Influence of habitat structures on fish abundances and diversity: comparing mainstream and tributary communities in the urban uMsunduze Catchment, KwaZulu-Natal, South Africa

Various factors drive the decline of freshwater vertebrate biodiversity. These include changing landscape and urbanisation, introduced invasive species, altered habitat, water quality deterioration, instream barriers, and climate change. In the present study, we evaluated the impact of different habitat features on the fish assemblages in an urban river using catch per unit effort (CPUE) as a proxy for fish assemblage per site and season. We sampled 17 main sites and 21 ad-hoc sites in the uMsunduze Catchment in Pietermaritzburg, uMgungundlovu District, KwaZulu-Natal, South Africa, during 2022-2023. We collected data using an electro-shocker, fyke nets, and gill nets, and we also recorded and calculated habitat features such as substrate types, hydraulic biotopes, in-situ water quality, ecohydraulics, average depth, and velocity. We used Generalised Linear Models to determine the habitat features driving fish communities. We calculated the Shannon-Weiner and Pielou diversity indices to compare between rivers. We used the Fish Response Assessment Index (FRAI) tool to understand each site's ecological integrity per season. Our results indicated that various features, including substrate (mud, sand, gravel), fast intermediate and fast deep ecohydraulics, electrical conductivity, habitat (glide, pool), and average velocity significantly impacted the CPUE of fish. There was no variation in diversity indices between tributaries, but there was a significant difference in fish diversity between the uMsunduze mainstream and its tributaries. The FRAI scores showed great deterioration in the system's ecological health, and most sites, especially the mainstream sites, were critically or extremely modified. We suggest that the relevant authorities take action to mitigate the pressures compromising the uMsunduze Catchment's ecological integrity. There is an urgent need for conservation measures for the two "near threatened" species, <i>Enteromius gurneyi</i> and <i>Amphilius natalensis</i>, the former now extirpated as per our study.