46 JUNE 2013 • WORLD AQUACULTURE • WWW.WAS.ORG lagoons. The College of Marshall Islands’ main focus shifted from providing general extension services to stabilizing the black-lip pearl oyster spat supply in the Republic. The lack of a reliable supply of spat is one of the main constraints on the development of the local pearl industry. Mortality may be excessive and growth can be highly variable for spat raised in lagoons. Restoration of hatchery operation and development of sustainable black-lip pearl oyster aquaculture in the RMI is an industry priority (CTSA 2003, MIMRA 2005). Hatchery rearing can offer consistent results, but requires resources and trained personnel. Black-lip pearl oyster hatcheries have attempted to find methods to yield the most oysters at low expense (Nair and Dybdahl 2006). Oyster Spat Production at the College of Marshall Islands Hatchery The hatchery is located on the Arrak Campus of the College of Marshall Islands on Majuro Atoll (Fig. 4). Oyster larvae (0.47 ± 0.07 mm) are collected 2.5 wk after spawning using 53-mm mesh screen from 3.75-m3 larval culture tanks in the same hatchery. Larvae are stocked into two second-phase larval rearing tanks (269 L) for an additional 1.5 wk before stocking in spat culture tanks (Fig. 5). Oyster densities are determined using four 2-mL sub-samples counted under a compound microscope. After stocking in spat culture tanks, oysters are fed with mixtures of cultured microalgae (Isochrysis galbana, Pavlova lutheri, Chaetocerus muelleri, and Tetraselmis suecii) twice daily on weekdays and once daily on weekends. Algal mixtures are cultured in 2-L glass flasks (Fig. 6) in autoclaved, 0.45-µm filtered, and ultraviolet treated seawater with a salinity of 33 to 34 ppt using f/2 algal nutrient medium (Guillard 1983). Algal density is maintained between 105 to 106 cells/mL in tanks. Equal volumes of I. galban, P. lutheri, and C. muelleri are added daily, with total volumes increasing from 300 mL at 12 days, 375 mL at 24 days and 800 mL at 48 days after stocking. T. suecii is added to the mixture after 48 days. Tank volume is maintained by supplemental additions of UV-filtered seawater from the nearby lagoon. Careful daily maintenance and a regular supply of quality microalgae diets enabled the CMI Hatchery to achieve a reliable supply of oyster spat (Doroudi et al. 1999). Density Study We conducted a study to assess the effect of spat stocking density on growth and survival of black-lip pearl oysters. Spat culture tanks were 1.32 m long by 0.71 m wide by 0.31 m deep (269 L; 0.94 m2). One-month old spat (0.5 mm; Fig. 7) were stocked at three densities to compare survival and growth: 500 FIGURE 3. Co-author and pearl oyster research scientist Tabwi Aini measures oyster growth. FIGURE 4. The lead author explains pearl oyster aquaculture to visiting students. FIGURE 5. Tanks (0.94 m2) used for spat culture density study. FIGURE 6. Cultured microalgae are used to feed oyster spat.
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