Studies on the improvement of the production efficiency of pearl culture industry in Nagasaki Prefecture

In order to stabilize the management of pearl culture industry in Nagasaki Prefecture, methods to improve production efficiency were investigated in this study. Firstly, the serum protein content of the Japanese pearl oyster Pinctada fucata martensii, an indicator of the selection of male and female parent oysters, was measured in 2002 and 2003, in order to develop a method of producing disease-resistant pearl oysters with high survival rates against the reddening adductor disease. The survival rate of 1-year-old oysters produced from parent oysters of the high content group was compared with those obtained from the low content group counterparts. The serum protein contents of parent oysters ranged from 0.53 to 1.62 mg/ml in 2002 and from 0.13 to 0.86 mg/ml in 2003. The high and low content groups were established from the higher 10% and the lower 17-35% oysters of the frequency histogram, respectively. The oysters obtained from the high content group showed high serum protein content and total body weight from autumn thereafter as compared to those of the low content group. At the end of the experiment, oysters obtained from the high content group were 5.0-20.4% higher in survival rates than those from the low content group. In addition, the survival rate of oysters produced from the high serum protein content group parents was compared with those obtained from the usual method group. Oysters obtained from the high serum protein content group showed 4.6-6.2% higher survival rates than those from the usual method group. Above results suggest that selection of parent oysters with high serum protein content can be effective in improving survival rate during seed production. Secondly, in order to find out the effect of low water temperature treatment in reducing the reddening adductor disease caused mortality of the Japanese pearl oyster in Nagasaki Prefecture, mortality and physiological traits were compared each month for one year between the different groups of oysters kept in various pearl farms where temperatures were below 13 degC during winter, whereas control animals were kept in a warmer area. As an indicator of low temperature burden, the low-water-temperature index [LTIsub(13)=sigma(13-Tn), Tn < 13] was calculated from the water temperature measured each day at 10 o'clock (am) for each group of oyster. As compared to control, the 100 degree-day group showed lower mortality, lower a-value (value obtained from the spectrophotometer and used as a color indicator for the reddening adductor), slower gonad maturation, higher glycogen content in the adductor muscle and higher phyto-pigment content in the digestive diverticula after wintering particularly from spring to autumn when the reddening adductor disease progressed. The high survival rate in the 100 degree-day group at the end of experiment may be attributed to the delayed onset of the reddening adductor disease which was a result of higher glycogen content, delayed maturation and increased food intake of the oysters. Thirdly, to improve the management of pearl culture, the production efficiency of mother-shells of difference ages was examined in Tsushima Island, Nagasaki Prefecture from June 2004 to January 2006. The survival rate, quality, value of production and price of pearls produced from 1-year-old oysters inserted 6.66 mm nuclei (1-year-old group) were compared with those from 2-years-old oysters inserted 7.27 mm nuclei (2-years-old group 1) and 2-years-old oysters inserted 7.42 mm nuclei (2-years-old group 2). Oysters in the 2-years-old group 2 were from the same group as the 1-year-old group, but nuclei were inserted in June 2005. The survival rate of the 1-year-old group was 32.0 % and 12.7% higher than those of the 2-years-old groups 1 and 2, respectively. The nacre thickness and rations of faultlessness and commercially valuable pearls produced from the 1-year-old group were 1.6 times and 2.2 times, 12.3% and 13.7%, 21.9% and 20.0% higher than those from the 2-years-old groups 1 and 2, respectively. The prices of 7 and 8 mm commercial pearls from the 1-year-old group were 1.3-3.4 times higher than those from the 2-years-old groups 1 and 2. The value of commercial pearl produced from the 1-year-old group was 2.6 times and 1.9 times higher than those from the 2-years-old groups 1 and 2, respectively. Above results suggest that using 1-year-old oysters for operation in June can be effective in improving the production efficiency of pearl culture. Finally, the a-value in the pearl layer of the pearl oyster, which is used as an indicator for selecting parent-shells, was measured by colorimetry method in June, 2004, and selected parent-shells were used in seed production. Seeds were then used in culture experiments at Nagasaki Prefectural Institute of Fisheries from Nov. 2004 to Dec. 2006. The purpose was to establish a method of producing sacrificed pearl oysters for graft (piece oyster) that produce high-quality pearl. The a-values in pearl layers and pearl qualities produced from piece oysters with parents from the plus field a-value [a(+)] group were compared with those from the minus field a-value [a(-)] counterpart. The a-values in pearl layers of seeds produced from the a (+) group were higher than values obtained from those of the a(-) group. Rates of production of pearls with white object color and pink interference color were significantly higher in the a(+) groups than those in the a(-) groups. Commercial values of pearls produced from the a(+) groups were 1.38-1.52 times higher than those of pearls obtained from the a(-) groups. Above results suggest that the a-value in the pearl layer is an effective indicator and selecting parent oysters with plus field during seed production can result in improved pearl quality. From above results, selection of parent-shells for use as mother and piece oysters, and culturing oysters for 100 degree-day in a culture farm, where the temperature drops below 13 degC during winter, can result in improved survival rate and pearl quality. Using 1-year-old oysters for the operation can be effective in improving the production efficiency of pearl culture.