The activities of microorganisms seriously affect the growth, survival and production rates of fish (including crustaceans and shellfish). In fact, there are useful and harmful microbes in the aquaculture environment. One of the main concerns of people engaged in aquaculture is the methods for how to keep pathogenic microorganisms, mainly bacteria and viruses, away from fish rearing water. To this end many techniques for sterilization are commonly adopted and people tend to believe that these processes can eventually eliminate all microbes in water, and produce and maintain a nearly pure water quality. However, with these treatments the occurrence of pathogenic microorganisms which cause fish disease cannot be permanently removed. In spite of that, after such treatment microorganisms grow very quickly because of the presence of less antagonism among the bacterial populations. Furthermore, no one can anticipate what kind of microbial species may grow in the vacant space produced by the above treatments. The antagonism among microbes is a naturally occurring phenomenon through which pathogens can be killed or reduced in number in the aquaculture environment. Thus method, which is called biological control or biocontrol, is already familiar in the field of agriculture. In order to apply such biocontrol to the aquaculture environment, biocontrol agents (BCAs) that can repress the growth of pathogenic bacteria and viruses have been sought by many researchers. Although some results indicate a need for further studies of how individual microbes inhibit or promote fish health and growth, many other studies show BCAs that repressed the growth of pathogens and also promoted the survival rates of fish, including the identification of the microbes utilized. In this article, studies on the use of microorganisms as BCAs are reviewed in an attempt to provide an all encompassing understanding of the BCAs's role in aquaculture.

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Year-to-year variations in salinity and in the DIP (dissolved inorganic phosphorus)·DIN (dissolved inorganic nitrogen) budgets in the head of Ariake Bay from 1990 to 2000 were studied using the data observed every month by the research institutions of the prefectures of Saga, Fukuoka and Kumamoto. The most striking result of our analyses showed a decrease of water exchange ratio, and an increase of mineralization rate of phosphorus and an increase of the denitrification rate in late 1990 compared to early 1990.

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To determine the effects of environmental management and the pollution load from land in the Seto Inland Sea, the water quality has been monitored for many years by the Ministry of Environment, Japan. By using this observed data, we proposed an index of water quality control in the coastal area, and clarified the differences in characteristics of various areas in the Seto Inland Sea. Consequently, it was shown that the effect of pollution load from land varied from area to area. This suggests that the degree of water quality improvement by control of total phosphorus and nitrogen loads differs from area to area in the Seto Inland Sea.

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Among the numerical simulations and data analysis published by several authors, a few different analytical results have been obtained with respect to the effect of the Isahaya sea dike on the decrease of M₂ tidal amplitude in Ariake Bay. Since the differences in the computed values are mainly caused by the setting of the open boundary conditions, the effect of them was studied on the basis of one dimensional theory of co-oscillating tide. The tidal observations as well as this theory show that the analytical results given by Nadaoka and Hanada (2002) and Unoki (2003) are more reasonable than those given by Tsukamoto and Yanagi (2002) and Fujiwara et al. (2004), who assumed too much decrease of amplitude at the open boundary after the construction of the dike. Thus it can be concluded that about half of the decrease of tidal amplitude in Ariake Bay has been caused by the closing of the dike accompanied by the Isahaya reclamation project. As evidence of this issue, the problems of the analytical method and its results given by Fujiwara et al. (2004) are discussed.

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Sea level response to changes in atmospheric pressure and its regional characteristics were investigated using sea level records from 85 tidal stations along the coasts of Japan and from 3 Korean stations facing the Japan Sea. The traditional inverted barometer (-1 cm hPa^-1) assumption is shown to be justified only near the coastal region facing the Pacific Ocean, except in the shallow bays. In the semi-enclosed Japan Sea and the Sea of Okhotsk, sea level lags behind synoptic atmospheric pressure variations by up to half a day. The lag along the coasts of the Japan Sea seems to be proportional to the distances from the Straits of Tsushima, Tsugaru, and Soya. We propose a new 'dynamic' barometric correction based on the response in different geographical locations. Using this correction, we have been successful in removing the barometric component from sea level records more accurately than the conventional inverted barometric correction, and in extracting a more realistic propagation of continental shelf waves along the San'in Coast.

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