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

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 2008been retained within the inshore lagoon. This suggests that heavy rains during cyclonesmay have a significant effect on the availability of spat.5.2.3 Silver-lip Pearl OystersNo information was found about the collection of wild silver-lip pearl oyster spat, probablybecause the industry is based on wild adult individuals of a particular size range orhatchery-derived spat.5.3 Intermediate Grow-out of Juveniles5.3.1 GeneralJuvenile pearl oysters originating from hatcheries or spat collectors are often placed inintermediate grow-out systems until they become a large enough (e.g. 50-60 mm shelllength) for transfer to the main grow-out system. These systems are usually deployed insheltered environments which provide protection from rough seas and adverse weatherconditions. Bays, inlets, channels and lagoons with gentle tides and an area of land nearbyfor establishing a land base are considered suitable grow-out locations (Passfield 1989).5.3.2 Black-lip Pearl OystersIn the Solomon Islands, the use of intermediate grow-out systems is essential to reducemortality caused by invertebrate and fish predators. The growth and survival of juvenileshas been assessed in lantern nets suspended from longlines at depths of 3-4 m at nineshallow water sites throughout the Solomon Islands and at depths of 9-12 m and 35-45 mwithin an area of the Gizo Lagoon, which had numerous passages and submerged reefslinking it to the ocean and an average depth of 40 m (Friedman and Southgate 1999).Juveniles with initial DVM of 8.3-51.5 mm increased in size by 20.4-24.8 mm in three monthsand 30.7 to 36.5 mm in five months. These growth rates are comparable with those reportedin Takapoto atoll, French Polynesia, where juveniles of 40-50 mm DVM grew 30 mm in sixmonths (Coeroli et al. 1984), and are much better than in the Cook Islands, where hatcheryproduced juveniles with a DVM of 10 mm grew 6.4 mm in three months (Braley 1997). Thesurvival of juveniles in lantern nets deployed in shallow water was poor due to predation byfish and invertebrates. Mortality due to reef-associated fish was reduced by moving theculture systems into deeper water. Friedman and Southgate (1999) pointed out that whilstthe high nutrient load in the Solomon Island lagoons may have a positive effect on rate ofgrowth, it also has an economic cost because it promotes algal fouling which needs to beremoved regularly from the culture units.The growth rates, survival and condition index of small black-lip pearl oysters over a 14month period has also been compared in two contrasting environments within the GreatBarrier Reef lagoon, a mainland bay (Bowling Green) and an area located adjacent to afringing reef on the leeward side of Orpheus Island approximately 20 km offshore (Yukihiraet al. 2006). The bay is subject to seasonal input of sediment from rivers and periodic resuspensionof fine material during strong winds, so the water is typically turbid andcontains large amounts of SPM and POM. The water at the offshore location, in contrast, isrelatively clear and characterised by small concentrations of SPM and POM, similar to thosefound within an oligotrophic French Polynesian lagoon (Takapoto). The temperature rangewithin the bay (19.8 - 30.7 °C) is also slightly greater than that at the offshore location (21.5 -29.6 °C). Small black-lip pearl oysters grew faster, suffered significantly less mortality andThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 23