| The characteristics of pools and weir fishways, such as drops between pools and water depth at weirs, are highly correlated with flow conditions at fishways, especially around weirs. The suitable flow for migrating fish varies with species, so it is necessary to consider multiple species when designing a fishway. In this study, the swimming behavior of char (Salvelinus richardson), ayu (Plecoglossus altivelis altivelis) and freshwater sculpin (Cottus pollux) around a weir under nine hydraulic conditions for two shapes of weir were studied. Five ultra-high-speed cameras were installed for the experimental study, for both tracing the swimming loch and analyzing the hydrological regime. The findings revealed that the flow conditions, which are defined by the combination of drops and overflow depth, affected the results of "approaching the flow from the weir top" and "passing over the weir," vary among each species. In each case, both char and ayu, which are free swimmers, could find the flow from the weir top and pass over the weir. But ayu, which are strong swimmers but smaller than char, require the over-flow condition for successful migration rather than the submerged over-flow condition. Freshwater sculpin, which is a small bottom-swimming fish, showed a completely different behavior. They did not migrate in each case, and only the submerged flow condition permitted them to migrate over the weir. It was assumed that particular flow conditions along the weir give this weak swimmer the chance to migrate under the submerged flow condition. |
