Yuichi Kayaba

Changes in fish assemblage structure with variability of flow in two different channel types

Despite the fact that flow and channel morphology are two critical factors that need to be taken into account when considering the conservation or restoration of river environments, there are no reports on the response of fish assemblages to changes involving both variables in combination. This study examined the response of fish assemblages to artificially induced changes in flow discharge in an experimental stream that had simple (a glide reach) and complex (pool–riffle reach) morphologies. When the flow of the glide reach was increased, the fish assemblage structure changed from one dominated by the demersal species Misgurnus anguillicaudatus and Cobitis sp. to one dominated by the water-column species Zacco platypus and Gnathopogon elongatus. In the pool–riffle reach, the fish assemblage structure changed from one dominated by Cobitis sp. and Z. platypus to one dominated by Plecoglossus altivelis altivelis. When the flow discharge was low, the fish assemblages of both reach types resembled each other in being dominated largely by demersal species, but when flow was increased these similarities faded and distinct assemblages emerged. In the glide reach, increasing the flow volume caused a linear increase in both water depth and velocity and a gradual increase in the diversity of water-column species, their relative abundance, and size. The number of Z. platypus increased fivefold whereas the number of M. anguillicaudatus decreased to less than a quarter of their original number. In the pool–riffle reach, the number of P. altivelis altivelis grew conspicuously although increased flow produced no clear increase in depth or velocity. Calculation of the availability of physical habitat environments suitable for Z. platypus and P. altivelis altivelis in each reach type indicated that preferable habitat for both species was more available in the glide reach. The lower abundance of P. altivelis altivelis in the glide reach was attributed to the relatively low availability of algal food resource due to the sand-predominated substrate whereas pebbles predominated in the pool–riffle reach providing good conditions for algal growth. Our findings suggest the need for a framework for considering environmental flow that takes into account variables such as channel morphology and food resource conditions.

Geographical patterns of flow-regime alteration by flood-control dams in Japan

Natural variability in water flow is an inherent feature of river ecosystems, but many rivers in the world have been dammed, altering the flow regime and leading to ecosystem degradation. Dam reoperation to maintain environmental flows has been proposed for ecosystem and biodiversity improvement. Reoperation requires an understanding of flow alteration due to dam operations. However, knowledge of natural flow regimes and their degree of alteration remain poor in Japan. The Japanese islands extend from the sub-Arctic to the subtropics with four seasons and thus are considered to show high spatial and temporal variation in flow regime and alterations caused by those variations. We examined the spatial and temporal dependency of flow alteration by flood-control dams in Japan. We confirmed that flood-control dams reduced the rate of change in discharge and peak discharge and that such dams fulfilled their primary role of preventing flood disaster by reducing and delaying flooding. However, other flow characteristics, such as high- and low-flow frequency and duration, were also altered. We demonstrated that the magnitude, frequency, duration, and seasonality of high and low flows in undammed flow regimes differed with latitude and that discharge alteration was spatially and temporally dependent, presumably because dam operation responded to the inherent differences in seasonal flow variability at different latitudes.

Flow-velocity-dependent effects of turbid water on periphyton structure and function in flowing water

Land-use change, human activities, and dam management supply highly turbid water with high flow velocity; however, little is known about its effect on river ecosystems. Here, we studied the effects of flow velocity and suspended silt concentration on periphyton structure and function using experimental channels. The effects of flow velocity on algal biomass in the periphyton community depended on the suspended silt concentration; flow velocity decreased chlorophyll a in low concentrations of suspended silt (clear water), but not in moderate or high concentrations (turbid water). Filamentous cyanobacteria density was significantly influenced by flow velocity, whereas densities of non-filamentous cyanobacteria, green algae, and diatoms were not. Thus, flushing effects on attached algae, especially filamentous cyanobacteria, were exerted in clear but not in turbid waters, perhaps because algal assemblages were resistant to water current through the binding of large amounts of silt by extracellular polymeric substances. Inorganic matter and its ratio in the periphyton community decreased with flow velocity, but increased with suspended silt concentration, irrespective of flow velocity. Periphyton function, assessed by maximum photosynthesis rates and light-use efficiency, was influenced by flow velocity and suspended silt concentration through changes in periphyton structure. Flood events would be expected to refresh the periphyton community by removing senescent algae and deposited fine particles. However, we demonstrated that algae and silt in the periphyton community exposed to turbid waters with human-induced silt may accumulate, irrespective of flow velocity.

Improvement of individual-based models for wild animals in river ecosystem

We developed an individual-based model (IBM) simulation program considering relationships between wild animals and spatial data. The IBMs have characteristics to simulate the wild animal behaviors based on the spatial preference observed using wild animal auto tracking system. The IBMs have an ability to accurately predict wild animals behavior using the spatial data. Previously to river improvement works, an IBM can be used for assessing the changes of wild animal behaviors after human impacts.

Effects of different bank types on aquatic organisms in an experimental stream: contrasting vegetation cover with a concrete revetment

Streamside vegetation distributed in the ecotone between terrestria1 and aquatic ecosystems provides a number o f eco1ogical functions for aquatic organisms (NAIMAN & DÉCAMPS 1997). For example, overhanging vegetation along stream channels supports terrestrial insects, which can fali into the stream and become an important food source for salmonids in summer (KAWAGUCHI & NAKANO 2001, KAWAGUCHI et al. 2003). Riparian forests can deposit wood into the stream channel to help create complex in-stream habitats, thereby increasing fish abundance in the stream (INOUE & NAKANO 1998, URABE & NAKANO 1998). Despite its wide array o f ecological functions, streamside vegetation is often removed by channelization and replaced with concrete revetrnents in urhan areas o f developed countries as part o f flood control efforts. Stream channelization simplifies the physical features of in-stream habitat. This simplification has been associated with declines in fish abundance and alterations of in-stream community composition (CHAPMAN & KNUDSEN 1980, PORTT et al. 1986, SWALES 1988, PUNCHARD et al. 2000). A variety of bank protection methods have been designed to prevent channel scouring in Japan (YAMAMOTO 2003). However, while these methods have been well examined ftom an engineering perspective (TAKEUCHI et al. 2002), their effects on in-stream communities have received less attention. Moreover, no study has explained the relationships between stream-bank structure, physical characteristics within the stream channel and the abundance of in-stream fauna by quantitatively comparing community profiles in streams with vegetated and concrete banks. The objective o f this study was to examine the influences o f different bank types on in-stream biota by quantifying the abundance of fish and crustaceans and measuring physical parameters of the in-stream habitat in vegetatedand concrete-bank reaches in an experimental stream.

EFFECTS OF HYDRAULIC CHARACTERISTICS IN FLOODS ON FISH BEHAVIOR

We tracked fish behavior using radio telemetry in experimental streams, and analyzed it with a behavior prediction model to understand the relation between fish behavior and flood flow.The results are summarized as follows. 1. Upward migration began w ith flood discharge, however, there was a threshold in flow rate for the start of the migration. 2. Experimental results were simulated successfully by a model that is driven by hydraulic characteristics such as velocity and acceleration. 3.Thus, hydraulic characteristics which can influence fish behavior seemed to be current velocity and acceleration.Upward migration of fishes during flood seems able to be explained by combining our fish behavioral prediction model and hydraulic model.

RELATIONSHIP BETWEEN THE CHANGE OF HABITATS AND INUNDATION FREQUENCY IN THE SHIZUKUISHI RIVER

近年、自然裸地の減少、樹林地の増加が日本各地の扇状地河川で報告されている。このような河道内のハビタットの変化は河原に依存する生物の減少等様々な問題を引き起こしている。本研究ではこの要因として冠水頻度を取り上げ、これとハビタットとの関連性を検討した。対象としたのは、ここ数十年で裸地の減少、樹林化が生じた雫石川で、空中写真から約50年間のハビタットの分布と河道形状の変化の把握、そして、過去の日流量データに基づく水位計算結果から、各ハビタットの冠水頻度を調べ、木本地及び裸地と冠水頻度との関連性を探った。この結果、1-5日/年程度の冠水頻度を境界とし、河道内の裸地及び木本地が占める面積の割合が変化することを示した。

A Study on Modeling of In-channel Woody Vegetation Processes by the Impact of Morphologic and Hydraulic Characteristics in Alluvial Rivers

Wildlife, landscape, aesthetics and other factors including bank stability are affected by riparian woody vegetation. But, trees in levees or In-channels can be create flooding hazards. The emphasis of tree management work in rivers should be on woody vegetation maintenance rather than trees and scrub removal. The forecast of In-channel woody vegetation processes such as germination, growth, competition and demolition is important for tree maintenance. This research attempts to present In-channel woody vegetation processes by the Impact of morphologic and hydraulic characteristics in alluvial rivers, suggesting that the model for forecasting trees and scrub processes could be made.

The basic study about the facters that cause the creation of Wando and their characteristics at Chikuma river

Separation zone is a type of backwater that occurs on banks of the main stream or on sandbars where water is pooled (M. Church, 1993). Flow diverges into these pools, with declining velocity and energy gradient. Thus, separation zones is suitable for the refuge for aquatic organisms (hydrobionts) such as small fish. This is called wando* in Japanese. Until recent days separation zones (wando) have been decreasing due to the old type river improvement or the development of river basin area. With these backgrounds, several activities to restore the separation zone have started. However, it is difficult to control the river dynamics and to keep the land in a form of a separation zone.In this study, the Chikuma River was selected as a study river and we analyzed that causes the creation of the separation zone based on river characteristics in order to effectively restore the separation zone. As a result, separation zones were classified into four types and the features of each type of separation zone were recognized.