Assessment of the current ecological integrity of the uMngeni River, KwaZulu-Natal, South Africa, using fish community structures and attributes of the Labeobarbus natalensis (Castelnau, 1861) populations.

Abstract

Rivers are the main source of freshwater water for human communities and provide people with numerous ecosystem services such as water purification, transportation, power generation, food supply, and water for domestic, agricultural and industrial use. Water resources, and the ecosystem services they provide, are particularly important in developing countries, such as South Africa. The uMngeni River, is a strategic resource that provides water to two of the largest cities in KwaZulu-Natal Province (the uMgungundlovu and eThekwini municipalities), with more than four million people, making it socio-economically important. As such, to maintain sustainability the protection of the river is important. However, in South Africa and KwaZulu-Natal, the impact of anthropogenic activities has made riverine ecosystems one of the most threatened types of ecosystems in the world. The use of fish as key indicators of the ecological state of aquatic ecosystems is well established as their vulnerability to environmental change, mobility, longevity and relative ease of species identification make them good indicators. This study evaluated the current ecological integrity of the uMngeni River in KwaZulu-Natal using multiple lines of evidence including fish communities and the state of Labeobarbus natalensis (the KwaZulu-Natal yellowfish) populations, and environmental variables. The research was undertaken in the major man-made lakes (dams) in the uMngeni River (namely Midmar, Albert Falls, Nagle and Inanda Dams) and in the rivers of the uMngeni Catchment. Abiotic lines of evidence investigated included water quality and habitat, while the biotic lines of evidence included fish community structures and attributes of the population of L. natalensis. Fish community structures at eight selected River Eco-status Monitoring Programme (REMP) sites in the uMngeni catchment were considered. This included consideration of how the fish communities responded to changes in a range of environmental variables and alien fishes using the Fish Response Assessment Index (FRAI), we were able to determine that the ecological integrity of the uMngeni River decreases in a downstream gradient from the upper reaches of the catchment to lower reaches, due the synergistic effect of multiple anthropogenic stressors. The multivariate analyses indicated that the anthropogenic impacts responsible for shifts in fish community structures, and the associated ecological integrity of the river were related to changes in instream habitats and water quality stressors primarily. Most of the environmental changes identified can be linked to flow modifications and land use activities throughout the uMngeni catchment. Assessments of attributes of the L. natalensis populations from large instream impoundments in the uMngeni River (namely Midmar, Albert Falls, Nagle and Inanda Dam) resulted in diminishing wellbeing of the populations of this endemic migratory fish progressively both in abundance and structure, down the length of the catchment. The quality and quantity of water diminished down the catchment gradient with this gradient and the effect of the barriers themselves can partially be attributed to the impaired state of the populations. Impoundments are not preferred by juvenile and young L. natalensis that prefer shallow riffle habitats that are lacking in dams, the occurrence of many predatory alien fishes in the dam can also be attributed to the absence of small yellowfish in the dams. The outcomes of this study can contribute to the sustainable management and development of conservation plans for the rivers and dams in the uMngeni catchment. Major stressors that should be mitigated include the barrier effect and operation or flow releases from the large dams and smaller weirs etc. that cause river fragmentation in the catchment. It is recommended that management plans for the conservation of the fishes in the catchment should be developed which is achievable as the current supply of resources in the catchment is balanced with the demand for use. Fish passages should be established in all of the dams in the uMngeni River to allow migratory fish free passage along the river and to re-establish river connectivity processes. Additionally, the removal of redundant weirs or partial man-made barriers is recommended to alleviate the effects of fragmentation particularly on the yellowfish in the catchment. More research is required to understand the migratory requirements of fishes in the catchments and the cost-benefit of mitigating river fragmentation to achieve a sustainable balance between the use and protection of resources in the catchment. Finally, the study has identified water quality and flow stressors that are negatively affecting the wellbeing of the fish communities in the catchment. The water quality stressors derived from land-based activities and associated management of flows in the catchment must be improved to attain a sustainable balance between the use and protection of the resources of the uMngeni Catchment.