Rhodeus atremius suigensis is an endangered bitterling species designated as a Nationally Endangered Species of Wild Fauna and Flora by the Ministry of Environment of Japan. In order to create preferable microhabitats for the threatened bitterling in agricultural channels, environmental factors affecting its occurrence need to be identified. The present study investigated environmental preference of the fish inhabiting an agricultural channel in southern Okayama Prefecture, western Japan, through monthly surveys of fishes and physical environments between June 2014 and May 2015. The effects of physical environmental factors on the occurrence of the bitterling were analyzed using generalized linear models （GLM） with binomial probability distribution or generalized estimating equations （GEE）, seasons separately. The best models yielded by model selection based on AIC or QICu revealed seasonal changes in the key environmental factors for the occurrence of the bitterling. The variables with positive effects on the bitterling occurrence included submerged macrophyte coverage during the spring and the summer to autumn period and presence of submerged structures and above-water cover during the winter. Water velocity had a negative effect on the occurrence during the spring and the summer to autumn periods. At the same time, mean hydraulic environment at the locations with the bitterling occurrence was consistent throughout the study period, with water velocity less than 11.5 cm/s （3.4 cm/s on average） and water depth ranging 20-50 cm （37.7 cm on average）. The findings insist on the importance of the above mentioned lentic environment, in addition to covering materials represented by submerged structures and above-water cover, when creating preferable microhabitats for the bitterling in agricultural channels.

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This study examined freshwater mussel habitats in excavated flood-channels with reference to initial ground heights and years elapsed since the excavations in the lowland Ibi River in order to facilitate efficient management of inter-levee floodplains using flood- channel excavation work. Mussels were captured in ponds at excavation sites with different ground heights and at different years since the excavations. Time-series data of cumulative sediment depth and the amount of ponds in the sites were also obtained. In the sites with lower excavation heights （but higher than drought water level）, mussel abundance and the proportion of the pond occupied by the mussels were higher, and sediment depths and deposition rates were lower. Mussel abundance and the amount of ponds increased and then decreased with peaks of 5 and 6-9 years after the excavations, respectively, in the context of continuing sedimentation. These results suggest that flood-channel excavations should be strategically implemented so that low excavation sites with 5-9 years elapsed since excavation are always present somewhere in a target river segment.

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Japan is known as a nation with high dam density. The mid- and downstream sections of most Japanese rivers are fringed by lateral embankments, causing serious dete rioration of aquatic connectivity in the upstream-downstream, the longitudinal direction and the channel-floodplain, the lateral direction. The present review summarizes general causes of the fragmentation, restoration methods for riverine connectivity, and current status and future challenges of the restoration in inter-levee and outside-levee floodplains of Japanese rivers. In the inter-levee floodplain, we detected four restoration types pertaining to longitu dinal fragmentation by physical and invisible barriers associated with water temperature and quality, and six restoration types concerning lateral connectivity. In marked contrast to the inter-levee floodplains, outside-levee floodplains have already been severed from a dy namic flooding process. In such altered systems, the management of an artificial habitat net work such as a paddy-field or a pond is a major mechanism for creating alternative disper sal pathways. We also found that many of above restoration techniques present practical issues that limit the restoration effects on specific species or life stages and risk further degradation due to inappropriate selection of sites or procedures. Human pressure on fresh waters and the resilience of the ecosystems have been changing gradually, suggesting that Japanese river managers need to select appropriate techniques depending on these changes to restore the connectivity more efficiently. Realistically, “Habitat Creation” and “Partial res toration” focusing on the artificial or semi-natural connectivity are major strategies in the short run. On the other hand, “Full restoration” should be the major goal in a time span of a few decades, in the context of future social changes such as population decline and deterioration of infrastructures. In addition, a database of the location of low-head dams is needed to prioritize the restoration of the sites for connectivity both in the inter- and outside-levee floodplains.

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Here, we review existing knowledge about step-pool structures in mountain rivers with reference to their morphology, process of formation, destruction, hydraulic function, restoration, and ecological function. First, we present the constructive factors of steppool structures and their appearance, focusing on channel bed profile. Second, we summarize the processes driving the formation and destruction of step-pools, concentrating on current and sedimentation. The untidune and keystone theories are discussed as the key concepts of step-pool formation in relation to the installation of artificial step-pool structures. Third, with reference to previous studies, we discuss the relationship between step-pool structures and river course characteristics. Fourth, we describe the law of resistance and velocity in a mountain river. Finally, with reference to their ecological function, we discuss the use of step-pool structures in restoration projects. Based on earlier studies of step-pool structures, we discuss future issues and propose studies to technologize and standardize step-pool structures for river improvement and restoration.

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Spur dikes have been constructed to redirect the stream flow, protect streambanks from erosion, and improve stream depth for navigation since long time ago. In addition, in recent years, spur dikes are expected to be used as one of the restoration methods on aquatic habitats as they produce local scours and sediment deposition areas around them. There have been significant number of papers about spur dikes and abutments, however, almost all the above-mentioned studies did not target the characteristics of sediment transport and bed deformation around spur dikes set on a inerodible bed field in great details. In this paper, the authors summarized the characteristics of flow, sediment transport, and bed deformation around both spur dikes and abatements set on erodible and inerodible bed fields in natural rivers, experimental flume, and numerical analysis. Subsequently, as a method to create sediment deposition areas along the both banks on inerodible bed field, acute angle of submerged spur dikes were introduced.

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[in Japanese]

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The essential purpose of agriculture is sustainable food production. Also agricultural ecosystem has another ecosystem functions including biodiversity conservation. Although traditional agricultural practices maintained biodiversity, recent modern agricultural activities may have negative consequences on biodiversity. Previous studies showed that land consolidation, which is a major recent land-use change in agricultural areas, decreases plant species diversity over the long term. However land consolidation is one of important component on effective food production. In this study we extracted areas which has whichever function ＂high food production＂or＂high biodiversity＂ based in previous studies & open data bases. We used heavy land consolidation and farmland areas per unit as index of food production, and un-consolidated area and occurrences of threated plant species as index of biodiversity, and extracted an agricultural areas which might have relatively high function from whole of Japan. These types of land classifying idea is necessary condition for sustainable agriculture with several types of ecosystem services.

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[in Japanese]

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An underwater line transect was used every May and October, from 2006 to 2012, to assess the population size of ayu, Plecoglossus altivelis altivels, in the Nahari River. Estimated numbers of ayu ranged from 145,000 to 1,540,000 in May, and from 55,000 to 420,000 in October. The mortality rates of ayu from May to October varied from 30 to 76 ％. The mortality rate was not correlated with the May population size of ayu, fishing intensity, precipitation, and the maximum of turbidity. In contrast, the mortality rate was positively correlated to low levels of turbidity （20-50 mg/L）. It was suggested that the long-term of the turbidity increases the mortality rate of ayu.

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Natural materials are often used to construct low-water shore protection works. We investigated fish habitats at six types of low-water shore protection structures and one vegetated bank in May, July, and October, 2012. Fish abundance and species diversity were higher in shallow vegetated areas with low water velocity throughout the study period. Free-swimming fish increased at the vegetated bank in May. In July and October, fish moved to deeper waters. They were therefore distributed widely in habitats with various water velocities and depths, particularly in October. Juvenile free-swimming fish were distributed only at the vegetated bank. Results suggest that the life history of juvenile free-swimming fish requires emergent plant banks for nursery and overwintering habitats. Adults need a bank habitat with various water velocities and depths. For benthic fishes dominated by Cottus pollux, the riverbed structure （i.e., loose stone） was apparently more important than the shore protection structure. These results indicate that straightening of rivers degrades the habitats of free-swimming fish and benthic fish troughs by decreasing the heterogeneity of water flow and benthic environments, even in rivers with low-water shore protection produced using natural materials.

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[in Japanese]

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