Invasive exotic fish control by application of encircling net and water level drawdown operation

In Japan, some of introduced exotic fish such as largemouth bass (Micropteraus salmoides) and bluegill (Lepomis macrochirus) are highly competitive over indigenous fish species, causing loss of biodiversity and degradation of aquatic ecosystem in a site. As this issue became apparent, many strategies have been implemented to control the exotic species. In a breading season of largemouth bass and bluegill, from May to June, they flock to shallow areas for spawning. In a front reservoir of Miharu dam, encircling nets were placed parallel to shores in order to lock the exotic fish in the target sites. As the reservoir level was lowered by a drawdown operation, fish in the areas were inescapably driven into the nets. This drawdown capturing method has been carried out from 2007 to 2011. Among the fish captured, the invasive exotic species, largemouth bass and bluegill, were eliminated and the others were released. Large sized individuals of largemouth bass were collected more in a joint area (about 5m width) between the front and main reservoir. Bluegill does not migrate around the reservoir, but they gather in shallow areas in spawning season. As a result of the consecutive practice, abundance of the largemouth bass and bluegill (2nd year and older) have been decreased. Whereas, the number of indigenous juveniles seen, such as Carassius auratus langsdorfii, has increased and increase of the indigenous populations has been affirmed.