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

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 2008maintained a better condition at Orpheus Island than in Bowling Green Bay. This impliesthat the environmental conditions were more suitable for the growth of juveniles offshorethan in the mainland bay.5.3.3 Silver-lip Pearl OystersYukihira et al. (2006) also compared the growth, survival and condition of small silver-lippearl oysters cultured in two dissimilar environments in the Great Barrier Reef lagoon (seeSection 5.3.2 for a brief description of these sites). These animals, in contrast to black-lipoysters, grew at similar rates at both sites and exhibited distinct seasonal differences with noor minimal growth occurring in winter. At the end of the study, there were no significantdifferences between sites in the condition of the oysters. The lack of difference in growthrates and condition between sites is surprising given the substantial differences in their SPMand POM concentrations. It suggests that there may be also be a difference in quality as wellas quantity of POM between sites and that the food at the offshore location may have beenof better quality than that in the bay. Small oysters failed to survive at both sites, with mostof the mortality occurring in winter. The rate of mortality was slightly smaller in themainland bay (50%) than offshore (60%). The seasonal difference in survival suggests thatpearl farmers should not transfer spat from hatcheries or warmer environments to siteswhere the temperature drops below 25°C in winter (Yukihira et al. 2006).No other information was found about the influence of habitat on grow-out of juvenilesilver-lip pearl oysters, presumably because most of the industry is based on wild caughtoysters of a target size (120-160 mm).5.4 Ongrowing of Adult Pearl Oysters5.4.1 Black-lip Pearl OystersIn the Cook Islands, the growth of black-lip pearl oysters has been investigated at differentsites within Manihiki Lagoon (Tauhunu, Paerangi and the Fishing Reserve), among lagoons(Rakahanga, Manihiki and Suwarrow), in different natural habitats (rock vs sandy bottom attwo depths 15-17 m and 33-35 m) and under different culture conditions (platform andlongline) (Sims 1994). The results, however, are difficult to interpret because of theexperimental design. At Paerangi, growth on long-lines was faster than that on the platformwith estimates for time to reach 120 mm DVM (T 120) being 1.2 and 1.4 years, respectively.The linear growth of oysters deployed on longlines and platforms was almost three timesgreater than those placed on rock bottom. At Tauhunu, linear growth rates of oysters in the100-119 mm size class were twice as great on the platform as on rock, but no such differencewas evident in the 120-139 mm size class. Linear growth of oysters in the 80-99 mm sizerange was faster on platforms at Paerangi than at Tauhunu. In Manihiki Lagoon, theestimated time to reach T 120ranged from 1.2 years on long-lines to over 6 years for oystersplaced on natural substrata in deep water. Linear increments at depths of 15-17 m weretwice as great as those at depths of 33-35 m. Growth rates also differed between substrata indeep water, with maximal length being significantly higher on rock than on sand and lineargrowth being slightly faster on rock, particularly in larger size classes. In mid-depths, therewas no difference in growth between sand and rock trials. Growth on platforms in Manihikiwas slightly faster than in Rakahanga. Growth of oysters on longlines was more than twiceas fast in Manihiki (T 120=1.2) as in Suwarrow (T 120= 2.9). Differences in growth between sitesin Manihiki were attributed to either differences in densities of oysters on platforms or site-The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 24