Stimulating investment in pearl farming in ... - World Fish Center

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 2008turbid conditions, silver-lip pearl oysters had significantly greater clearance and ingestionrates and absorbed food more efficiently, but produced pseudofaeces at a slower rate thanblack-lip oysters. Silver-lip oysters also had a greater SFG under turbid conditions. Underthese conditions, silver-lip pearl oysters are clearly more efficient suspension feeders thanblack-lip oysters. Black-lip oysters, in contrast, had a higher clearance rate, retained smallerparticles and ingested a wider size range of particles at low SMP concentrations than silverlipoysters. This study indicated that the optimum SPM concentrations for black-lip andsilver-lip oysters were < 5 mg l -1 and 3-15 mg l -1 , respectively and their optimum particle sizeranges were > 3 µm and > 4µm, respectively. This information may be useful for selectingoptimum pearl culture sites.The feeding responses of black-lip oysters has also been assessed in Takapoto atoll lagoon,French Polynesia, an oligotrophic system, dominated by small (< 2 µm) primary producersand characterised by very small quantities ( 1 µm. The retention efficiency of the oysters was found to be 98% for particles > 5 µm, butto decline to 35% and 15% for 1.7 µm and 1 µm particles, respectively (Pouvreau et al.(1999b). In this system, high clearance rates (200 to > 1500 l d -1 for animals aged 1-4 years)appear to compensate for the poor food quality. The feeding responses of black-lip oysters,however, may differ in Polynesian atolls with larger quantities of POM Pouvreau et al.(2000a). It is also worth noting that black-lip oysters from the GBR retain about 70% of 1.7µm particles (Yukihira et al. 1999) and exhibit a lower clearance rate than those in Takapotoaatoll (Yukihira et al. 1998).Loret et al. (2000a) have shown that the black-lip oysters in this lagoon are selective feeders,consuming large amounts of nanoflagellates, one of the most abundant groups of primaryproducers, but only small quantities of picocyanobacteria, the other dominant group,probably because they are too small to be retained. They also showed that cryptophytes,one of the less abundant phytoplankton groups, were consumed preferentially and that thedigestion efficiency for chlorophytes and dinoflagellates was poor. These oysters are alsocapable of consuming and retaining ciliates (Loret et al 2000b). These results conflict withGervis and Sims (1992) suggestion that pearl oysters are non-specific feeders.2.9 ConclusionsBlack-lip pearl oysters have a wider geographic distribution and are presumably adapted toa greater range of temperatures than silver-lip pearl oysters. The latter species, however,apparently occupies a greater variety of habitats and has a more extensive verticaldistribution than its black-lip counterpart. The response of these two species to fluctuationsin environmental factors is poorly documented and studies comparing the responses of thetwo species and different developmental stages of each species are rare. Comparisons arealso hindered by the lack of information on latitudinal differences in physiological functionswithin each species. In Australia, adult silver-lip pearl oysters appear to be less tolerant oflower temperatures, but more tolerant of higher temperatures than black-lip pearl oysters.They are also capable of coping with a wider range of food concentrations and more turbidconditions than black-lip pearl oysters. This suggests that silver-lip pearl oysters have thepotential to be cultured under a greater variety of environmental conditions.The above review also suggests that little is known about the environmental conditionsunder which natural populations of black-lip and silver-lip pearl oysters grow in the SouthPacific region. Most of the information obtained about the environmental needs of pearloysters came from Australia or French Polynesia.The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 6