Fish Diversity and Community Structures of the Lower Phongolo River Following Four Decades of Human Impacts

Dams are essential in securing adequate water supply to human populations but at the expense of significant environmental impacts. Understanding the ecological consequences of dams is crucial for sustainable water management. Freshwater ecosystems already overexploited for various human activities, such as damming, are particularly vulnerable to climate change and drought events. This study assessed historical fish population data from the Phongolo River floodplain (PRF) and the only protected area of the region, Ndumo Game Reserve (NGR), in northern KwaZulu-Natal, South Africa, to determine whether there have been any shifts in the fish communities since the construction of the Pongolapoort Dam in the 1970s as well as the occurrence of recent droughts. A total of 46 fish species from 12 different families have been documented in the PRF since the 1970s. The number of species and families have varied over time, with the highest records occurring during periods of high flow (42 species) and the lowest records during periods of extreme droughts (30 species). Nonetheless, the fish diversity has remained high, with 37 species recorded in the most recent surveys of 2019, highlighting the resilience of this ecosystem and its conservation value. However, fish abundance in the river and its associated floodplains is low, likely due to overexploitation by local communities and threats from flood releases, poor water quality, and water abstraction outside NGR. Additionally, the invasive carp, Cyprinus carpio, has also been recorded in this ecosystem since the 1990s, potentially disrupting native biodiversity through destructive feeding habits and introduction of invasive parasites. Ndumo Game Reserve and its floodplain habitats act as import refugia for fish, preserving the biodiversity of the PRF and protecting them from external anthropogenic influences. Continued conservation and appropriate management of this small game reserve is crucial to maintain the ecological integrity of this ecosystem.