Effective Population Control of Invasive Largemouth Bass in Japanese Reservoirs, in Relation to Water Level Management

The reservoirs behind the numerous dams constructed along flowing rivers in Japan provide conditions suitable for the reproduction of introduced largemouth bass. In spite of popularity for recreational fishing, this fish has been officially designated an Invasive Alien Species in Japan. Since reservoirs have served as major foci in its range expansion through river systems, the population control of largemouth bass is an important task in reservoir management. In preparation for the rainy season’s floods, the water level of many reservoirs in Japan is drawn down during the breeding period of bass. In Miharu Dam (Fukushima), we have developed a stepwise drawing-down procedure, by which the spawning sites of bass are successively dried up. In other reservoirs, the water level is constantly maintained at the maximum level through overflowing. We evaluated the environmental conditions for spawning of bass along the stable shoreline in Tomata Dam (Okayama) and selected the areas of highest potential reproductive success as the places to focus our eradicatory efforts such as installation of artificial devices on which spawning is effectively induced. As a result, its population density in the dam has been decreasing. Next, we are planning to test new biochemical approaches in both reservoirs.