An investigation was carried out in order to estimate the optimum yield (OY) from the fisheries exploiting multiple demersal fish stocks in Japan's coastal and offshore waters. In this paper, OY is the maximum static net economic yield from multiple stocks that are simultaneously exploited by various fisheries. Dynamics of the size of the whole stocks are approximated using a discrete surplus production model that is expressed by an algebraic function of the depletion level of the whole stocks. Estimates of biological parameters in the model and depletion levels of the whole stocks over the last several decades are available from a previous study and fish price and cost parameters were estimated using catch and economic statistics for 1973-2013. Estimates of OY calculated using those parameter values increased from 40 billion yen in 1973 to 196 billion yen in 2013. The level of fishing coefficient for realizing the OY in 2013 was 84% of the actual level in 2013. Application of the results to stock management and issues to be resolved in the near future are discussed.

　To clarify the population structure based on the genetic differences in three species of yellowtail amberjacks (Seriola lalandi, S. dorsalis and S. aureovittata) distributed in the high seas, we genetically analyzed mtDNA regions (ND4, CR and COI) of samples from the Northwest Pacific Ocean (coastal areas of Japan, high seas of the Northwest Pacific Ocean and the waters around Emperor Seamounts), and the Tasman Sea. As a result, there was no genetic difference between the samples of the Northwest Pacific Ocean, but there was a significant difference between samples of the Northwest Pacific Ocean and the Tasman Sea.

　To clarify the natural distribution of Japanese eels in Japan, otolith oxygen and carbon stable isotope analyses were conducted by analyzing collections of data on Japanese eels in scientific archives. The stable isotope analyses can segregate naturally recruited eels from stocked eels. As a result, coastal regions of the whole area of Kyushu, the Seto Inland Sea, the Pacific side south of Aomori Prefecture, and the Sea of Japan side south of Kyoto Prefecture were identified as main areas of natural distribution, and coastal regions from Fukui Prefecture to Aomori Prefecture facing the Sea of Japan were identified as marginal areas of natural distribution. Even in the main area, some rates of naturally recruited eels were less than 30%, suggesting that the eel resources inhabiting Japanese rivers and lakes are dependent on stocked eels.

　A harmful bloom of the raphidophycean flagellate Chattonella spp. occurred in the Yatsushiro Sea, Japan in summer 2016. The mechanism of occurrence of the bloom and the relationship to mass mortality of cultured yellowtail Seriola quinqueradiata were examined by analyzing field monitoring data. After nutrient concentration increased in the surface layer due to vertical mixing and intrusion of upwelled offshore water induced by meteorological disturbance, the population of Chattonella rapidly developed from the central to the southern area during early to mid-September when stratification formed again. The rate and timing of mortality of cultured yellowtail corresponded to cell density and the timing of increase of Chattonella spp., which were influenced by the strength of stratification in each fish farm. The present study suggests that real-time monitoring of oceanographic conditions is important for predicting short-term dynamics of Chattonella and for conducting countermeasures such as stopping feeding at appropriate times.