| Despite the fact that flow and channel morphology are 2 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 the one dominated by the demersal species Misgurnus anguillicaudatus and Cobitis sp. to the one dominated by the water-column species Zacco platypus and Gnathopogon elongatus. In the pool–riffle reach, the fish assemblage structure changed from the one dominated by Cobitis sp. and Z. platypus to the 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, those 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 5-fold whereas the number of M. anguillicaudatus decreased to less than a quarter of their original one. 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 relatively low availability of algal food resource due to sand-predominated substrate whereas pebbles predominated in the pool–riffle reach providing decent conditions for algal growth. Our findings suggest the need for a framework for considering environmental flow that takes into account such variables as channel morphology and food resource conditions. |
