402 research outputs found
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 ~ yr, 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
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A model of fall chinook salmon (Onchorhynchus tshawytscha) life history
The research involved development of two ecological
simulation models to explain the complex dimensionality of
chinook salmon (Oncorhynchus tshawytscha) life history
structure (represented by the age composition of the
spawning stock) and management difficulties entailed in the
complexity.
Since different sizes of chinook salmon are thought to
adapt differently to heterogeneous habitats, age composition
of the spawning stock is determined by characteristics of
the habitats of the substocks. Numerical properties of
substocks result from the incorporation of individual
spawners in different age classes and each substock performs
differently because their age compositions are distinctive.
A stock or population consists of substocks whose age
compositions are concordant with their habitats. The
productive capacity of a population will result from the
incorporation of substocks. If habitat structures of
streams are different, the age and size compositions and
productive capacity of the populations may differ.
Selective harvesting affects spawners in different
ways, so that age compositions must be deformed differently
by fishing pressure. Once the age composition deviates from
the natural age composition, the productivity of the
population will decrease.
Population dynamics are strongly correlated with
substock structure which is determined by habitat structure
in a stream system. Hierarchical population structure make
fisheries management difficult and requires not only
quantitative but also qualitative analysis on the
populations in relation to habitat classification
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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
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