Fish community in a fish-retreat ditch created in a paddy irrigation and drainage system near the sea

Abstract

The importance of biodiversity conservation in paddy field ecosystems is widely recognised, as is the importance of maintaining a water body in the paddy field throughout the year. This study reports on the fish community in a fish-retreat ditch constructed in a paddy field near the sea in Kunitomi District, Obama, Fukui Prefecture, Japan. The fish-retreat ditch contained purely freshwater, migratory, 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 (pure freshwater fish) use the fish-retreat ditch mainly as a spawning ground during the irrigation season; their numbers dropped during the non-irrigation season. T. hakonensis, G. urotaenia (migratory 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. As the biodiversity conservation function of the fish-retreat ditch was confirmed, I propose increasing this function by forming a network of multiple fish-retreat ditches in the future.