石礫河床への大量の覆砂が魚類生息密度に及ぼす影響について

抄録

We studied a response of fish density to an artificial sand cover on stony riverbed at downstream of the Yahagi-daini dam. Two study sites with stony substrate were set at ca. 0.7 km downstream of the dam. Riverbed at the lower study site was covered by a large amount of sand (ca. 20 m³) to prepare the sand-covered study area (25 m²), whereas riverbed at the upper was not covered as reference area. Density of fishes (mainly composed of benthic gobies, Rhinogobius spp.) and environmental factors (current velocity, depth, and cover degree of each particle size) were monitored at the two areas before and after the sand-cover. The density was decreased at the sand-covered area although it had not been different between the two areas. The sand-cover simultaneously changed the environmental factors (increase in current velocity and finer substrate and decrease in depth). These results suggest that a lot of sand coverage should bury stones on the riverbed originally and make flow higher on the sandy riverbed. As most fish had been found in interstices under/between stones, decrease in the interstices associated with sand cover may partially contribute the decrease in fish density. The further studies are necessary to detect the threshold of sand coverage causing decrease in density of fishes dramatically.