Fish-Retreat Ditch Survey

Abstract

AbstractThe importance of biodiversity conservation functions in paddy ecosystems has become widely recognized. Many surveys of fish and dragonflies inhabiting paddy fields, irrigation and drainage canals have been conducted. It has been highlighted that it is important to secure flowing water in drainage canals year round. If drainage canals and paddy fields can be flooded, then they will contribute to the improvement of biodiversity conservation functions of birds and frogs. As in the above case, although there are structures similar to fish-retreat ditches and swales across Japan, there are few studies that confirm their effects. In this study, to address this issue, the abundance of fish inhabiting the fish-retreat ditch created in a paddy field near the sea in Kunitomi District, Obama City, Fukui Prefecture, was investigated. The fish-retreat ditch contained purely freshwater, amphidromous, brackish water and marine fish. The specimens collected in this ditch included 565 Misgurnus anguillicaudatus, 306 Cobitis sp. BIWAE type A, 132 Tribolodon hakonensis, 122 Acanthogobius flavimanus, 33 Gymnogobius urotaenia and 34 other fish. M. anguillicaudatus and Cobitis sp. BIWAE type A (purely freshwater fish) use the fish-retreat ditch mainly as a spawning ground during the irrigation season; their numbers dropped during the nonirrigation season. T. hakonensis, G. urotaenia (amphidromous fish) and A. flavimanus (brackish water/marine fish) used the fish-retreat ditch mostly as a growing ground. They swim upstream to the fish-retreat ditch in spring, grow from summer to autumn and remain until spawning. In addition, according to the survey that monitors the ascending and descending of a fish community in the fish-retreat ditch, the fish-retreat ditch is not only a spawning ground but also an overwintering ground for M. anguillicaudatus and Cobitis sp. BIWAE type A (purely freshwater fish). And the ditch was not only a growing ground but also an overwintering ground for T. hakonensis, G. urotaenia (amphidromous fish) and A. flavimanus (brackish water/marine fish). The fish-retreat ditch, which has had less environmental changes than rivers, drainage canals and oceans, has become a nursery for many fish. The biodiversity conservation function of the fish-retreat ditch was confirmed, and to increase this function, a network of multiple fish-retreat ditches should be constructed in the future.KeywordsAmphidromous fishBrackish and marine fishGrowing groundNurseryOverwintering groundPurely freshwater fishSpawning ground