Monitoring the efficacy of a lowland instream barrier on the lower uThukela River and the importance of river connectivity.
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Date
2023
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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 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. 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.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.