Abstract
The consolidated paddy field of Japan will remain extensively in the future and continue to have a crucial influence on regional environment. The purpose of this study is to show an ecological significance of connecting irrigation and drainage canals in a consolidated paddy field, and a practical plan based on a field survey of fish distribution at four sites in a paddy field of Shimodate City, Ibaraki Prefecture. The sampling interval was one week during the period of May 2002 to June 2003. The results clarified the followings: (1) The ten species of fishes caught in the canals can be classified into two groups; fishes living mainly in drainage canals, such as Misgurnus anguillicaudatus (Dojou) and Silurus asotus (Namazu), and those living both in irrigation and drainage canals, such as Gnathopogon elongatus elongatus (Tamoroko). (2) The fishes in the irrigation canals were fewer because they had just found their way into the canals from the source river, while some were found overwintering in the pools of the canals after irrigation was stopped. (3) An effective way of attaining both the conservation of fishes and the high productivity is to employ a shallow farm drain by using under-drain system for sub-surface drainage. The minimized difference in elevation between farm drain and paddy plot allows the first group of fishes to enter the shallow water on the paddy, and the connection of lateral canal and shallow farm drain with water flow allows the second group of fishes to go and come between the irrigation and drainage canals. (4) A simple and effective way of conserving fishes in the irrigation canal is to evacuate them to the farm drain before stopping irrigation.
