Monday, August 6, 2007

PS 2-13: Flow-dependent effects of channel morphology on fish assemblages: Evidence from an experimental stream

Shiro Sagawa, Junjiro Negishi, and Yuichi Kayaba. Public Works Research Institute

Despite the fact that flow and channel morphology are 2 critical factors that need to be taken into account when considering the river restoration, there are no reports on the response of fish assemblages to changes involving both variables in combination. In this study, we aimed to contribute to future river management by elucidating the response of fish assemblages to artificially induced changes in flow over time in an experimental stream featuring a simple river morphology (a glide reach) and complex morphology (pool–riffle reach). When the flow volume was small, the fish assemblages of both reach types resembled each other in being dominated largely by demersal species, but when flow was increased, those similarities faded and very different assemblages emerged. In the glide reach, increasing the flow volume caused a linear increase in both water depth and velocity and a progressive increase in the number of water-column species and their proportion, numbers and size (total length). The number of Zacco platypus (freshwater minnow) increased 5-fold, while Misgurnus anguillicaudatus (oriental weatherfish) numbers declined to less than a quarter of their initial number. In the pool–riffle reach on the other hand, increased flow produced no clear increase in depth or velocity, but the number of Plecoglossus altivelis altivelis (Ayu) grew conspicuously. Concrete attempts are being made in various countries to achieve environmental flow, but the results of our research suggests the need for a framework for considering environmental flow that takes into account such variables as channel morphology condition.