大井川河口養鰻地域の展開過程と河況変動

抄録

Since modern embankments were built along the main river in Shizuoka Prefecture, much of the river bed has risen higher than the neighbouring plain. But the post-war construction of dams has sharply decreased the drift supply to downstream. Moreover, the increasing demand for gravel as construction material has led to the removal of a great deal of gravel, resulting in the rapid lowering of the river beds in recent years. This lowering of river beds and underground water level have produced various changes in the water utilization and the land utilization of the river basin in line with geological features which are characteristic of an alluvial fan.
The eel cultivating area of the Oi river's downstream is not an exception; namely, the eel cultivating operation which had been established in the disused paddy fields along the Oi River led to the management practice of cultivation by exchanging water, making use of the spring water and the sloping land. Later, after overcoming the water shortage and the problems of pollution, this area developed into the largest eel cultivating area in Japan.
Thus, the influence of water supply on establishing productivity deteriorated in the downstream area of the Oi River. However, this change for the worse gave us an important opportunity to accellerate the establishment and development of eel cultivation. On the other hand, the increased demand for drinking and industrial water which kept pace with the increase of eel cultivating ponds brought about increased competition between them.
Faced with the increased demand for drinking and industrial water, eel cultivators tried unsuccessfully to get water rights. Their lack of success may be due to the fact they were snarled in national and prefectural bureaucratic procedures as well as in the peculiar local problems relevant to water rights.
In the eel cultivating area in the downstream of the Oi River, with Yoshida-cho on the right side of the river, and Oigawa-cho on the left side, we find an asymmetrical development of the geographical distribution of eel cultivating ponds and the significance of eel cultivation with respect to the industrial structure of these two towns.
The differences between the two towns may be traceable to the differences of so-called historical conditions governing their formation as well as the differences of their geological features.
Thus, these differences, which determined the extent of competitive utilization of underground water of both eel cultivating areas, and the consequent change in the attitude of eel cultivators of the local authorities, have become the main reason for the recent switchover to the use of ponds for eel cultivation.