Changes in the performance of ayu Plecoglossus altivelis altivelis angling in a river fragmented by dams

摘要

Ayu Plecoglossus altivelis altivelis (Plecoglossidae, Salmoniformes) is an amphidromous fish with a 1-year life span. The distribution of this species is Japan, the Korean Peninsula and the eastern part of China. It possesses two unique ecological characteristics: first, it is an herbivorous species feeding on epilithic algae during young and adult stages; second, it has a feeding territory with an area of about 1 m~2. The territory is usually established in lotic part of rivers with cobbles. Using the habit requirements of the fish, anglers catch ayu by a traditional method called Tomozuri. In this angling method, instead of bait, live ayu is used as a decoy and is rigged to the line by a nose ring so as not to be damaged. The rod is as long as 9 m to lead the decoy to wide areas. When the decoy enters a territory of ayu, the territorial ayu attacks and is caught by a hook floating just behind the decoy. The Yahagi River, a middle-scale river in Japan, has a main stream channel length of 117 km~2, a drainage basin area of 1830 km~2 and a human population along the drainage basin of 1.2 million. The alluvial plain located around the middle and lower reaches of the Yahagi River is a large agricultural region as well as a well-known industrial area of mainly automotive industries. The flow of the river is heavily used for agricultural irrigation, industrial water supply and for service water supply through the construction of 7 dams. Therefore, the amount of water flow at the lower stream of this river is relatively low. The Yahagi River is locally famous for ayu angling; however, the catch success of ayu angling has declined rapidly for quite some time. Here we report on the remarkable changes of the catch success of ayu and concomitant changes of the riverine condition since 1980, referring to a diary that describes the record of ayu angling of the author in detail.