Source to sea conservation of the fish communities of the greater Incomati river catchment, Southern Africa.

Abstract

South Africa is a water-scarce country, with its water resources under constant strain due to ongoing resource use and development. With high levels of poverty, inequality and unemployment, many people in South Africa rely on natural river systems to meet their daily water and natural product requirements. The National Water Act of South Africa made provision for this by legislating ''fresh" or "clean" water required to meet domestic use as a basic human need. But, continued population growth, land transformations, intensified agriculture and industrial diversification coupled with climate change and unsuccessful law enforcement are threatening and degrading the ecosystems and the biodiversity that depends on them. Degradation of freshwater systems at the current rate may cause two-thirds of the human population to face severe water shortage and water stress as soon as 2030 in the region and losses of associated ecosystem services. Freshwater fishes and their populations and community structures are indicators of key ecological attributes of complex ecosystems. A lot is known about fishes and this information is used to understand, monitor and manage ecosystems. Changes in ecosystem function and associated condition drives changes in fish communities that is characterised and used to determine the amount of degradation that specific ecosystem has undergone. Multiple Lines of Evidence (LoEs) can be implemented to evaluate the present condition of biotic communities in important catchments, characterise drivers of changes in communities and evaluate the threat of multiple stressors affecting biotic communities and the ecosystems they live in. The study aimed to characterise and evaluate the effects of multiple anthropogenic stressors affecting the rivers of the lncomati River Catchment from source to sea, using fish communities and migratory species as ecological indicators of the rivers through multiple lines of evidence. The greater Incomati River Catchment is a semi-arid environment located in the southeastern parts of South Africa, northern parts of Eswatini and southern parts of Mozambique. The greater Incomati River Catchment is identified as an ecologically important Water Management Area (WMA) in South Africa due to the high number of endemic fish species and conservational fish protected areas. The greater Incomati River Catchment is home to 70 known expected species throughout the catchment. The greater Incomati River Catchment is also one of the most stressed ecosystems in South Africa, partly due to land use transformations, agriculture and urbanisation. Currently, many of these environmental stressors are driving the fish communities negatively that could lead to the degradation of the 2 ecosystem and the provided ecosystem services. Fifty of the expected 70 fish species has been collected throughout the catchment, along with four invasive fish species. Using multiple lines of evidence and exciting biotic indices, the current state of the fish communities in the greater Incomati River Catchment is in a moderately-modified state with a low to moderate relative risk assessment score. Various anthropogenic activities in the catchment are leading to environmental drivers such as barrier formation, sedimentation, flow alteration, water quality alterations and habitat modification to occur. Water quality modifications and alterations in substrate diversity contributed significantly to affect the fish communities and the ecological and social services throughout the greater Incomati River Catchment. The risk assessment of the different risk regions supports the effects of these anthropogenic activities and identified areas of concern due to adverse ecological effects that occur or will occur due to the exposer of one or more stressors. Management considerations are needed to addressed flow alterations and limited water quality and habitat stressors in the Uanetze and Sabie River sub-catchments, considerable water quality, flow alterations, habitat changes and alien invasive species in the Crocodile River sub-catchment, the extreme effect of barriers and altered flows with alien invasive species affecting fish communities in the Komati River sub-catchment and lastly, invasive species, overexploitation and agricultural water abstraction in the Incomati River sub-catchment. Additionally, anguillid eels were used as an ecological indicator species to the effects of anthropogenic stressors in the greater Incomati River Catchment. A 40.2% decline in anguillid eel populations indicate the drastic effects of environmental dysconnectivity and overexploitation. A Relative Risk Assessment indicate the risk associated to the different life cycle stages of anguillid eels as well as the different anthropogenic stressors that affects the populations at each stage. This study successfully characterized and evaluated the effects of multiple anthropogenic stressors on the rivers of the greater Incomati River Catchment, from source to sea. By utilizing fish communities and migratory species as ecological indicators, I have provided valuable insights into the health and resilience of these aquatic ecosystems. Through multiple lines of evidence, the findings highlight the extent of human-induced pressures and their implications for riverine biodiversity and ecosystem functioning.