Abstract
This study is to elucidate the characteristics of habitat for Medaka Oryzias latipes in both irrigated and unirrigated seasons through field investigation made over 71 km at the suburban irrigation canal in Tokushima City.
In unirrigated season, the water flow and velocity had decreased and about half of the irrigation canal, especially narrow one had dried up. As a result, organic matter pollution had increased in bayous. 15 fish species were found in irrigated season and 13 in the other. In unirrigated season, there was some canal with no fish even the canal wasn't dried up. The number of Medaka found also reduced, however the number found of other fish did much more. Other fish than Medaka may have more influence of dried up. It may be difficult for Medaka to return to canal which had dried up once, because of Medaka's lack of locomotive. Medaka needs an environment with the velocity under 10 cm/sec, and in irrigation canal, submerged plants help forming bayous. Also, submerged plants are considered important because of its role of spawning ground. Through our investigation, no relationship was found between habitat for Medaka and organic matter pollution.
