Hatchery culture potential of the scallop Chlamys australis in Western Australia

Derek A. Cropp Aquatech Australia Pty Ltd, (Accepted 9 March 1993)

ABSTRACT

Adult scallops (Chlamys australis) were obtained from a scallop trawler operating in Shark Bay, Western Australia, and maintained in a 6000-1 pool using raw seawater. Processed adult scallops, from the bay, averaged 20.6 g meat weight (16.9% of live weight), a figure comparable with yields from commercially fished scallops of other species. Adults were induced to spawn by the addition of sperm and a water temperature increase. 12.55 million eggs were produced from four females. 76.25% of the larvae developing from these fertilised eggs were reared to metamorphosis after 12 days. Approximately 2.4 million spat resulted at the completion of the metamorphosis/settlement stage. The meat content of adult C.australis and the success of hatchery spat production both indicate a potential for commercial culture of this scallop species.

INTRODUCTION

There have been a number of studies on the hatchery production of various species of Australian scallops in recent years (Dix and Sjardin, 1975; Dix, 1981; Rose and Dix, 1984; Cropp and Frankish, 1988; Rose et al., 1988). Only one (Rose and Dix, 1984) has dealt with a Chlamys species as their commercial importance in Australia has been minimal. The latter paper provides information on larvae of the doughboy scallop, Chlamys asperrimus, which appears morphologically and genetically to be a similar species to Chlamys australis from Western Australia. The ecological niches that the two species occupy appear to be similar but the natural temperature regimes of their environs are quite different; 9-20°C for C. asperrimus compared with 1725°C for C. australis. C. australis larvae were reared mainly at 23-24°C in a subtropical area, whilst C. asperrimus larvae were reared at 17-18°C in a cool-temperate area.

Trawlers in Shark Bay, Western Australia, target the species Amusium balloti. A by-catch of approximately 1-5% of C. australis (and C. scabricostata ) is generally landed but returned to the sea as processing is deemed to be difficult and markets have not been established. However, the meat and gonad from processed C. australis is almost identical to that from the cooler water species C. asperrimus, which is common in Tasmania and well received on the market. Hence a potential would appear to exist for marketing of C. australis. The second Chlamys species which is caught with A. balloti, C. scabricostata, does not grow to the same adult size that C. australis or C. asperrimus does (Slack-Smith, 1990). C. australis was thereby targeted as having a better potential for aquaculture.

The most common spat production technique for overseas scallop culture is currently based around collection of natural spat at sea (Bull, 1988; Ito, 1988). From this perspective alone, it is necessary to be able to distinguish between larvae which are likely to be present in the water column at similar times. For this reason a small-scale hatchery trial involving C. scabricostata was conducted prior to the study documented here. Adults of this species were induced to spawn and the larvae reared under the same conditions as for C. australis. Larvae produced by A. balloti adults have been also reared under similar conditions and are distinguishable from both C. australis and C. scabricostata; details of that research are in press.

Various Chlamys species are cultured in hatcheries and in some areas grown-out in culture operations overseas (Broom and Mason, 1978; Mason, 1983; Cropp,1988). Chlamys species generally attach firmly to substrates upon settlement and remain attached for several months. However, the Amusium species exhibit a weak temporary attachment (byssus) only (Dredge, 1981; Gwyther et al., 1991). Interception of the natural settlement and spat attachment process (with artificial substrates) has been found to be both viable (Hortle and Cropp, 1987) and economically feasible on a large scale (Rhodes and Widman, 1980; Maru, 1985; Cropp, 1987). Unfortunately, the small population of C. australis in Shark Bay suggests that natural spat-fall would probably be minimal. The situation implies that if an aquaculture industry was to develop around this species, hatchery culture of the spat would probably be necessary. The present study was an examination of the potential for hatchery rearing of the scallop Chlamys australis.

MATERIALS AND METHODS

Adult scallops for this study were obtained from a scallop trawler operating in Shark Bay, Western Australia, at approximately 24 ° 54' S, 113 ° 12' E in 20 m of water near Bernier Island on 8 July 1991. The broodstock were transported to a nearby hatchery facility (Carnarvon, Western Australia) and maintained in a 6000-1 above-ground swimming pool, at approximately 21 ° C, for a period of 10 days prior to spawning. Over this period, water changes of 2000 I were completed every 2 days, and the alga Chaetoceros gracilis was added daily in a volume sufficient to establish a food cell density in the holding pool of about 40 000 cells ml-'.

On the spawning day, five male and ten female scallops with well developed gonads were removed from the holding tank at 21.7°C.
They were gently cleaned, washed and air dried briefly before placement on to a spawning table in water at 18.5°C. Also on the spawning day, a sample of 30 scallops was measured for shell size. Height of the shell was taken as the perpendicular length from the tip of the umbo to the ventral edge of the shell; the maximum diameter was the maximum shell size measured from the edge of one 'ear' of the shell to the opposing side. Maximum shell diameter (in preference to the length) was used owing to the irregular shape of C. australis adults.

Most of the shells of the adult scallops exhibited a light covering of fouling organisms, principally a sponge. This appeared to have a negligible affect on the health of the scallops (given the meat quality and yield) although it could inhibit their swimming capabilities. The majority of sponge was removed from the shells prior to the recording of biological data (or spawning). The live weight of the animals was recorded before each adductor muscle and gonad was removed from the shells to obtain relevant meat recovery data.

The scallops to be spawned were maintained in recirculating aerated water for 20 min before heating commenced. The table water, (21.5 ° C) containing faecal matter, was drained after 1.5 h of heating and replaced with clean water at 22.2 ° C. After a further 2 h of heating and the addition of sperm (extracted from a sixth male) the first male (M1) began to spawn at 25°C; the heater was then turned off.

This male was left to spawn on the table for several min before being isolated in a 3-1 spawning container. Eight min after M1 began spawning, the first female (F1) began to spawn. F1 was immediately placed in her own 3-1 spawning container. Over the following 15 min another five females (F2-F6) and two males spawned.

Eighteen min after F1 began to spawn, her eggs were poured into a 20-1 bucket through a 141-,µm screen (to remove debris). Several ml of sperm suspension from M1 was added (in the bucket) to the 4.26 million eggs produced by F1. That resulted in a ratio of approximately 5 sperm egg- ~. The egg and sperm suspension was mixed using a small plunger. A 0.7-ml volume of water was randomly sampled from the bucket with a 1-ml pipette. A stereo microscope was used to count the sample of eggs in a Sedgwick-Rafter cell; the total number of eggs from Fl was then calculated. This procedure was repeated for all other egg counts; a similar sub-sampling technique was used at a later stage when conducting larval counts. The number of sperm per egg was visually assessed under the microscope.

The fertilised eggs from F 1 were poured into a 4000-1 tank (T2) which had been filled with water at 24.2°C and a salinity of 35 ppt. This process was repeated for eggs from F2, F3 and F4 (into T2 and T1). Eggs spawned by other females were examined separately but were deemed to be of poor quality (unsatisfactory shape, size and stage of maturity) and therefore discarded.

The mean size of larvae was ascertained regularly by measuring samples of 30 larvae under the dissecting microscope, using an eyepiece graticule. In this study the larval measurement (in micrometres, ,µm ) perpendicular from the umbone to the ventral margin of the shell is referred to as the height. The measurement taken at a right angle to this and from one shell margin to the opposing margin, is referred to as the length.

Larvae were cultured in 4000-1 larval tanks with the temperature maintained at approximately 23-24°C. Diet was composed of Chaetoceros calcitrans, Pavlova lutheri and Tahitian Isochrysis (aff.) galbana. Food cell density (in larval tanks) was maintained at 10000 cells ml-' from day 2, up to 15000 cells ml-' at day 15. The water in the larval tanks was changed every 2 days, and on day 12 the larvae were placed in a 6000-1 settling tank with 60 red mesh spat collectors. These bags had a drawstring and were 780 mm long and 360 mm wide with mesh measuring O.9XO.9 mm internally or 1.2 mm diagonally. They were filled with approximately 4 m of old (hardened ) monofilament shark netting to which the settling spat could attach at metamorphosis.

Even though C. scabricostata was deemed to be unsuitable for commercial culture operations owing to its limited size, larvae from adults induced to spawn were reared to settlement at the same hatchery facility as used for the rearing of C. australis larvae. As no recorded hatchery work has been conducted on this species previously, results are briefly mentioned to allow for comparison with C. australis and other species.

Four adult females (C. scabricostata), measuring 43.4, 64.4, 65.8 and 72.2 mm in maximum shell diameter, were induced to spawn by immersion in solar heated water of 23.5°C. Two adult males, measuring 62 and 66.1 mm, were also induced to spawn at a similar time using the same technique. Eggs and the resulting larvae were cultured in a 1000-1 larval tank at 20.2 + 0.8 °C and total water changes were carried out every 2 days during larval rearing. Larvae were fed daily on a mixture of Chaetoceros calcitrans, Pavlova lutheri and Tahitian Isochrysis ( aff. ) galbana at a concentration which increased from 15 000 to 25 000 cells ml-'. Although not recorded herein, a visual assessment of larval shape and size was made daily. Spat collecting bags were placed in the larval tank on day 17 to allow for spat attachment.

Figure 1., Egg, larvae and spat of Chlamys australis. (bar = 100µm)
(a) Egg	(b) 4-day-old larvae
(c) 8-day-old larvae	(d) 12-day-old larvae
(e) 14-day-old swimming pediveliger	(f) 15-day-old settled spat

RESULTS

The mature eggs of C. australis (Fig. la) were spherical, 62.2+2.2 ,um ( n = 30) and the first D-shaped larvae were 108.5 + 4.1 ,um in length (mean + s.d. ). The total number of eggs produced from the four females was 12.55 million, of which 8.25 million were in T2 and 4.3 million in T1. The individual details of egg release for the four females are shown in Table 1.

By day 4 the larvae measured 124.1 +5.0,um in length (Fig. lb). The D-shaped larvae developed rapidly up to day 8 when a characteristic scallop larval shape was displayed (Fig. lc).

Identification of the precise size of fully developed pediveligers is difficult to ascertain; for C. australis, a sample of swimming pediveligers taken on day 12 (Fig. ld) had a mean length of 203.6_ 12.1 ,um. At that stage approximately 50% of the larvae had an actively motile foot. Some smaller swimming larvae (198.4+10.3,um) were still present on day 14 (Fig. le).A sample of settled spat on day 15 (Fig. lf) had a length of 296.9+48.3,um. Thin new (dissoconch) shell is clearly evident on the outer edge of spat in Fig. lf. A sample of five collectors was washed and spat counted on day 16. The mean count per collector was 37 800+6058. A total of approximately 2.4 million spat (39.3%) settled from 6.1 million eyed larvae at day 12. The spat count included an estimation of those spat attached to the wall and bottom of the tank.

The survival and development of larvae up to the day 12 settlement stage is shown in Fig. 2. Numbers of larvae and spat are difficult to assess during the metamorphosis stage and sampling tends to increase mortalities, hence no estimates of total numbers were available until settlement was completed.

The relationship between larval shell length and height is shown for the 2day-old to 12-day-old growing period in Fig. 3 ( n = 180). The growth of larvae during rearing in the hatchery is shown as shell length in Fig. 4. Standard deviations are shown for each measurement and n = 30 for all points.

The survival and development rates shown in this hatchery trial are extremely high for scallops, 63.7% of eggs developed to D-shaped larvae; 76.25% of D-shaped larvae developed to metamorphosis, which, overall was 48.6% of eggs developing to metamorphosis (Fig. 2). Combined with this was the high growth rate of 9.51,um day-' for the entire larval stage from the first D-shaped larvae to metamorphosis; 7.7 ,um day-' to umbonal stage and 10.7 ~m day-' from umbonal to pediveliger stage (Fig. 4 ).

The sample of wild-caught adult scallops had attained a commercially viable size with a mean diameter of 108.5 + 6.5 mm and height of 96.9 + 5.6 mm. Final processing of these scallops produced a mean of 20.6 + 2.9 g wet meat weight (muscle plus gonad) per scallop with a percentage recovery (meat weight/total weight X 100) of 16.9%.

During the C. scabricostata trial, a total of 1.54 million eggs (diameter 6063 ~m ) were produced and to these were added sperm giving a ratio of 4-5 sperm per egg. After 46 h (day 2) at 21.8°C,800 OOO larvae (51.95% of eggs) had developed into D-shaped veligers with a mean size of 103.4 ~m in length and 82.2 ~m in height (no standard deviation data available). At day 13 the larvae were 197 ~m in length and exhibited a prominent eye-spot. By day 17 numerous pediveligers were evident. The 75 000 remaining larvae were 220 ~m in length at that stage and had grown at a rate of 8.33 ~m d~ ~-' since becoming D-shaped larvae at day 2. Metamorphosis and settlement occurred over 3 days, days 17-20.

DISCUSSION

Rose and Dix (1984) found that the mean egg diameter for C. asperrimus was 61.5 + 0.4 ~m, the first D-shaped larval stage with a prodissoconch I shell occurred after 2 days and was 108 ,um in length, and that fully developed pediveligers occurred on day 19, when larvae were 194,um in length. Corresponding data for C. australis were 62.2+2.2 ,um, 108.5+4.1 ~m and 203.6 + 12.1 ~m respectively. Therefore, larval development appears to be very similar for C. asperrimus and C. australis and the spat settle at a similar size.

Canadian research (Thompson et al., l 985) on the Japanese scallop Patinopecten yessoensis indicated survival rates of 10% from fertilised eggs to D-shaped larvae and 10% from D-shaped larvae to metamorphosis; the growth rate of larvae was shown to be 4.3 ~m day- ', which is acceptable for culture. Rose et al. (1988) recorded a growth of 5.2 ,um day-' up to the umbonal veliger then 6.3 ,um day-' until the pediveliger stage, for Amusium balloti. For C. asperrimus, Rose and Dix ( 1984) observed a growth rate of 3.6 ,um day-' up to the umbonal stage and 5.6 ,um day-' until the pediveliger stage. Clearly, the hatchery techniques used in the present study have resulted in relatively high growth and survival rates for C. australis. The success of hatchery larval culture detailed here would allow for the economically feasible production of large numbers of spat.

Although the current potential for catching natural settlements of C. australis spat in Shark Bay appears to be small, the situation may change in future years. If this was to occur, the presence in Shark Bay of C. scabricostata larvae of a similar size to C. australis larvae, would be of significant importance. It would be virtually impossible to separate the larvae by size or visual identification as they are almost identical; additionally, the eggs are similar in size and both species undergo metamorphosis at a similar size. These aspects provide further support for the production of C. australis spat in a hatchery rather than collection of natural spat from the sea.

The processing figures for meat yields ( 16.9% ) from the C. australis adults sampled in this study indicate that commercial returns are possible with this species. However, as the age at a suitable harvest size has not yet been confirmed as 2 years (as determined by the author), more research is required on the grow-out of hatchery-produced spat to determine the species culture potential. The meat yield is comparable with commercial processing data from C. asperrimus in Tasmania. The meat recovery rate, as compared with live weight, from adults harvested in the D'Entrecasteaux Channel (Southern Tasmania) was 17.9% (Zacharin, 1988 ). Overall, the results indicate considerable potential for the commercial culture of C. australis, subject to further grow-out research.

ACKNOWLEDGEMENTS

The late Dr. Jeremy Langdon is thanked for the photographs and his constructive comments on the text. I also wish to thank the master and crew of the fishing vessel "Slaven" for the broodstock. Bob Shaw and Trevor Sweetman are thanked for their valuable assistance with broodstock management and hatchery operations.

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