The rice fish medaka has been genetically differentiated into the Southern and Northern populations. Both populations have parapatric contacts in the Yura River, Kyoto Prefecture, Japan. However, each population has genetically remained almost pure. This phenomenon suggests the reproductive isolation between the Northern and Southern populations. The ecological differentiation between both populations could be demonstrated by fish schooling behavior. They showed a significant difference in fish school radius, with the Northern Population forming a fish school with a larger radius than the Southern Population under conventional behavioral conditions. When a small number of individuals were analyzed, fish school radius did not show a significant difference between both populations. Moreover, the Southern Population congregated near the edge of a circular aquarium, which is a behavior caused by a stimulus, while the Northern Population moved actively. In addition, the Northern Population showed changes of behavior in December. These findings suggest that both populations have unique characteristic fish schooling behaviors, thereby the endemism of local populations may be threatened if they are released outside their original range.

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During the years 2004-2008, investigate of the oxygen concentration distribution and seasonal changes of thermocline form in hypolimnetic water were conducted in the Sanbe reservoir. In 2006, high - oxygen - concentration water was supplied into hypolimnetic water using a new type of oxygenator (WEP system). As a result, the thermocline formed at 12 m depth, and water temperature was 7°C throughout the year in the hypolimnion. The amount of dissolved oxygen in the hypolimnetic water (528,000 m³) after the turn over was observed at about 5,000 kgO₂ (Feb. 2007) - 6,000 kgO₂ (Feb. 2008) and the oxygen consumption was 58.6-32.0 kgO₂·d^-1. For this reason, it was predicted that the dissolved oxygen depletion would occur every summer. Several weeks after the WEP system was used, the high - oxygen - concentration water widely spread to 800 m (dam's bottom area: 8.7 ha) from the WEP system without destratification. The oxygen transfer efficiency of the WEP system was observed to be between 82 and 87%.

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To understand the phenomena occurring in Lake Biwa and predict the effects of measures for improving water quality, constructing a simulation model that can calculate the hydrological and material cycle and reaction process in the basin with high accuracy is the most effective way. In this study, a Lake Biwa Basin simulation model with the integration of three components of land, lake flow, and lake ecosystem is constructed and named "Lake Biwa Basin Hydrological and Material Cycle Simulation Model". Distributed and 3-dimensional models are adopted; these models can simulate water quality and quantity unsteadily by considering weather, social conditions, and so on. As a result, river discharge and quality in main rivers, such as Yasu River and Hino River, and water temperature, average quality and seasonal variation in the lake are adequately calculated. On the other hand, it is insufficient to calculate water quality of the time series and distribution in Southern Lake; thus, it is a future challenge to evaluate spatial-temporal lake water quality precisely.

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We investigated the effect of preservation measures at Yamaguchi Bay where a nature restoration project was carried out for fish resource recovery of the Manila clam. The presence of many predatory species of the clam was considered as one of the obstructive factors for such resource recovery. The use of protective net setting was found to be an effective measures against feeding damage, and adult clams were maintained in the net setting area for more than one year. Many juvenile clams survived in the net setting area with cultivation, and it was thought that a decrease in sediment hardness induced by cultivation relieved the estival increase in heat. We designed a resource management model that incorporated feeding damage and fishery catchment to examine the process of resource recovery. As a result of our calculation when feeding damage was eliminated, we estimated that the quantity of resources increased to about 10 times greater than the present quantity. In addition, to restore the resources to a level equal in quantity to that in the past, it was suggested that it is necessary to reduce feeding damage below 10%. For sustainable fishery resources, it was shown that the management of fishery catches is important.

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