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 2008A number of operational problems were encountered during the censuses, including strongcurrents that exceeded optimal imaging speed, irregular terrain that resulted in snaggingand/or flipping of the camera and battery failure, the last of which resulted in the loss ofdata. Problems were also encountered when some of the video material from Malaita andSandfly Passage was transcribed onto DVD. Operating this complex electronic equipmentfrom a small boat was demanding when rain fell. The number of transects surveyed at eachlocation and the quality of the images consequently varied markedly (Appendix 2).On return to Australia, DVDs were viewed on a computer screen using the softwarepackage Power-DVD. The quality of the video material varied, with some sections beingeither difficult or impossible to analyse, because the camera had been towed too fast or hadbeen positioned too high above the substratum or the water was silty and visibility waspoor. The types of substrata observed along each transect were recorded and any organismsresembling pearl oysters were counted and measured. Measurements were taken relative tothe 160 mm distance between the two laser spots projected from the camera, when thesewere visible.2.4 Statistical MethodsThe survey was designed so that nested analyses of variance could be used to assess thesignificance of variation in the density of silver-lip pearl oysters among and within fishinggrounds. Due to the absence of pearl oysters on some fishing grounds and their scarcity atothers, these analyses were restricted to Mboli Passage, Kia and Waghina, where pearloysters were relatively abundant. In these analyses, location was considered a fixed factor,whereas sites and transects were random factors. These analyses were based on densities ofoysters per 400 m 2 transect (i.e. they were standardised to compensate for differences in thelength of transect surveyed). Data were checked for homogeneity of variances usingCochran’s C Test prior to doing the nested ANOVA. Student Newman Keuls (SNK) testswere used to identify which levels of significant factors differed. Further details of theseanalytical procedures are available in Underwood (1997). One-way analysis of variance wasused to compare the mean lengths of oysters among the three locations.2.5 Estimates of Population SizeThe size of the silver-lip oyster populations at Kia, Mboli Passage and Waghina, the fishinggrounds with the most abundant stocks, were estimated as follows:1. First, raw counts per transect were corrected for sampling error on this type of seafloor by multiplying by 2.7. This is based on the assumption that collecting efficiencyduring the survey was similar to that for Australian divers working over similarhabitats (i.e. 37%) (Hart and Friedman 2004), in practice the efficiency of the diversused, who were inexperienced in pearl survey my have been less than this resultingin an underestimate of abundance;2. Second, corrected counts per 400 m 2 transect were multiplied by 25 to give estimatesper hectare; and3. Finally, densities per hectare were multiplied by estimates of the total habitat areaavailable at each locality derived from MaxSea digital bathymetric charts.These calculations were based solely on the density estimates derived from underwatervisual census, because of the uncertainty about the identification of oysters on the remotevideo censuses.The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 7