Nw. Quinn et al., An index for the management of South African estuaries for juvenile fish recruitment from the marine environment, FISH MA EC, 6(5), 1999, pp. 421-436
The release of fresh water from major impoundments to meet the requirements
of estuaries has become a recognized need and methodologies to define appr
opriate release strategies have become an urgent priority in some parts of
the world. A recruitment index for juvenile marine fish is presented, based
on information relating to the preferred recruitment periods for 27 specie
s, as well as the extent of dependency of these species on estuaries. The s
tatus of the estuary mouth (i,e, open or closed) and the longitudinal salin
ity differences (i,e, the difference between the salinity at the estuary he
ad and the mouth) in the estuary are obtained from a hydrodynamic model of
the estuary. The recruitment index integrates this information to provide a
n indication of potential recruitment (or recruitment opportunity). This in
dex was applied in a case study of the Great Brak Estuary, South Africa, an
d a distinct decrease in the recruitment index was associated with a reduct
ion in freshwater inflow. In the estuary modelled, annual run-off could be
halved without an appreciable decrease in the fish recruitment index (FRI),
but with a potentially sharp decline in fish recruitment thereafter. While
the index represented recruitment opportunity rather than actual recruitme
nt, it does at least provide an indication of where ecological risk is like
ly to increase substantially. Since the index integrates the current unders
tanding with respect to the key processes which regulate recruitment of juv
enile marine fish into estuaries, it is suggested that it has the potential
to fulfil a vital role in identifying appropriate freshwater release polic
ies for impoundments in South Africa.