PHYSICAL HABITAT ASSESSMENT FOR AYU SPAWNING HABITAT FOCUSING ON THE RIVERBED ENVIRONMENT

Abstract

 The aims of this study are to propose suitable habitat criteria for extracting ayu (Plecoglossus altivelis) spawning habitat and to develop a general method for extracting suitable spawning habitat formed by flood disturbance using a planar two-dimensional sediment transport model. The method was applied to the alluvial fan section of the Nagara River in mid central Japan, and its effectiveness and issues were examined. A numerical model that evaluates flow resistance for a wide range of flow conditions in a gravel-bed river was subjected to a runoff that causes riverbed disturbance during the ayu spawning period, and then suitable sites were extracted based on the suitable habitat criteria under normal water flow condition. The proposed habitat criteria identify unstable gravel beds by the combination of two conditions: the grain size distribution of the material in the streambed and the shear stress acting during normal flow, it was able to identify more limited set of riffles suitable for ayu spawning habitat than conventional methods.