This review places the life-history styles of fishes associated with South
African estuaries in a global context and presents a classification system
incorporating all the major life-history categories for estuary-associated
fish species around the world. In addition, it documents the early life his
tories of the major fish groups in South African estuaries, with particular
emphasis on the differing modes of estuarine utilization by marine, estuar
ine and freshwater taxa.
This review details factors influencing the ichthyofaunal community structu
re in South African estuaries. The availability of fish for recruitment int
o an estuary depends primarily upon the distributional range of euryhaline
marine and estuarine species, with tropical and temperate taxa showing mark
ed abundance trends. Within a particular biogeographic region, however, est
uarine type and prevailing salinity regime have a major influence on the de
tailed ichthyofaunal structure that develops. There is an increasing prepon
derance of marine fish taxa when moving from a freshwater-dominated towards
a seawater-dominated type of system, and a decline in species diversity be
tween subtropical estuaries in the north-east and cool temperate systems in
the south-west. Similar declines in fish species diversity between tropica
l and temperate estuaries in other parts of the world are highlighted.
Fish assemblages in estuaries adjust constantly in response to changing sea
sons, salinities, turbidities, etc. Despite persistent fluctuations in both
the biotic and abiotic environment, the basic ichthyofaunal structure appe
ars to have an underlying stability and to be predictable in terms of the r
esponse of individual species to specific conditions. This stability seems
to be governed by factors such as the dominance of eurytopic taxa within es
tuarine assemblages and the robust nature of food webs within these systems
. The predictability arises from factors such as the seasonality associated
with estuarine spawning cycles and juvenile fish recruitment patterns. The
se patterns, together with a well-documented resilience to a wide range of
physico-chemical and biotic perturbations, appear to be an underlying featu
re of fish assemblages in estuaries around the world.
In contrast to marine fish species, estuary-associated taxa have received l
ittle conservation attention. Apart from the designation of protected areas
, the main direct means of conserving estuary-associated fish stocks includ
e habitat conservation and controls over fishing methods, effort, efficienc
y and seasonality. Of these, the conservation of fish habitats, the most im
portant, because healthy aquatic environments invariably support healthy fi
sh populations. The use of estuarine sanctuaries for fish conservation is b
riefly reviewed, as well as the legislation governing the USA National Estu
arine Research Reserve System (NERRS) and the Australian Marine and Estuari
ne Protected Area (MEPA) system. It is concluded that South Africa requires
an expansion of the existing Estuarine Protected Area (EPA) network, as we
ll as the upgrading of selected `estuarine reserves' where fishing is permi
tted, into `estuarine sanctuaries' where no exploitation of biological reso
urces is allowed.