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

Abundance, Size Structure and Quality of Silver-lip Pearl Oysters in the Solomon Islands Final, June 20083.3 Remote Video CensusEleven objects that could be “possible” pearl oysters were sighted across the video transectsanalysed. Two of these were dead silver-lip pearl oyster shells lying open on the seabed.“Possible” oysters that appeared to be alive were seen on only nine transects, with only onespecimen being evident per transect. Three “possible” oysters were found along the set oftransects surveyed at Kia and Waghina, whereas two “possible” oysters were observed offMalaita and Sandfly Passage. The identity of the “possible” oysters could not be confirmed,because the objects were overgrown by other biota. These animals were found on silty sand,living reef, a mixture of coral sand and rubble and on coral sand with live coral rubble. Onlyseven of the “possible” oysters could be measured relative to the laser guides; the five livespecimens varied in length from 68 - 116 mm, whereas the two dead specimens were 36 mmand 140 mm long. Overall this yields a maximum video census estimate of 8 possibleoysters per 400 m 2 .Ten types of benthic substrata were distinguished during the analysis of remote videotransects. The primary components identified were silty sand, coral sand, dead reef, lowrubble, coarse rubble, medium rubble, heavy rubble and living reef. The substratum at twosites off Malaita was dominated by silty sand, except for one of the transects at Site 1, whichwas composed of dead reef. The substratum at Site 3, Malaita was characterised by coralsand. The substratum at Sites 1 and 2, Sandfly Passage were also dominated by silty sand,but that off Site 3 consisted of either coral sand, a mixture of coral sand and rubble or livingreef. The substratum at Site 1, Kia consisted of silty sand or silty sand plus low rubble, whilethat at Site 3 was composed of either silty sand or silty sand plus heavy rubble. Thesubstratum at Site 2, Kia was quite different, consisting of either coarse sand and mediumrubble, coral sand and living coral or living reef. The substratum at Waghina consisted of amixture of coral sand and various grades of rubble.3.4 Estimates of Population SizeThe total area of potential pearl oyster habitat at Kia, Mboli Passage and Waghina wasestimated to be approximately 2000, 1250 and 4163 hectares, respectively (Figures 9 - 11).These areas were estimated to support a total of 136,000 ±, 42,016, 55,688 ± 14,598 and345,529 ± 141, 353 silver-lip pearl oysters, respectively. It should, however, be noted thatoysters < 175 mm, the size preferred by pearl farmers, comprised about 3.6% of the total,with total estimates being 1114 (4.3%) at Kia, 0 in Mboli Passage and 6,910 (5.4%) atWaghina.3.5 Number of Sustainable QuotasThe first step in a sustainable quota management system for silver-lip oysters is the settingof an annual total allowable catch (TAC). The latter is normally based on the informationthat is available about the abundance, size structure, recruitment, growth, reproduction, andmortality of the species. For the silver-lip oyster fishery to be sustainable, the loss ofbiomass due to natural mortality plus fishing mortality must not exceed the increase inbiomass due to recruitment of smaller individuals into the fishery, growth andreproduction. Given the small size of the silver-lip oysters stocks (and large size ofindividuals within the stocks) at present and lack of information on recruitment, growth,reproduction and mortality, it is not feasible to determine a TAC or allocate sustainablequotas to potential fishers at this time.The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 10