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

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 2008Silver-lip oysters are found in a variety of environments, ranging from shallow seagrassbeds to deepwater reefs and on sedimentary substrata adjacent to continents and largeislands, where the water is often turbid due to large inputs of terrigenous sediments andnutrients and higher productivity levels (Gervis and Sims 1992). They have been found atdepths varying from 0 - 80 m (Gervis and Sims (1992) and occur at depths of 10 - 60 m in theSolomon Islands (G Tiroba pers. comm. in Richards et al. 1994).2.6 Type of SubstratumThe distribution of pearl oysters is thought to be limited by the availability of suitablesubstrata (Gervis and Sims 1992). Black-lip oysters are scarce or absent in some FrenchPolynesian lagoons, because of limited substratum availability (Service de Pêche 1970 inGervis and Sims 1992). Some workers indicate that this species is absent from soft bottoms,however, there is a report of them occurring only on sand in one of the Kiribati atolls. Spatof silver-lip oysters only settle on hard substrata and an aggregate or seabed with a hardcrust covering a soft substratum is considered an ideal substratum in Western Australia.They also mentioned that adult silver-lip oysters occur on mud or sand in association withseagrass beds and that they may be shifted to these areas when their byssus detaches.In Western Australia, silver-lip pearl oysters are found on areas of seabed consisting of flatbasement rock with crevices which are generally covered by a few millimetres of finesediment and a variety of organisms which extend up to 1 m off the bottom (Wells andJernakoff 2006). Several distinct habitats are recognised within the pearl fishing groundsand these have been assigned the names “potato” bottom, “garden” bottom, “collar“ and“asparagus”, reflecting the form of the dominant inhabitants. Commercial quantities ofsilver-lip oyster are found only on potato bottom and garden bottom habitats. “Potato”bottom is dominated by a low, round densely-packed ascidian species, followed by a varietyof sponges ranging in size from a few centimeters to 0.5 m high. “Garden” bottom ischaracterised by large hydroids (up to 1 m high) covered by a variety of other, colourfulorganisms, hence the name. Sponges, soft corals, sea pens, crinoids and a large species ofascidian are also found on “garden” bottoms. The composition of the habitat varies withdepth, with “potato/asparagus” habitat predominating at 9-12 m, “potato/garden” bottomsoccurring at 13-16 m and depths of 16-34 m being characterised by “garden bottom” (Hartand Friedman 2004).In Solomon Islands, silver-lip pearl oysters have been found in a variety of habitats,including mud, sand, gravel, coral rubble and various mixtures thereof (The Ecology Lab2008)2.7 CurrentsGervis and Sims (1992) pointed out that silver-lip oyster beds often occur in areas with verystrong currents and that growth of both species is greater in areas with strong currents.2.8 Food AvailabilityPearl oysters are filter-feeders and obtain their food resources by actively clearing organicparticles, such as phytoplankton, from the water column. The quantity and quality ofnatural SPM has been shown to affect feeding and food utilization in black-lip and silver-lippearl oysters from the Great Barrier Reef (GBR) (Yukihara et al. 1999). Both species reducedtheir clearance rates and absorption efficiency, but increased their production ofpseudofaeces when exposed to large concentrations of SPM (i.e. high turbidity). UnderThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 5