Successful mechanical eradication of spotted bass (Micropterus punctulatus) from a South African river

1. Following the discovery in 2007 of nonā€native spotted bass (Micropterus punctulatus (Rafinesque, 1819)) in the Thee River, Olifantsā€Doring River system, Western Cape Province, South Africa, a mechanical removal project was initiated in 2010 to eradicate them to protect the unique native fish assemblage in the river.

2. A temporary gabion barrier was constructed to halt spotted bass invasion into upstream reaches and a variety of mechanical removal techniques were used throughout the project.

3. Three hundred and ninetyā€nine spotted bass were removed from the river during the project. Three hundred (75%) of these were captured by chasing them into gill nets or by catching them with hand nets. The remainder were removed using spearguns, seine nets and by backā€pack electrofishing.

4. Spotted bass had been depleted to below detection levels downstream of the temporary barrier in the second year of the project in 2013. A subsequent survey of the river in 2014 revealed a bass population in a reach above the temporary barrier that had previously been considered uninvaded. This population of spotted bass was removed in 2014. Subsequent annual surveys of the Thee River in 2015, 2016 and 2017 have not detected spotted bass and the population is considered to have been extirpated.

5. Three years after the extirpation of spotted bass from below the temporary barrier, native fiery redfin (Pseudobarbus phlegethon (Barnard, 1938)) and Cape galaxias (Galaxias zebratus Castelnau, 1861) were observed in pools where they had been absent during the bass invasion.