Abstract
The Yoshino River is known to have a very long spawning section of ayu up to about 50km (Ito et al, 1962), so ayu fishing was prosperous from 1950's to 1980's. However, after that, loss of spawning grounds gets remarkable. The reason can be at-tributed to decrease of sediment supply and flow rate by the construction and operation of several dams, and riverbed degrada-tion by gravel gathering for construction of structures. Nowadays situation is getting worth and is worried so much.
In this study, inquiring and questionnaire survey was performed to ayu fishermen engaged in the Yoshino River in the section of 14~20km from the rivermouth. In addition, river channel changes are quantified using aerial photos and geological survey date of the channel. Changes in the number and position of the spawning grounds of ayu from 1950's, and: river environmental factors affecting the loss of the grounds, were examined. As a result, it was suggested that the opinion of the fishermen who watched a river for a long time included useful information. In addition, river environmental factors affecting the loss of the grounds was suggested the reduction of the discharge and immobilization of the river bed about the factor to have an influence on the laying eggs ground of the ayu.
