Taylor, Joseph James Uel (1999) Juvenile production and culture of the silver-lip pearl oyster, Pinctada maxima (Jameson). PhD thesis, James Cook University.

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Abstract

The silver-lip (or gold-lip) pearl oyster, Pinctada maxima (Jameson) is the most highly prized of all pearl oysters. Through production of "south sea pearls", P. maxima is the basis of Australia's most lucrative aquaculture industry and is of great economic importance throughout south east Asia. In recent years there has been a rapid expansion in the cultivation of P. maxima throughout the region brought about by improvements in hatchery technology. Despite this, there is limited published information concerning rearing of P. maxima. This study aims to address this problem by establishing appropriate culture criteria for P. maxima in the specific areas of collection (settlement) and maintenance of spat in the hatchery, and the early nursery rearing and the grow-out of P. maxima at sea.

In the first of a series of settlement experiments, the influence of pediveliger stocking density on settlement and subsequent growth of spat was investigated. Pediveligers were placed in 15 L vessels at densities of 0.5, 1.0, 1.5 or 2.0 larvae per mL. The number of spat per 100 cm² in the 1.0 larva per mL treatment was over double, and significantly greater (P< 0.05) than that in the 0.5 larva per rid, treatment. Increases in the number of spat were not significant (P >0.05) between larval densities of 1.0 and 1.5 larvae per mL and 1.5 and 2.0 larvae per mL. Stocking density did not significantly influence survival (P>0.05). By day 44, spat from the 0.5 larva per mL treatment were significantly larger (P <0.05) than spat from all other treatments and spat in the 1.0 larva per mL treatment were significantly larger (P <0.05) than in the 1.5 or 2.0 larvae per mL treatments. Differences in size were most likely due to competition for space and food.

Further settlement experiments assessed artificial substrata for the collection of hatchery-reared P. maxima. Pediveliger larvae were settled onto collectors made from: curved PVC slats; polypropylene rope; a combination of PVC slats and polypropylene rope; and mono-filament nylon. Rope and the combined PVC slat and rope collectors had significantly more spat (P <0.05) than either nylon or PVC slat collectors. In a second experiment, significantly more spat (P <0.001) were counted on horizontally positioned PVC slat collectors than those vertically positioned. The concave surface of PVC slats had significantly more (P <0.001) spat than the convex surface, regardless of orientation. In a third experiment, significantly more (P <0.001) spat attached to PVC slats with an epifloral biofilm than clean PVC slats.

An adequate diet for settled spat is essential and seventy-five day-old P. maxima were fed for 21 days on the following monospecific micro-algal diets: Isochrysis aff. galbana, (T-ISO), Pavlova lutheri, Chaetoceros muelleri, C. calcitrans, and Tetraselmis suecica. The largest increase in ash free dry weight (AFDW) was for spat fed C. muelleri, which was significantly greater (P <0.05) than for any other species. The mean AFDW of spat fed T suecica and T-ISO did not differ significantly from each other, but were significantly greater than for spat fed C. calcitrans and P. lutheri (P <0.05). The final AFDW of spat fed P. lutheri was not significantly different from that of unfed spat (P >0.05).

P. maxima that were ready for transfer to sea were re-settled onto PVC slats at a mean density of 340 per 100 cm² and either left exposed (control) or covered with a mesh sleeve of varying aperture sizes (0.75 mm, 1.5 mm or 3.0 mm) before being suspended from a raft. Two weeks later, there was no significant difference (P >0.05) among the number of spat retained on the covered slats; however, all covered slats had significantly greater (P <0.001) spat retention than controls. Spat were significantly larger (P <0.05) with each increase in mesh size. There was no advantage in using sleeves with a mesh size small enough to retain dislodged.

Spat require detaching from their point of attachment during grading. The following stress factors were tested as potential inducers of detachment in P. maxima: salinities of 45 ‰, 40 ‰, 30 ‰ and 25 ‰; pH of 10 and 4, and air exposure. High mortality (>80%) resulted from the use of pH 10 and it was abandoned after 1 h. Hypersaline sea water (45 ‰) resulted in significantly more spat detaching (92.3 ± 0.6 %, mean ± s.e., P<0.05) than in any other treatment. A pH of 4 resulted in 85.6 ± 2.3 % (mean ± s.e.) detaching after 1 h. Exposure to the treatments beyond 1 h, except in the case of exposure to air, did not yield significant increases (P> 0.05) in numbers of detached spat. Spat that had detached in the treatment baths after the first hour began to re-attach during the second hour. After 24 h exposure to treatments, excluding pH 10 and air exposure, spat had firmly re-attached with 100 % survival and no mortality was recorded 24 h after the spat were returned to normal sea water.

Byssus regeneration following detachment was studied in P. maxima of six different age classes. In the first experiment, 75 or 120 day old P. maxima were removed from their points of attachment by severing the byssus and byssal thread production and oyster behaviour were monitored for 120 hours. Younger juveniles re-attached faster than older juveniles, but older juveniles produced significantly more (P <0.001) byssal threads after 12 h and significantly more (P <0.001) byssal threads over the 120 h period. Byssus production for the younger juveniles did not increase significantly (P >0.05) after 48 h whereas byssus production from older animals continued to increase significantly (P <0.001). P. maxima were observed to eject the byssal apparatus, move and reattach within 24 h. Re-attachment following ejection of the byssus was faster than that following mechanical severing. In the second experiment, older P. maxima aged 7, 9, 11 or 13 months were placed in nets in strong (2.5-3.5 knots per h) or mild (<1 knot per h) current. Pearl oysters re-attached faster in the mild current. However, after 5 days, oysters aged 13 and 11 months in strong current had produced significantly more threads (P< 0.05) than oysters in mild current. This trend continued after day 5, but was not significant (P>0.05) for pearl oysters aged 9 and 7 months. By day 11, 9-month-old oysters had produced significantly more byssal threads than any other age class.

P. maxima are grown in tropical regions affected by monsoonal rains which can alter salinity for extended periods. Juvenile P. maxima were held over a period of 20 days in the following salinities: 45 ‰, 40 ‰, 34 ‰ (ambient), 30 ‰ and 25 ‰. There was no significant difference (P> 0.05) in survival of spat from the different treatments; however, growth was significantly depressed (P< 0.05) at 45 ‰, 40 ‰ and 25 ‰. The best growth was recorded at 30 ‰, where spat were significantly larger (P< 0.05) than those held at ambient salinity.

Several experiments investigated the effects of stocking density directly on growth and survival of juvenile P. maxima. Spat were held in suspended nursery culture for six weeks at four stocking densities: 10 juveniles per slat (133 juveniles per m²); 50 juveniles per slat (670 juveniles per m²); 100 juveniles per slat (1330 juveniles per m²) and 150 juveniles per slat (2,000 juveniles per m²). Best growth and survival was recorded at a stocking density of 10 juveniles per slat (80 ± 4.36%: mean ± s.e.), which was significantly higher than the other densities tested (P < 0.05). Survival did not differ significantly between the other densities tested (P >0.05). The incidence of growth deformities increased with increasing stocking density.

In a second experiment, juvenile growth was compared at two stocking densities (28 individuals per net: 66 oysters per m² or 48 individuals per net: 99 oysters per m²) with animals held either in suspended or bottom culture. Mean (± s.e.) survival in 28-pocket nets in suspended culture (99.0 ± 1.6 %) was significantly better than any other treatment (P <0.01). Survival was also high in the 48-pocket nets in suspended culture (94.8 ± 3.6 %). Mean survival in bottom culture was significantly lower (P <0.05), being 15.8 ± 7.8 % and 13.3 ± 3.6 %, respectively, for 28 and 48-pocket nets. Oysters held in suspended culture grew significantly larger (P <0.001) than those in bottom culture. In both suspended and bottom culture, P. maxima in the 28-pocket nets grew significantly larger (P <0.001) than those held in 48-pocket nets. The dry weight of suspended solids, and phytoplankton number and diversity were all greater in surface waters indicating greater food availability. In a third experiment, seven month-old P. maxima were graded into four size classes (G1 to G4: largest to smallest, respectively). Three replicates for each size class were stocked into 28-pocket nets and 8-pocket nets (19 pearl oysters per m2). Dorso-ventral shell height (SH), SL and wet weight (WW) were measured monthly for five months. For G2 to G3 no differences in survival or growth were recorded during the experiment. Survival for GI in 8-pocket panels was 100% and significantly better (P<0.01) than GI in 28-pocket panels. Further, by the end of the second month, GI in 8-pocket panels were significantly larger (P<0.001) than G1 in 28-pocket panels. This size advantage was maintained during the course of the experiment. The highest percentage of `runts' resulted from G1 in 28-pocket panels.

Cleaning is the major activity on pearl farms. A comparison was made of the growth of one-year-old P. maxima, cleaned every 2, 4 or 8 weeks or after 16 weeks. The diversity of fouling animals was recorded and their dry weight (DW) estimated. The DW of fouling animals increased steadily over the first 10 weeks of the experiment before declining during weeks 10 to 16. Significant (P< 0.05) differences in the DW of fouling animals between treatments was observed and pearl oyster growth was affected by fouling. SH, SL and WW of pearl oysters cleaned every 2 or 4 weeks was significantly greater (P <0.05) than that of pearl oysters cleaned every 8 weeks or after 16 weeks.

Item ID:	33794
Item Type:	Thesis (PhD)
Keywords:	aquaculture; cultured pearls; gold-lipped pearl oysters; growth; juveniles; pearl cultivation; pearl farming; pearling; Pinctada; silver-lipped pearl oysters; South Sea pearls
Date Deposited:	21 Jul 2015 05:31
FoR Codes:	07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 100%
SEO Codes:	83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8305 Primary Animal Products > 830504 Pearls @ 10% 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830104 Aquaculture Oysters @ 90%
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