Yasuhiro Obata

Allozyme variation of littleneck clam Ruditapes philippinarum and genetic mixture analysis of foreign clams in Ariake Sea and Shiranui Sea off Kyushu Island, Japan

Allozyme variation of the littleneck clam Ruditapes philippinarum was evaluated in four samples from Nameishi and Matsuo in the Ariake Sea, Ryugatake and Ushibuka in the Shiranui Sea off Kyushu Island, Japan, and in one sample from Jinzhou, China, in the Bohai Sea. A Ruditapes bruguieri sample imported from the Korean Bay off Nampo, North Korea was also studied. Among the R. philippinarum samples, heterozygosity varied from 0.265 to 0.301 and Fis estimates indicated significant homozygosity excess in 15 of 40 loci analyzed. Deviations from Hardy-Weinberg equilibrium were significant in all samples (P<0.05). Pairwise FST estimates indicate that genetic differences between the Chinese and Japanese samples were very low, but significantly different from zero. Mixture proportions with 95% confidence intervals of Chinese R. philippinarum in Nameishi and Matsuo were estimated at 0.4098 [0.2512, 0.5705] and 0.4899 [0.3262, 0.6540], respectively. However, genetic invasion of stocked Chinese R. philippinarum into wild populations in the Ariake Sea remains uncertain due to the low precision of the estimates caused by the high similarity of allele frequencies between Jinzhou and the Ariake Sea.

Evaluation of Stocking Effectiveness of the Japanese Spanish Mackerel in the Eastern Seto Inland Sea, Japan

This study evaluated the stocking effectiveness of two release groups of Japanese Spanish mackerel, Scomberomorus niphonius, in the eastern Seto Inland Sea, Japan. Juveniles of about 40 mm in total length (TL) that were directly harvested from seed production tanks and 100 mm TL that were reared in nursery facilities were released from early June to early July. The otoliths of released juveniles were marked with alizarin complexone. In 2002, 51,000 40-mm and 82,929 100-mm juveniles and, in 2003, 94,000 40-mm and 77,193 100-mm juveniles were released. A total of 2,761 individuals were sampled at the 14 main fish landing markets between 2002 and 2004, from September in the release year to June the next year. Recapture rates, yields from the released juveniles, contribution rates to the commercial landings, and the economic efficiency were estimated with standard errors. The variance estimators were derived by assuming simple random sampling of the commercial landings. All estimates for evaluating stocking effectiveness showed significantly higher values in the 100-mm release group than in the 40-mm release group. Consequently, releasing 100-mm juveniles reared in nursery facilities was more efficient than direct release of 40 mm juveniles.

Simultaneous Estimation of Mixing Rates and Genetic Drift under Successive Sampling of Genetic Markers with Application to the Mud Crab (Scylla Paramamosain) in Japan

In stock enhancement programs, it is important to assess mixing rates of released individuals in stocks. For this purpose, genetic stock identification has been applied. The allele frequencies in a composite population are expressed as a mixture of the allele frequencies in the natural and released populations. The estimation of mixing rates is possible, under successive sampling from the composite population, on the basis of temporal changes in allele frequencies. The allele frequencies in the natural population may be estimated from those of the composite population in the preceding year. However, it should be noted that these frequencies can vary between generations due to genetic drift. In this article, we develop a new method for simultaneous estimation of mixing rates and genetic drift in a stock enhancement program. Numerical simulation shows that our procedure estimates the mixing rate with little bias. Although the genetic drift is underestimated when the amount of information is small, reduction of the bias is possible by analyzing multiple unlinked loci. The method was applied to real data on mud crab stocking, and the result showed a yearly variation in the mixing rate.

Species Composition, Reproduction, and Body Size of Mud Crabs, Scylla spp., Caught in Urado Bay, Japan

ABSTRACT Species composition, reproduction and body size of mud crabs, genus Scylla de Haan, 1833, were investigated using gill nets from October 2008 to October 2009 in Urado Bay, Japan. Three mud crab species were identified in the area, with S. paramamosain Estampador, 1949 being the dominant species (74% of the catch), followed by S. serrata (Forskål, 1775) (23%) and S. olivacea (Herbst, 1796) (3%). We found temporal changes in species composition with abundances of S. serrata increasing towards the summer season. Ovigerous females of S. paramamosain and S. olivacea occur between January and October, peaking in the warm (May-July) and rainy (June-July) seasons, but we found no berried females of S. serrata. The female-biased sex ratios of S. paramamosain and S. serrata, and the larger body size of females during the period from autumn (November) to early spring (April), suggest that large gravid females may have migrated offshore before the spawning season. The mean body size of females and males of S. paramamosain and S. serrata tended to increase and the mating activity was high between May and October, showing the moulting and growth season.