Abstract
Conventional paddy farmland consolidation in Japan, which aims to increase farming efficiency by improving the drainage conditions of paddy fields and independently creating irrigation and drainage canals, is thought to have negative impacts on biodiversity in rural areas. The Land Improvement Act of Japan was amended in June 2001 and requires agricultural and rural development projects to be harmonized with the environment. It is widely recognized that transforming concrete irrigation and drainage canals into earthen canals and minimizing the differences in elevation between paddy plots and drainage canals aid in the preservation of aquatic animals. However, most paddy fields that have been consolidated using conventional standards will inevitably remain intact, thus continuing to have a substantial influence on the regional environment. Therefore, at least the minimum environmental measures should be effectively implemented in consolidated paddy fields. To this end, the distribution of the aquatic animals associated with these fields requires clarification. In addition, this knowledge will be useful in determining how to effectively distribute water during the non-irrigation season. The purpose of this study was to clarify the distribution of aquatic animals in the canal systems of main, lateral, and farm drains in the consolidated paddy fields, with a special focus on canal structure and year-round water flow in the canals. A field survey at six sites, which were selected for their different canal levels, was carried out in Shimodate City, Ibaraki Prefecture, Japan (36°21'N, 139°59'E). Sampling was conducted at monthly intervals from April 2001 to March 2002. A survey of fishes revealed that Zacco platypus (Oikawa) was concentrated in the main drains, while Misgurnus anguillicaudatus (Dojou) was found mainly in the lateral and farm drains. Among aquatic insects, Calopteryx atlata (Hagurotombo) was concentrated in the lateral drains, while Orthetrum albistyrum speciosum (Shiokaratombo) was observed primarily in the farm drains. Zacco platypus preferred gravel-bottom main drains to those made of concrete. Calopteryx atlata and Orthetrum albistyrum speciosum preferred year-round water flow to seasonal flow in lateral and farm drains, respectively. In contrast, Sympetrum infuscatum (Noshimetombo) preferred seasonal water flow to year-round flow in farm drains. The drainage systems in the consolidated paddy fields are clearly composed of different levels of drains with peculiar physical conditions, e.g., water depth and flow velocity, each of which attracts certain aquatic animals. To enrich the biodiversity of the paddy fields, our results suggest the importance of year-round water flow and natural materials for canal beds.
