Assessment of the ecological integrity and fish community structures of the uMngeni River, KwaZulu-Natal, South Africa.

The uMngeni River is economically important as it provides water to two of the largest cities in KwaZulu-Natal Province, South Africa. As such, protecting the river and the life within it is also of great importance. In this study, we used fish community structure as an indicator of ecosystem health by assessing how fish communities responded to changes in habitat composition and water quality as a consequence of anthropogenic activities, using the Fish Response Assessment Index (FRAI). We used multivariate statistical analyses to determine differences in fish communities and drivers of change in these communities. We found that the ecological integrity of the uMngeni River (and its tributaries) tended to degrade from upper to lower reaches in response to various anthropogenic activities. Examples of degradation included flow modification and migration barriers from instream structures, the introduction of invasive fish species, and water quality alterations from rural and urban settlements. Multivariate analyses showed that variation among the sites selected in this study was significantly driven by changes in velocity-depth classes, substrate type, and water quality, all of which can be influenced by flow modifications. The FRAI ecological scores and multivariate analyses presented in the study provide a baseline for managing the uMngeni River's fish communities, highlighting that flow regime, river fragmentation, and alien invasive species negatively impact these.