Fishes and freshwater in southern African estuaries – A review
The functioning of estuaries relies on a natural dynamism imposed on these systems by riverine and marine influences. The increasing abstraction of fresh water from both large and small river catchments in southern Africa has had the effect of forcing some estuaries into artificial cycles, i.e. natural successions now have human imposed trajectories that are changing estuarine variability and forcing some systems into extreme states. The ichthyofauna has responded to these changes in a variety of ways. Where river flow has declined considerably, or ceased altogether for extended periods, fish recruitment has shown a considerable decrease. This can be related to the collapse in planktonic productivity which negatively affects zooplanktivorous fishes, as well as decreased amounts of olfactory cues entering the sea for the attraction of larval and juvenile marine fishes into these estuaries. Hypersaline conditions can result in both a reduced species diversity and abundance. However, where estuaries lose their normal estuarine salinity gradient and become “arms” of the sea, there is often an increase in fish species diversity due to stenohaline marine taxa entering the estuary. Unfortunately the gain in small numbers of marine stragglers is insufficient to compensate for the decline in estuarine dependent fishes that usually dominate these systems. Conversely, major river flooding often causes temporary decreases in both species diversity and abundance due to a rapid decline in salinity, increased suspended sediments, reduced dissolved oxygen levels, and a collapse in the availability of pelagic and benthic food resources. However, the “resetting” of estuaries by episodic events is part of the essential cycle that maintains and enhances estuarine productivity and habitat diversity. Recovery by estuary associated fishes from such events is usually rapid and linked to a variety of factors, especially estuary morphometry which has a direct influence on the flushing or retention of estuarine biota. Freshwater flows interact directly and indirectly with the fishes that inhabit estuaries, e.g. river floods directly influence estuarine morphometry, water temperature, salinity, pH, turbidity, nutrient status, organic inputs, dissolved oxygen concentrations and olfactory cues; and indirectly affect mouth status, tidal prism, habitat diversity, primary and secondary productivity, fish recruitment, food availability and competition. Depletion or removal of components of river flow to an estuary have major short and long-term negative impacts on the ichthyofauna, some of which can be ameliorated by the provision of an environmental freshwater allocation that is appropriate to that particular system.