Recruitment and presence of native migratory fish species in the uThukela River estuary and lower uMngeni River catchment, KwaZulu-Natal, South Africa

Abstract

African rivers provide significant sources of freshwater and are ecologically and economically important to human livelihoods, especially regarding income and food sources. In Africa, ~200 million people consume freshwater fish daily, and ~10 million people are gaining an income associated with freshwater fisheries. Unfortunately, African rivers are some of the most unpredictable rivers as they fall victim to extreme variability in precipitation. These rivers and streams are under threat to anthropogenic activities, such as the building of dams, weirs and locks, water extraction, modified river embankments, water quality issues, flow modifications from hydropower dams, and climate issues, that are significant threats to all fish species. Estuaries are unique and highly productive systems that are rich in biodiversity, as they are tidal-driven. An intricate flow dynamic connecting freshwater systems, estuaries, and the sea needs to be maintained for optimum ecosystem functioning. Thus, rivers are fundamental to the connectivity they provide. Physical barriers decrease the ecological connectivity of river systems. This has a major impact on various fish species, especially migratory fish. Migratory fish are major ecological drivers that can shape the structure and function of ecosystems. They are fundamental in maintaining food webs as predators or prey. Many flagship migratory species (i.e. yellowfish (Labeobarbus spp.) and eels (Anguilla spp.) in South Africa are prevented from reaching their breeding grounds. Four of the globally nine recognised species of anguillid eels are located and documented along the East flowing African Rivers, including those in South Africa. A few studies have been conducted on the recruitment of glass eels into the rivers and estuaries in the Western Indian Ocean. There are limited studies on the location of spawning areas of African freshwater eels. Relatively little is known about the migratory requirements of African fish species; only a few quantitative studies have been conducted to support the understanding of the migratory habits of these fish species and their dependency on free-flowing and connected rivers. This study investigated the recruitment of African freshwater glass eels into the uThuleka River. Environmental data were collected. Glass eel species were identified by their tail and caudal fin pigmentation together with DNA barcoding. The findings included seasonal variation in recruitment, but the majority of glass eels were caught in the wet season, in warmer temperatures, during high spring tides and at night. These findings should encourage more frequent sampling in estuaries across all seasons along the Western Indian Ocean coast, as the timing of glass eel recruitment can be used to locate where the African Freshwater glass eels’ breeding area is. Additionally, this study aimed to characterise the risk of multiple stressors, including instream anthropogenic barriers, on the ecology of fish communities and their environmental drivers in the lower part of the uMngeni River catchment. Various fish sampling methods were used to capture fish and investigate their species diversity and evenness. Statistical analyses were used to evaluate the relationship between fish communities and environmental variables, including barriers. The findings showed distinct fish community structures found across all sites in the catchment, which were susceptible to multiple stressors impacting migratory fish, especially velocity and mean depth. These stressors result in impaired river connectivity, which reduces a fish’s ability to recolonise a river's range sufficiently. Restoring and considering river connectivity in the lower uMngeni catchment can potentially improve fish communities.