Breeding control of the invasive alien fish Micropterus salmoides by stepped water level drawdown operation

The limited water-level method is used for the reservoir of the Miharu Dam, located in upstream of the Abukuma River, which flows into the Pacific Ocean, and drawdown of the water level is routinely practiced just before the flood season (June to October). At this time of the year, the surface water temperature of the reservoir reaches around 15℃ at which spawning of largemouth bass, an invasive species, is thought to begin. Hence, the drawdown period coincides with the spawning period of largemouth bass. Several patterns of drawdown were performed in order to evaluate the effects of the drawdown on the spawning of largemouth bass. In an ordinary drawdown, the water level is lowered at a constant rate till the onset of the flood season. In the Miharu Dam reservoir, drawdown is temporarily paused for 2 to 4 days in the midst of drawdown to promote the spawning of largemouth bass. Then, drawdown is resumed to dry up the spawning beds established at the maintained water level. As a result, a considerable number of exposed spawning beds were detected by setting a few maintained periods during a drawdown, called “Stepped Water-level Drawdown”, and exposed spawning beds were observed at depths of 0.5 to 2.0 m from the maintained water level. Fries of largemouth bass were captured and the dates of their spawning were estimated from the relationship between length and daily rings (otoliths). The analysis revealed that spawning during the drawdown period scarcely led to hatching. It was also found that 4-stepped drawdown is more effective for exposing spawning beds than 3-stepped drawdown. Population growth simulated by the Leslie matrix given a scenario of 4-stepped drawdown yielded a stable trend over 50 years. These analyses suggest that 4-stepped drawdown before the flood season could be an effective management method to control the recruitment of largemouth bass.