Assessing instream structures to evaluate river connectivity of urban streams in the city of Pietermaritzburg, KwaZulu-Natal, South Africa

Abstract

Artificial barriers in freshwater ecosystems may hinder fish movement by altering their habitat and fragmenting rivers. We assessed the impact of anthropogenic structures on river connectivity in Pietermaritzburg, KwaZulu-Natal, South Africa. We mapped the barriers on eight watercourses using the Adaptive Management of Barriers in European Rivers (AMBER) ‘Barrier Tracker’ application between 2021 and 2022. We logged different types of instream obstacles, recorded their description, and uploaded photographs. We found that the Wilgerfontein and Dorpspruit streams had a higher density of barriers per kilometre stretch and greater disconnectivity in their longitudinal waterways. Culverts were the main cause of disconnectivity and the most abundant after structures categorised as ‘other’, mainly road bridges. The number of barriers in the Dorpspruit, Wilgerfontein streams and the uMsunduze River were similar, although the assessed lengths differed. The uMsunduze is the main stream and is four times longer than either the Dorpspruit and Wilgerfontein lengths as the two longest tributaries assessed. The high number of instream barriers, particularly dysfunctional culverts, needs to be mitigated. Removal of some instream barriers, installation of fishways, and the upgrading and maintenance of other instream structures can improve river connectivity. Poor river connectivity impedes the persistence of the freshwater fauna in these rivers, especially those that migrate seasonally.