WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2013 47 spat/tank (532 spat/m2), 1000 spat/tank (1064 spat/m2) and 2000 spat/tank (2128 spat/m2). Each density was replicated in three tanks. Haws and Ellis (2000) recommend 12 cm2 of tank area for each 1-cm spat, equivalent to 785 spat for the 0.94-m2 tanks at the hatchery. Larval oysters were stocked into spat culture tanks and reared for 6 weeks on cultured algae. The harvest size (ca. 3 mm) is suitable for pearl production. As spat density increased, oyster growth rate decreased (Table 1), although differences in final size were modest. Survival in most tanks, irrespective of density, was 40-60 percent. Variability in survival increased with density (Table 1). If culture facilities are limiting, the higher densities of those tested are recommended. If ample facilities are available, spat can be stocked at the lower density to maximize growth rate. Temperature, pH, salinity and conductivity were fairly stable throughout the study (Table 2). However, low dissolved oxygen concentration may have affected growth and survival. Dissolved oxygen concentration was less than 5 mg/L for 35 percent of measurements. Challenges of Hatchery Management at the College of the Marshall Islands Operation of the oyster hatchery revealed some emergent issues that require attention: • Numerous power outages at the hatchery (twice weekly for about 2-6 hours each day) made it difficult to maintain constant water exchange and circulation for dissolved oxygen management. • Theft of stocks caused a decline in large oysters for pearl production. This was detrimental because our intention was to develop an inexpensive method to expand the hatchery oyster population with limited resources and trained personnel. • Our team lacked multiple trained personnel; only one technician knew how to culture algae under sterile conditions. Communication was poor regarding certain details and needs for hatchery production. • There was limited monitoring equipment and other technological advantages associated with large-scale hatcheries. However, careful management and well-trained personnel are likely more important than technology to successful hatchery operations. Personnel at CMI were generally efficient and accurate. • There is limited information exchange among people in industry, academia, and government. These groups do not always work together as a team to help ensure oyster production. These issues need attention and resolution for the hatchery to serve as the primary supplier to the pearl industry in the RMI. Despite these challenges, black-lip pearl oysters are a prolific species that should continue to be a focus of economic development in the RMI. FIGURE 7. Black-lip pearl oyster larvae (0.5 mm) used to stock spat culture tanks for a density study. FIGURE 8. Layout of larval and spat culture facilities at the College of the Marshall Islands black-lip pearl hatchery. Larval Collection (CONTINUED ON PAGE 48) TABLE 2. Minimum and maximum values of water quality parameters measured during a study to evaluate the effect of stocking density on spat growth and survival. Temperature, salinity, dissolved oxygen, pH, conductivity in each tank was measured daily with a YSI 556 multiprobe and total alkalinity and total phosphate concentration were measured weekly with a YSI 995 photometer. Parameter Minimum Maximum Temperature (C) 27.5 31.9 Salinity (ppt) 32.8 34.1 Dissolved oxygen (mg/L) 3.7 20.4 pH 7.6 8.0 Alkalinity (mg/L as CaCO3) 60 115 Total phosphate (mg/L) 0.04 0.92 TABLE 1. Final length and survival of black-lip pearl oyster spat stocked at three densities in 0.94-m2 culture tanks and grown for 6 weeks. Density Final length Survival (spat/tank) (mm) (%) 500 3.5 ± 1.4 61 ± 3 1000 2.9 ± 1.3 51 ± 11 2000 2.7 ± 0.8 75 ± 30
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