Monitoring the efficacy of a lowland instream barrier on the lower uThukela River and the importance of river connectivity.

Abstract

Freshwater is the key to life on earth, with rivers being the pathway that allows it to run from mountains to the ocean, performing many important functions along its way. Humans rely on the many ecosystem services that freshwater provides, such as water quantity provision for domestic, industrial, and agricultural processes, food supply, water purification, and recreation and tourism, to name a few. Water-scarce countries, such as South Africa, are particularly vulnerable to water shortage issues and require many water supply solutions, like dams, weirs, and transfer schemes, to harvest the available rainfall. The uThukela River in KwaZulu-Natal is not only an important resource within its catchment but also to external catchments through inter-basin water transfer schemes, including that of the Thukela-Vaal transfer, which feeds the economically important hub of South Africa, the Gauteng Province. The lower uThukela River is a highly stressed system, with synergistic effects from multiple stressors relating to water quality, water quantity, habitat alterations, and wildlife disturbance affecting it. Additionally, the Lower Thukela Bulk Water Supply Scheme (LTBWSS) weir was recently constructed in its lower reaches, approximately 20 km upstream of the uThukela Mouth to the Indian Ocean, to provide bulk water to surrounding local municipalities. Due to the rich diversity of fish species in the region, particularly those with marine, estuarine, and freshwater migratory patterns, a fishway was incorporated into the design of the weir to facilitate fish movements over the weir. This study evaluated the impact that the LTBWSS weir has on fish community structures in the region and additionally assessed the efficacy of the fishway in allowing fish movements through it. Various sites were selected upstream, downstream, and on the tributary Mandeni ii Stream to assess environmental variables driving fish community structures, with passive and active sampling methods used to assess the fishway's functionality. Fish collection occurred at three sites upstream of the LTBWSS weir, four sites downstream of it on the uThukela River, and two sites on the Mandeni Stream. Abiotic variables relating to water quality, velocity, depth, and habitat were collected along with fish to determine which environmental variables were driving the fish communities at these sites. Multivariate analyses indicated that available substrate and cover, the average depth, and temperature were drivers of the fish communities in the study. Upstream sites showed lower species richness compared with downstream sites, with fish communities largely made of freshwater species and few euryhaline species. Additionally, individual species showed different responses to different environmental variables. Furthermore, since the construction of the LTBWSS, the loss of previously highly abundant cichlid species has occurred in the region. This is likely because of the synergistic effects of stressors created by it, such as the disruption of fine sediment transport, water abstraction, and pollution. Passive assessment of the fishway's efficacy in catering for migratory species used PIT telemetry. Budget constraints only allowed the installation of a single PIT antenna at the upstream entrance of the fishway, which was able to assess the upstream migration of fish from downstream. The results found that only eight individuals representing three species managed to navigate the fishway during the study successfully. This included Oreochromis mossambicus, Labeo molybdinus, and Clarias gariepinus. Active sampling involved electrofishing three key locations in the fishway on a monthly basis. Results showed that small-size classes of fish largely dominated the fishway and that the downstream entrance had the highest abundances and species richness. Further research on the role of the fishway in maintaining river connectivity is recommended. iii The outcomes of this study showed the importance that water resource managers have in maintaining the resource for humans and the environment. Knowing individual species' responses to environmental variables allows their populations to be better managed. Additionally, the outcomes of this study showed the importance of river connectivity past a barrier and highlighted the need for effective fish passage solutions in South Africa. It emphasised the need to better understand the migratory requirements of local fish to build better fish passage structures. Major stressors to be addressed include the impacts caused by barriers relating to flow releases, migration blocks, and habitat alteration upstream and downstream of them. Furthermore, the proper management of fish passage structures is essential to their functionality, which includes regular monitoring of the fishway for issues such as debris blockages and swiftly finding solutions to them to ensure that no undue delays or stress may occur for migratory fish.