Recovery of a subtropical headwater fish community following a large flood, Klaserie River, Limpopo River System, South Africa

Headwaters are important refuges for threatened fishes and play an important role in their conservation. The effects of large flooding events on headwater fish assemblages are under studied in southern Africa. In January 2012, heavy rainfall resulted in a large flood in the upper Klaserie River, Olifants River, and Limpopo River System, South Africa. This flood had an estimated return level of 225 years and caused significant ecological and economic damage. This study aimed to explore the impact of a large flood on the fish assemblage and substrate in the headwaters of a subtropical stream. The fish communities and selected substrate variables were collected at 10 sites. Sites were sampled at six monthly intervals thrice before and thrice after the flood. Fish were collected by electrofishing and substrate quantified visually. Changes in habitat resulted from the flood included the scouring of gravel sand and mud, greater exposure of bedrock and boulder substrates, increased stream width, and decreased stream depth in all zones. The fish community showed an increase in abundance for all but three species, colonisation of upstream sites, and colonisation of the study area by six species. The majority of fish species have opportunistic life-history traits, which could explain the rapid colonisation and increase in abundance. The flood occurred in the middle of the breeding season for most the fish species, and this resulted in an increase of abundance for these species and provided opportunities for upstream colonisation. The impact of late season and aseasonal large floods requires further study.