Response of Endemic Species in South Africa's Rondegat River to Removal of Smallmouth Bass

In South Africa’s Cape Floristic Region, predation by alien fishes has severely impacted native fish populations and their removal from conservation areas is a priority. In February 2012, South Africa’s first alien fish eradication using rotenone took place in the Rondegat River, a small headwater stream that had been invaded by Smallmouth Bass (Micropterus dolomieu). We assessed fish densities using snorkel surveys, electrofishing and underwater video analysis from 2011 to 2015. Prior to the eradication the fish population comprised low densities of alien Smallmouth Bass (2.29 ± 0.56 fish/100 m²) and native Clanwilliam yellowfish (Labeobarbus capensis; 0.68 ± 0.33 fish/100 m²). All Clanwilliam yellowfish co-occurring with Smallmouth Bass were too large to be preyed upon by the bass. In 2013, one year after the eradication of Smallmouth Bass native fishes (4 species, 38.7 ± 7.0 fish/100 m²) had begun recolonizing from upstream source populations. Post-treatment monitoring in 2014 and 2015 continues to demonstrate this rapid recovery with native fish diversity, densities and length structure beginning to approximate those in previously non-invaded sections. The Rondegat River experience is therefore that native fishes are able to rapidly recolonize river sections following removal of the alien predators.