Abundance of larval and 0+ juvenile marine fishes in the lower reaches of three southern African estuaries with differing freshwater inputs

Abstract

Although numerous studies have been conducted on the biology and ecology of larval and juvenile marine fishes in estuaries, information on the factors influencing abundance, which in turn reflects the magnitude of immigration into these systems, is generally lacking. In an effort to provide some insight into the posslble role of freshwater discharge on the abundance of marine larval and juvenile fishes, 3 permanently open eastern Cape Province estuaries with similar tidal prisms and mouth dimensions, but differing riverine inputs and turbidity characteristics, were selected as study sites. The Great Fish system can be characterized as a turbid estuary (>50 NTU) with a moderate axial salinity gradient (> 20 g kg-'), the Sundays as a semi-turbid estuary (> 10 NTU) with a strong axial salinity gradient (>30 g kg-'), and the Kariega as a relatively clear estuary (< 10 NTU) with a weak axial salinity gradient (< 10 g kg-'). Although the number of marine fish taxa entering the 3 estuaries was similar (18 to 20 species), Littoral densities of larvae and 0+ juveniles in the lower reaches of the Great Fish (280 ind. 100 m-2) and Sundays (290 ind. 100 m-2) estuaries were significantly (p < 0.01) greater than those in the Kariega system (50 ind. 100 m-'). lchthyoplankton densities in the lower reaches of the 3 estuaries were highest in the Great Fish estuary (13 ind. 100 m-3), followed by the Sundays Estuary (7 ind. 100 m-3) and Kariega Estuary (4 ind. 100 m-3). Regression analyses indicated that several factors were associated wlth the abundance of the early life stages at the ichthyonekton study sites, the most important of which were estuarine axial salinity gradient, water temperature and axial turbidity gradient. The possible significance of these and other associated variables in influencing migration processes into eastern Cape estuaries is discussed.