A fragmented fish community: evaluation of the present distribution and drivers of fish communities in the lower uThukela River, KwaZulu-Natal, South Africa.

Abstract

In South Africa, a water-scarce country, the construction of dams and weirs is increasing to ensure water security for a growing nation. The human need for this infrastructure is generally prioritised over its environmental impact, particularly for aquatic fauna with migratory needs. Such infrastructure can block migratory pathways for fish, trap sediment, and alter river flow regimes. The Lower Thukela Bulk Water Supply Scheme (LTBWSS) abstraction weir was constructed on the lower uThukela River in KwaZulu-Natal Province, South Africa, in 2017, near the town of Mandini. The riverine stretch where it is constructed is a critical interface for marine, estuarine, and freshwater migrant local fish species. As such, a vertical-slot fishway was incorporated into its design to facilitate the movement of fish over the weir. This study utilised data collected from field surveys at three sites upstream of the weir, four sites downstream of it, and two sites on the Mandini Stream, a tributary of the uThukela, to assess the impact of the weir on local fish community structures. Multivariate statistical analyses were employed to identify the environmental variables influencing fish community structures and select fish species in the region. The results showed that the dominant cover types, dominant substrate, average depth, and water temperature significantly affected fish communities in the study. Individual species were driven by different habitats, water velocity-depth profiles, and physical water-quality variables. Furthermore, the presence of the LTBWSS has influenced three cichlid species, particularly <i>Tilapia sparrmanii,</i> which was previously shown to favour silt substrate. The resulting loss of downstream sedimentation because of the LTBWSS weir has led to the absence of the species, along with synergistic effects on water quality and quantity in the downstream sections of the uThukela River.<h4>Supplementary information</h4>The online version contains supplementary material available at 10.1007/s10641-026-01822-y.