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.