:Presentation of working hypotheses concerning the adaptive management
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概要
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The objective of this study was to establish working hypotheses in order to improve the structure and function of river regulation works. We examined the choices for spawning grounds of rainbow smelt (Osmerus mordax eperlanus) and the characteristics of spring habitat of other fish species (especially, amphidromous fish). The survey was made from May to June 2000, in the Warui and Pon-warui streams, located in southern Hokkaido, Japan. We analyzed the density and mortality of the rainbow smelt egg, and fish density in relation to environmental variables. The results of the analyses showed that rainbow smelt spawning grounds are distributed from the downstream to the upstream reach, and mainly located downstream from the altered reaches. Other fish species had own habitat choices respectively (e.g. masu salmon Oncorhynchus masou selected pools with riparian forest cover), and a lot of amphidromous fish species also inhabit all reaches, however fish density declines with distance from the river mouth. Furthermore, rainbow smelt egg density and mortality were related to riparian forest cover. Without canopy cover (increased light levels) epilithic periphyton grows into a very thick covering, trapping fertilized rainbow smelt eggs as well as fine sediments on the streambed. Egg mortality was higher in the areas that had high epilithic periphyton biomass because of the thick deposition of fine sediment. Finally, two working hypotheses were presented for improvement of river regulation works. 1: Controlling the flow velocity over the altered section will facilitate upstream and downstream migration of fish populations, and will maintain the distribution of rainbow smelt spawning grounds and other fish species habitat downstream. 2: Replanting of riparian forests in the altered reaches will reduce egg mortality of rainbow smelt, and maintaining the meandering nature provided by geomorphic processes in the altered reaches will recreate habitats for masu salmon and other fish species.
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