P. maxima spat grown near the surface on long lines, which generally achieve an average increment of 50 mm per year in their first 2 years at farms in the Northwest and North of Australia (D. Mills, pers. comm.). Cooler temperatures and surface boundary effects creating slower flow rates are hypothesized to be the main causes of slower growth in spat from 80 Mile beach stocks. Rose et al. (1990) recorded winter water temperatures dipping to 20[degrees]C, which is close to the calculated temperature of 0 growth (18[degrees]C) in P. maxima (Mills 2000, Pass et al. 1987). Also, spat were collected from wild adult oysters living in benthic habitats where friction effects naturally slow the average water movement, in comparison with cultured spat that are positioned individually on surface long lines. This practice provides optimal, rather than normal, conditions for growth. Other studies of tropical bivalves also confirm the growth enhancing effect of surface long lines; for example Hart et al. (1999) showed that juvenile growth of a giant clam (Tridacnidae) (0-2 y) was considerably improved on surface long lines.

Rose et al. (1990) reported that males in the wild mature at 1 y with a shell height of 110 mm, however our growth results indicate this in incorrect. At 110 mm DVM, the size when males start to mature, their age would be 2 y, and most would be in their third year of life, at least on the 80 Mile Beach stocks. Females do not mature until 4 or 5 y of age at 140 mm DVM. These results show clearly, for the first time, that growth in wild stocks of P. maxima is less than that achieved on farms.

In Western Australia, unfished shells grow into the "Mother-of-Pearl" or MOP size-class at 170 mm DVM + and remain unexploited at a low natural mortality rate. Estimates of the age of an exceptionally large oyster found on the 80 Mile Beach fishing grounds (252 mm DVM) are between 15 and 30 y (Hart, unpublished data), hence it is possible that P. maxima, which escape commercial exploitation may breed for another 10-15 y. Overall, Research surveys of MOP have confirmed high abundance of stocks (Hart & Friedman 2004) and preliminary analyses suggest that the balance between recruitment and fishing mortality is generally tipped towards recruitment, except in exceptionally poor recruitment years. Stock surveys however, should be carried out every 5 y to measure any change of the breeding stock.

Comparative estimates of the demographic parameters K and [L.sub.[infinity]] for pearl oysters are sparse in literature, owing presumably to the fact that pearl fisheries were among the earliest marine animals exploited commercially, most of which occurred prior to the development of modern, quantitative biological methods. Saucedo et al. (1998) for example, reports that the natural pearl beds of Pinctada mazatlanica in Mexico were subject to 400 y of uncontrolled exploitation, leading to virtual extinction by 1939. Herdman (1903) reports on the great antiquity of the Ceylon pearl oyster fishery, which targeted Pinctada imbricata and details individual yearly catches and revenues of this fishery throughout the 1800s at a very fine spatial scale, including an assessment of normal and stunted growth in the presence of overcrowding, however this was before von Bertalanffy's (1938) seminal work. Sims (1992) study on the black-lip pearl oyster (Pinctada margaritifera) in the Cook Islands yielded an Ls infinity of 183 mm, and K of 0.26, which is slightly smaller and slower growing than P. maxima. This is to be expected because P. maxima is recognized as the largest of all pearl oysters in the world (Shirai 1994), although Saucedo et al. (1998) presented data on individual P. mazatlanica growing to 190 mm DVM. Considerable spatial and individual variation in growth of P. maxima in Western Australia was noted, with a standard deviation of 16 mm from the Zone 2 stocks, and exceptional individuals have been observed as large as 300 mm from the Lacepede Islands. This is confirmed by Wada (1953), who reports that a 300 mm specimen was not uncommon in some fishing areas along the north of Australia and Arafura Sea.

Natural mortality (M) of 170 mm + oysters appeared to be location specific, and generally low, with indirect estimates from length-converted catch-curve analysis (0.11-0.18) being higher than direct estimates from tag-recapture studies (0.02). Sims (1992) estimated natural mortality at 0.11 for Pinctada margaritifera, which suggests that low rates of M for pearl oysters are to be expected. The differences between M was primarily related to depth, and shallow areas of the pearl stocks may exhibit a higher rate of natural mortality caused by increased turbidity and greater water movement and susceptibility to annual extreme storm events, such as cyclones and perhaps higher predation. We hypothesize that P. maxima in Western Australia, a sedentary species having evolved under the influence of huge tides and cyclones would be favored by having low natural mortality as a life-history strategy, indicating a robustness to cope with extreme environmental conditions.

Historically, varying catch per unit effort (CPUE in shells caught per diver hour) in the pearl oyster fishery (Fig. 11), suggests that recruitment is quite variable, a conclusion in common with many marine invertebrate species. Whereas the introduction of GPS (Global Positioning System) in 1994 might have confounded the changes in CPUE occurring at that time, subsequent changes in CPUE suggest that fishery performance can reflect recruitment variation. Understanding this natural variation in recruitment will greatly enhance our management capability of this fishery, and our preliminary work on using the "piggyback" spat recruitment index has yielded promising results. Firstly, it was possible to collect adequate samples of newly settled spat as part of the on-board vessel monitoring program and divide them up into two age classes (0+ and 1+). The results proved consistent from year to year because the fishery largely operates within the same 3 mo each year (mid-March to mid-June). Secondly, preliminary predictions detected a future temporal variability in CPUE, which is consistent with the past history of CPUE in the fishery, namely that the general pattern has been 2-4 y of "baseline" CPUE around 30 shells per hour, followed by 2-3 y of increased CPUE, up to 60% above the baseline and then a drop back to normal levels. Future work will quantify effects of habitat and depth and other environmental factors, such as variables representing larger ocearnograhic conditions (e.g., ENSO phenomena) and provide a formal predictive index to assist in managing level of catch.

ACKNOWLEDGMENTS

The authors thank Arrow Pearls, Broome Pearls and Paspaley Pearls Pty Ltd for use of their vessels and divers to conduct the various sampling regimes. Craig Skepper, Clinton Syers, Frank Fabris, Kim Friedman, Dave Murphy, Rod Pearn and Jamie Colquhouri participated in the field sampling and took the photographs. This work was supported by grants from the Western Australian Fisheries Department, and the Fisheries Research and Development Corporation (FRDC) of Australia.

LITERATURE CITED

Beer, A. C. & P. C. Southgate. 2000. Collection of pearl oyster (Family Pteriidae) spat at Orpheus Island, Great Barrier Reef (Australia). J. Shellfish Res. 19(2):1-6.

Francis, R. I. C. C. 1988. Maximum likelihood estimation of growth and growth variability from tagging data. NZ J. Mar. Freshw. Res. 22:42-51.

Haddon, M. 2001. Modelling and quantitative methods in fisheries. Chapman & Hall/CRC. 406 pp.

Hancock, A. 1993. Reproduction in wild stocks of the Silverlipped pearl oyster, Pinctada maxima (Jameson) (Mollusca: Pteriidea) in Western Australia. Honours thesis, Murdoch University, Western Australia.

Hart, A. M. & K. J. Friedman. (editors) 2004. Mother of pearl shell (Pinctada maxima): stock evaluation for management and future harvesting in Western Australia, FRDC Project 1998/153. Fisheries Research Contract Report No. 10, Department of Fisheries, Western Australia. 84pp.

Hart, A. M., J. D. Bell, I. Lane & T. P. Foyle. 1999. Improving culture techniques for village-based farming of giant clams (Tridacnidae). Aquacult. Res. 30:175-190.

Herdman, W. A. 1903. Report to the Government of Ceylon on the Pearl Oyster Fisheries of the Gulf of Manaar. Part 1. London: Harrison and Sons. 307 pp.

Joll, L. M. 1994. An assessment of stocks of the pearl oyster Pinctada maxima. Final report to the Fisheries Research and Development Corporation (FRCP). Project No. 88/93.47 pp.

Knuckey, I. A. 1995. Settlement of Pinctada maxima (Jameson) and other bivalves on artificial collectors in the Timor Sea, Northern Australia. J. Shellfish Res. 14:411-416.

Mills, D. 2000. Combined effects of temperature and algal concentration on survival, growth and feeding physiology of Pinctada maxima (Jameson) spat. J. Shellfish Res. 19(1):159-166.

Mills, D. 1997. Evaluation of histological cassettes as holding containers for individual spat, and a weekly handling protocol to assess growth of the silver-lip pearl oyster, Pinctada maxima (Jameson). J. Shellfish Res. 16:555-559.

Pass, D. A., R. Dybdahl & M. M. Mannion. 1987. Investigations into the causes of mortality of the pearl oyster, Pinctada maxima (Jameson), in Western Australia. Aquaculture 65:149-169.

Pauly, D. 1984. Fish population dynamics in tropical waters: a manual for use with programmable calculators. ICLARM Studies and Reviews 8. 325 pp.

Rose, R. A. & S. B. Baker. 1994. Larval and spat culture of the Western Australian silver- or gold-lip oyster, Pinctada maxima (Jameson) (Mollusca: Pteriidae). Aquaculture 126:35-50.

Rose, R. A., R. E. Dybdahl & S. Harriers. 1990. Reproductive cycle of the Western Australian silverlip pearl oyster, Pinctada maxima (Jameson) (Mollusca: Pteriidae). J. Shellfish Res. 9:261-272.

Saucedo, P., M. Monteforte & F. Blanc. 1998. Changes in shell dimensions of pearl oysters, Pinctada mazatlanica (Hanley 1856) and Pteria sterna (Gould 1851), during growth as criteria for Mabe pearl implants. Aquacult. Res. 29:801 814.

Shirai, S. 1994. Pearls and Pearl Oysters of the world. Marine Planning Co. Okinawa. 108 pp.

Sims, N. 1992. Size, age, and growth of the black-lip pearl oyster, Pinctada margaritifera (L.) (Bivalvia; Pteriidae). J. Shellfish Res. 12(2):223-228.

Takemura, Y. & T. Okutani. 1955. Notes on animals attached to the shells of the silver-lip pearl oyster, Pincvtada maxima (Jameson), collected from the "East" fishing ground of the Arafura Sea. Bull. Japanese Soc. Sci. Fisheries 21:92-100.

Takemura, Y. & T. Okutani. 1958. On the identification of species of Pinctada found attached to Pinctada maxima (Jameson) in the Arafura sea. Bull. Tokai Reg. Fish. Res. Lab. 20:47-59.

Taylor, J. J., R. A. Rose, P. C. Southgate & C. E. Taylor. 1997. Effects of stocking density on growth and survival of early juvenile silver-lip pearl oysters Pinctada maxima, (Jameson), held in suspended nursery culture. Aquaculture 153:41-49.

von Bertalanffy, L. 1938. A quantitative theory of organic growth. Hum. Biol. 10:181-213.

Wada, S. K. 1953. Biology and fisheries of the silverlip pearl oyster. Unpublished report in the library CSIRO Marine Laboratories, Hobart Tasmania. pp. 1-86.

Yukihira, H., D. W. Klumpp & J. S. Lucas. 1998. Effects of body size on suspension feeding and energy budgets of the pearl oysters Pinctada margaritifera and P.maxima. Mar. Ecol. Prog. Ser. 170:119-130.

ANTHONY M. HART * AND LINDSAY M. JOLL

Western Australian Fisheries and Marine Research Laboratories, PO Box 20, North Beach, WA 6920, Australia

* Corresponding author. E-mail: [e-mail]

TABLE 1.

Period at liberty (years) and number of recapture increments used
to assess growth of Pinctada maxima in Western
Australian populations.

  Location                             Year

80 Mile Beach   Period at
                  Liberty        89/90   90/91   91/92   92/93   93/94
                No. Recaptures    204     362     553     343     391
Lacepede        Period at        96/97   97/98
  Channel         Liberty
                No. Recaptures    293     84
Exmouth Gulf    Period at
                  Liberty        96/97   97/98
                No. Recaptures    275     212

TABLE 2.

Recruitment (piggyback spat) monitoring of Pinctada maxima in
Western Australia, showing the number (n) and percent (%) of
oysters caught commercially that were examined for the presence
of spat.

                       Location
        Sampling     80 Mile Beach    Lacepede Islands
Year    Regime         (Zone 2)           (Zone 3)

1991        n            17,867
            %                 5%
1992        n            20,950            15,536
            %                 6%               18%
1993        n            31,252            13,238
            %                 8%               19%
1994        n            72,284            15,008
            %                19%               19%
1995        n            83,134            21,576
            %                20%               24%
2001        n           132,920             5,020
            %                28%               18%
2002        n           123,660             9,340
            %                27%               53%
2003        n           107,670            11,608
            %                25%               50%
2004        n           101,833             6,593
            %                27%               26%

TABLE 3.

Growth parameters and their variability for the silver-lipped pearl
oysters (Pinctada maxima) at three locations in Western Australia.

                                    Lacepede     80 Mile     Exmouth
Growth Parameters (1)                Islands      Beach       Gulf

[alpha] (mm)                           50          60          65
[beta] (mm)                           170         160         180
[g.sub.a] (mm [year.sup.-1])           42          30          37.5
[g.sub.[beta]] (mm [year.sup.-1])      10           8           9.5
[L.sub.x]                             210         200         194
Standard deviation of [L.sub.x]         5.0        16.2         5.3
K                                       0.74        0.79        0.72

(1) The parameters are derived from the Francis (1988) reformulation
of the von Bertalanffy growth curve. See methods for more details.