Actual achievements of the four fishways of the Nagara Rivermouth Barrage for a catadromous mitten crab, Eriocheir japonica (de Haan), were investigated based on the data of five-year monitoring by the Japanese Ministry of Construction on environmental changes after operation of the Barrage in 1995. Juveniles and adults of the crab under upstream migration were collected using mini-traps at upstream sites of each fishway and at downstream sites of each riverside of the Barrage in April and May for the five years. Significantly more juveniles migrated upstream through the brook-type fishway than the other fishways particularly in 1998, whereas the adults used the guide-flow fishways more frequently than the other fishways. The highest rate of juvenile upstream migration at the brook-type fishway showed its exceeding performance as a migration passage for small benthic animals. In spite of the remarkable amount of juveniles migrated upstream through the fishways at the barrage, the results of Nobori-ochi fishing at 46km and 57km from the rivermouth showed a significant reduction in the crab population. It is probable that the long distance of backwater area of the barrage may hinder the crab from safe migration.

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The daily total number of ayu fry under upstream migration was compared among the three fishways of different types at the Nagara Rivermouth Barrage, based on the monitoring data taken over a five-year period from 1995 to 1999 by the Japanese Ministry of Construction. The number of migrating fry through the guide-flow fishway on the right side tended to be fewer than the other two fishways. There was no significant difference in the number of migrating fry between the guide-flow fishway on the left side and the brook-type fishway. It should be noted, however, that temporal concentration of upstream migration was observed both hourly and daily in the brook-type fishway. The peak date of the migration corresponded to the neap tide period. This correspondence was only observed in the brook-type fishway. Under natural conditions, the water at the mouths of rivers becomes brackish at the neap tide and the range of brackish water extends upstream farther as the tide advances. The data suggests that the environment created in the watercourse of the brook-type fishway resembled natural conditions. Relations between the number of migrating ayu fry through the guide-flow fishway and the total water flow at the barrage showed that the upstream migration of ayu fry reached the maximum when the total water flow was near 170 m3/sec and decreased when the flow exceeded this volume. It is likely that the effect of the guide-flow is weakened as the water flow increases at the barrage. In contrast, the number of migrating ayu fry through the brook-type fishway did not show such a decrease and increased with increasing total water flow. These results suggested that the brook-type fishway surpassed the guide-flow fishway in its function.

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These years, environmental conservation has been regarded as a more important part of irrigation projects than before. Accordingly, it has been required that many fishes including non-fishery species are able to make migration through irrigation barrages. It has been also required, however, to harmonize improvement of fishway performance with operations of the irrigation barrage. In this study, we investigated on spatial patterns of local flow velocity in a boulder-paved fishway, aiming at examination of its performance for upstream migration. The local flow regimes were compared through hydraulic tests among several kinds of fishways differed in boulder shape, patterns of boulder arrangement, and fishway slope. Results indicated that a boulder-paved fishway could guarantee easier maintenance against partial blockage with drift stuff by submerged boulders and migration routes for many fish species by separation of their routes, serving both upper routes for large fishes and bottom routes with enough resting areas behind boulders for benthic and small fishes.

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Based upon the "Model projects for promotion of enhanced environments for ascent of fish" that was begun in 1991, obstacles to upriver migration of fishes from cross-river structures such as weirs have been gradually improved. Along with these changes improvements have also been made along rivers that are not among the rivers in the model project. However, there remain many issues such as entrainment countermeasures, responses to high dams, and evaluations of type of fishways that require research and technology development, because some of this technology is still under development, and the spread and use of the technology in the field is still insufficient. In addition there are other types of issues, such as the fact that means for verifying the improvement results have not been systematized. When pushing ahead with future projects, spreading the experience and knowledge gained on the model rivers, conducting further surveys and investigations, and supporting flexible responses and devices at actual field sites will be indispensable.

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Evaluation methods for fishways in Japan were reviewed and some proposals were made on their improvement. A major problem in the methods was lack of a holistic view in the role of fishways. Most of previous fishway evaluations were based on experiments using stocked fish in the fishway to examine possibility of their upstream movement. In the case of the fishway of Komaki dam with 79.2 m in height, fish passing through the dam were few in number in spite of the effectiveness of the fishway itself. The monitoring data on the fishways of the Nagara Rivermouth Barrage have been examined by several scientists and arguments on the environmental influence of the barrage were made based on their evaluation. They claimed that environmental change accompanied with the construction of the barrage made degradation of the fish population in spite of the ability of the fishways. These results suggested that we should consider a number of factors for estimating on fishway performance, including those outside the fishway. This means fishways should be evaluated from the aspect of the survival of fish populations. The function of fish passage is not the purpose of a fishway but a part of conservation of fish population. We should include dam effect estimation in a fishways evaluation program. For this purpose, required are approaches from fish population ecology such as investigation of longitudinal distribution and seasonal pattern of migration in each age of the fish population. Now practical guidelines for the method of evaluation should be developed.

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The effects of differences in substrate characteristics on benthic macroinvertebrate communities were studied in two small catchments with different land usage (agriculture and forestry) in central Hokkaido, northern Japan. Permeable sediment/substrate samplers were installed on streambeds within each catchment from June to November, 1998. The proportion of substrate particle size was clearly different between the catchments. The amount of fine sand (particle size : 0.1-1 mm) that was collected in the agricultural catchment was 50-60 % of the total sediment, while that in the forested catchment was only 30 % of the total. The density of macro invertebrates in the agricultural catchment was only 10-20 % of that in the forested catchment. The results of canonical correspondence analysis showed fine sand was the most important variable in explaining the habitat characteristics in the agricultural catchment. Extensive agricultural land use in this region has induced gully erosion, resulting in a continuous sediment supply to the streambed. Effective stream restoration will require both construction measures for preventing slopes from eroding and the regulation of land use activities.

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