Out of the pot and into the fire: explaining the vulnerability of an endangered small headwater stream fish to black-bass Micropterus spp. invasion

Introduced predatory fishes have had consistently severe consequences for native fishes in stream environments around the world, although the drivers of these effects are often unclear. In the Swartkops River headwaters in South Africa, native Eastern Cape redfin Pseudobarbus afer were always absent from sites occupied by non‐native black basses Micropterus salmoides and Micropterus dolomieu, but generally co‐occurred with the native predators Anguilla marmorata and Anguilla mossambica. A natural experiment provided by flood‐mediated recolonization of black‐bass occupied sites by P. afer demonstrated depletion in black‐bass invaded sites. Field behavioural observations of P. afer indicated that they foraged among benthic cover during the day, but suspended in open water at night. As the nocturnal A. marmorata and A. mossambica foraged actively within structural cover at night and M. dolomieu and M. salmoides are diurnal or crepuscular predators, P. afer is thus optimized to avoid predation by native anguillid predators and not the functionally unique predatory black basses. The integration of distributional, temporal population dynamics and behavioural data suggests that the severe effects of Micropterus spp. are probably a consequence of prey naïveté and behaviour evolved to evade native predators.