Integrated Population Control of Largemouth Bass, an Invasive Alien Species, in Reservoirs in Japan

Largemouth bass, Micropterus salmoides, is now established almost nationwide in Japan, where it has been designated an “Invasive Alien Species” under the Invasive Alien Species Act. Among other habitats, it has proliferated in dam reservoirs, whence it may spread via the out-flowing rivers and/or irrigation systems. Its control is thus an important concern in reservoir management. In a majority of Japanese reservoirs, the water level is systematically drawn down in early summer before the onset of the rainy season. For many reservoirs this period overlaps the spawning period of largemouth bass, and water-level manipulation may result in the desiccation of bass spawning sites. In addition, when the water is drawn down, many fish aggregate in the shallow, shore-side zone to forage, and this may facilitate their collection and removal by nets or other means. We have developed the following methods for the integrated control of largemouth bass populations in reservoirs around Miharu Dam in Fukushima Prefecture: 1) stepwise lowering of the water level, with stable intervals; 2) installation of a set net to isolate the shallows with aggregating fish before the draw-down; and 3) installation of artificial spawning-beds to attract parental males irrespective of water-level fluctuations.