Abstract
The daily total number of ayu fry under upstream migration was compared among the three fishways of different types at the Nagara Rivermouth Barrage, based on the monitoring data taken over a five-year period from 1995 to 1999 by the Japanese Ministry of Construction. The number of migrating fry through the guide-flow fishway on the right side tended to be fewer than the other two fishways. There was no significant difference in the number of migrating fry between the guide-flow fishway on the left side and the brook-type fishway. It should be noted, however, that temporal concentration of upstream migration was observed both hourly and daily in the brook-type fishway. The peak date of the migration corresponded to the neap tide period. This correspondence was only observed in the brook-type fishway. Under natural conditions, the water at the mouths of rivers becomes brackish at the neap tide and the range of brackish water extends upstream farther as the tide advances. The data suggests that the environment created in the watercourse of the brook-type fishway resembled natural conditions. Relations between the number of migrating ayu fry through the guide-flow fishway and the total water flow at the barrage showed that the upstream migration of ayu fry reached the maximum when the total water flow was near 170 m3/sec and decreased when the flow exceeded this volume. It is likely that the effect of the guide-flow is weakened as the water flow increases at the barrage. In contrast, the number of migrating ayu fry through the brook-type fishway did not show such a decrease and increased with increasing total water flow. These results suggested that the brook-type fishway surpassed the guide-flow fishway in its function.
