Influence of altered freshwater flow regimes on abundance of larval and juvenile Gilchristella aestuaria (Pisces : Clupeidae) in the upper reaches of two South African estuaries

Past investigations into South African estuaries that are subject to altered freshwater flow regimes have concentrated on the effects of reduced river flow on recruiting estuary-dependent marine fish species. This study compares larval and juvenile abundance of an estuary resident, Gilchristella aestuaria, in the upper reaches of the 'freshwater-deprived' Kariega Estuary and the 'freshwater-rich' Great Fish Estuary. Catches in the Kariega Estuary were significantly greater than those in the Great Fish Estuary. Larval and juvenile densities were correlated and regressed with salinity, temperature, turbidity and river flow in each system. River flow was found to be the most probable defining factor in terms of the contrasting catches in these systems. G. aestuaria larvae and juveniles appear to be flushed out of the Great Fish system as a result of the excessive amount of river flow received from an inter-basin water transfer scheme. Although freshwater input into estuarine systems is vital for the nursery function of the lower reaches of South African estuaries, excessive supply can alter the larval composition of estuary-resident fish species in the upper reaches.