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

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 20086.0 CONCLUSIONS6.1 Black-lip pearl oystersMost of the existing black-pearl farms in the Cook Islands and French Polynesia are locatedin fairly deep, ‘closed’ or ‘semi-enclosed’ atoll lagoons, some of which contain natural blacklippearl oyster populations. The limited exchange of water between these systems and theopen ocean results in the retention of pelagic larvae produced by the broodstock andfacilitates the collection of wild spat. These lagoons are typically oligotrophic and have lowturbidity levels (Pouvreau et al. 1999). Black-lip pearl oysters appear to compensate for thelow food concentrations by having a high clearance rate, retaining a wide size range of foodparticles and, in some circumstances, feeding selectively on certain components of thephytoplankton (Pouvreau et al. 1999b; Loret et al. 2000a).This species is also found throughout Melanesia and off the north Australian coast, wheremore ‘open’ coral reef habitats predominate. In these systems, there is much greaterexchange of water with surrounding habitats than in atoll lagoons. Spat are consequentlyless likely to be retained within ‘open’ systems. Despite this, it is clearly feasible to collectwild spat from ‘open’ reef habitats in the Solomon Islands, particularly from sites with clearwater, moderate currents that are at least 35 m away from the nearest reef (Friedman et al.1998). Surprisingly, collectors deployed in lagoons within barrier reefs, the reef habitatmost comparable to atoll lagoons in terms of enclosure, yielded the fewest spat. The lowyield appeared to be due to collectors becoming clogged with sediment or overgrown byalgae during the rainy season, when there is considerable runoff from the adjacent landmass. The lack of a suitable hard substratum within the barrier reef lagoon may also havecontributed to the low spat yields.Friedman and Southgate (1999) have shown that the growth rate of juvenile black-lip pearloysters cultured in the ‘open’ reef systems of the Solomon Islands compares favourablywith that observed in some of the atolls in French Polynesia and the Cook Islands. Theysuggest that the high nutrient load in the Solomon Island lagoons, resulting from runoffduring the rainy season, may have contributed to the good growth rates.In north-east Australia, black-lip oysters grown in a sheltered mainland bay took 14 monthslonger to reach a size appropriate for the first pearl nucleus implantation than those grownoffshore of a fringing reef site located 20km offshore (Yukihira et al. 2006). The poorerperformance of oysters in the mainland bay was attributed to the adverse effects of highconcentrations of SPM and POM, derived from the seasonal input of sediment from riversand periodic re-suspension of fine material deposited on the bottom by waves during strongwinds, on the species energetic processes. The poor performance of black-lip pearl oystersunder turbid conditions suggests that commercial culture may be less viable in this type ofenvironment.6.2 Silver-lip pearl oystersSilver-lip pearl oysters are often cultured in near-shore areas subject to large fluctuations insalinity and high inputs of terrigenous sediment and nutrients during seasonal rainfallevents. Reductions in salinity to levels as low as 25 ‰ during the rainy season has anadverse effect on growth rate and results in detachment of juveniles, but has no effect onsurvival of spat (Taylor et al. 1997 and 2004). This suggests that hatchery-reared spat shouldnot be transferred to these environments during the rainy season. The poorer survival andThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 26