Enrichment of r-process elements in dwarf spheroidal galaxies in chemo-dynamical evolution model

The rapid neutron-capture process (r-process) is a major process to synthesize elements heavier than iron, but the astrophysical site(s) of r-process is not identified yet. Neutron star mergers (NSMs) are suggested to be a major r-process site from nucleosynthesis studies. Previous chemical evolution studies however require unlikely short merger time of NSMs to reproduce the observed large star-to-star scatters in the abundance ratios of r-process elements relative to iron, [Eu/Fe], of extremely metal-poor stars in the Milky Way (MW) halo. This problem can be solved by considering chemical evolution in dwarf spheroidal galaxies (dSphs) which would be building blocks of the MW and have lower star formation efficiencies than the MW halo. We demonstrate that enrichment of r-process elements in dSphs by NSMs using an N-body/smoothed particle hydrodynamics code. Our high-resolution model reproduces the observed [Eu/Fe] by NSMs with a merger time of 100 Myr when the effect of metal mixing is taken into account. This is because metallicity is not correlated with time up to ~ 300 Myr from the start of the simulation due to low star formation efficiency in dSphs. We also confirm that this model is consistent with observed properties of dSphs such as radial profiles and metallicity distribution. The merger time and the Galactic rate of NSMs are suggested to be <~ 300 Myr and ~ $10^{-4}$ yr$^{-1}$, which are consistent with the values suggested by population synthesis and nucleosynthesis studies. This study supports that NSMs are the major astrophysical site of r-process.Comment: 16 pages, 16 figures, accepted for publication in Ap

A model for understanding the influence of selective and nonselective harvest on fish populations

To understand the influence of selective harvesting on the adaptive capacity of fish populations in changing environments, a computer simulation model was developed. In the model, hypothetical populations were composed of five different life histories, which were genetically determined. Each life history type had its own rate of survival and reproduction that varied with changing environment and population density. In nonselective harvesting, life history types were equally vulnerable to harvest. In selective harvesting some life history types, were more intensively harvested than others. Population life history composition changed continuously in response to both changing environmental conditions and harvesting. Changes in life history composition were closely related to changes in numerical population performances such as density and yield. In general, selective harvesting of the simulated populations reduced mean and terminal abundance, total catch, and life history diversity. Nonselective harvesting tended to eliminate individuals evenly from the life history distribution so that the life history diversity was not greatly influenced. Selective harvesting tended to severely reduce or eliminate the life history types that were more intensively harvested. This generally resulted in extreme reduction of life history diversity. Variation in abundance, as measured by the absolute value of the residuals, was consistently larger in selectively harvested populations. Increased variation was probably related to reduction in life history diversity. Perhaps the most potentially alarming impact was the dramatic increase in incidences of extinction of selectively harvested populations at the highest exploitation rate. High selective harvest rate led to severe reduction in life history diversity which diminished the population's ability to persist in a changing environment. Furthermore, the extreme reduction of life history diversity made it difficult for populations that were able to persist to recover in abundance after termination of harvest. It is important to note, however, that the above conclusions are general outcomes based on 20 different environmental regimes. The results from a few individual regimes were not in accordance with some of the general outcomes, illustrating simply that particular results are highly contingent on the particular environmental pattern

Status of resource enhancement and sustainable aquaculture practices in Japan

Contrary to the rapid increase in the world aquaculture production, fish production in Japan has been decreasing slightly due to the decreasing trend in seafood consumption of Japanese. Aquaculture production is approximately 20% in terms of yield, and 30% in terms of market value, of the country s total fisheries production. In Japan, about 80 species are targeted for release for sea ranching and resource enhancement purposes. The local governments (prefectures) are the main driving force in resource enhancement programs. Chum salmon, Oncorhynchus keta, and scallop Mizuhopecten yessoensisis are examples of successful resource enhancement in Japan. Japanese flounder, Paralichthys olivaceus, and red seabream, Pagrus major, represent intensely released fish species in Japan, and around 10% of the total catch of those species are estimated as released fish. The low price of products and increasing costs of production, such as costs of fuel and fish meal, are the major pressing issues in coastal fisheries and aquaculture in Japan. For aquaculture, the guarantee of food safety, minimization of environmental impact, and management of natural stock populations are highly necessary in order to achieve the sustainability of the industry. For resource enhancement, budget constraint is the major issue, and possible impact on natural stocks caused by released fish should also be considered. The Government of Japan (GOJ) is implementing some measures to rectify unstable business practices of aquaculture and to improve production techniques in aquaculture. For resource enhancement, the GOJ encourages cooperation among local governments (prefectures) for seed production and release of certain targeted species in order to reduce the cost and improve the efficiency of stock enhancement. In Japan, traditionally, the purpose for release was mainly sea ranching, namely harvesting all released animals. Nowadays, actual resource enhancement, i.e. the integrated release program including resource management and development of suitable nursery for released fish, is encouraged by the government. The evaluation and counter measures for the negative impact of stocked fish on genetic diversity of the wild population are also implemented. Recently, marked progress was achieved in seed production technologies of two important tropical fish species, namely coral trout, Plectropomus leopardus, and humphead wrasse, Cheilinus undulatus. These technologies are expected to contribute to the advancement of the aquaculture industry in the South East Asian region